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Sourcecode: postgresql-8.4 version File versions

xlog.c

/*-------------------------------------------------------------------------
 *
 * xlog.c
 *          PostgreSQL transaction log manager
 *
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.345 2009/06/26 20:29:04 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <ctype.h>
#include <signal.h>
#include <time.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <unistd.h>

#include "access/clog.h"
#include "access/multixact.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/tuptoaster.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogutils.h"
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/pmsignal.h"
#include "storage/procarray.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/flatfiles.h"
#include "utils/guc.h"
#include "utils/ps_status.h"
#include "pg_trace.h"


/* File path names (all relative to $PGDATA) */
#define BACKUP_LABEL_FILE           "backup_label"
#define BACKUP_LABEL_OLD            "backup_label.old"
#define RECOVERY_COMMAND_FILE "recovery.conf"
#define RECOVERY_COMMAND_DONE "recovery.done"


/* User-settable parameters */
int               CheckPointSegments = 3;
int               XLOGbuffers = 8;
int               XLogArchiveTimeout = 0;
bool        XLogArchiveMode = false;
char     *XLogArchiveCommand = NULL;
bool        fullPageWrites = true;
bool        log_checkpoints = false;
int               sync_method = DEFAULT_SYNC_METHOD;

#ifdef WAL_DEBUG
bool        XLOG_DEBUG = false;
#endif

/*
 * XLOGfileslop is the maximum number of preallocated future XLOG segments.
 * When we are done with an old XLOG segment file, we will recycle it as a
 * future XLOG segment as long as there aren't already XLOGfileslop future
 * segments; else we'll delete it.  This could be made a separate GUC
 * variable, but at present I think it's sufficient to hardwire it as
 * 2*CheckPointSegments+1.    Under normal conditions, a checkpoint will free
 * no more than 2*CheckPointSegments log segments, and we want to recycle all
 * of them; the +1 allows boundary cases to happen without wasting a
 * delete/create-segment cycle.
 */
#define XLOGfileslop    (2*CheckPointSegments + 1)

/*
 * GUC support
 */
const struct config_enum_entry sync_method_options[] = {
      {"fsync", SYNC_METHOD_FSYNC, false},
#ifdef HAVE_FSYNC_WRITETHROUGH
      {"fsync_writethrough", SYNC_METHOD_FSYNC_WRITETHROUGH, false},
#endif
#ifdef HAVE_FDATASYNC
      {"fdatasync", SYNC_METHOD_FDATASYNC, false},
#endif
#ifdef OPEN_SYNC_FLAG
      {"open_sync", SYNC_METHOD_OPEN, false},
#endif
#ifdef OPEN_DATASYNC_FLAG
      {"open_datasync", SYNC_METHOD_OPEN_DSYNC, false},
#endif
      {NULL, 0, false}
};

/*
 * Statistics for current checkpoint are collected in this global struct.
 * Because only the background writer or a stand-alone backend can perform
 * checkpoints, this will be unused in normal backends.
 */
CheckpointStatsData CheckpointStats;

/*
 * ThisTimeLineID will be same in all backends --- it identifies current
 * WAL timeline for the database system.
 */
TimeLineID  ThisTimeLineID = 0;

/*
 * Are we doing recovery from XLOG?
 *
 * This is only ever true in the startup process; it should be read as meaning
 * "this process is replaying WAL records", rather than "the system is in
 * recovery mode".  It should be examined primarily by functions that need
 * to act differently when called from a WAL redo function (e.g., to skip WAL
 * logging).  To check whether the system is in recovery regardless of which
 * process you're running in, use RecoveryInProgress().
 */
bool        InRecovery = false;

/*
 * Local copy of SharedRecoveryInProgress variable. True actually means "not
 * known, need to check the shared state".
 */
static bool LocalRecoveryInProgress = true;

/*
 * Local state for XLogInsertAllowed():
 *          1: unconditionally allowed to insert XLOG
 *          0: unconditionally not allowed to insert XLOG
 *          -1: must check RecoveryInProgress(); disallow until it is false
 * Most processes start with -1 and transition to 1 after seeing that recovery
 * is not in progress.  But we can also force the value for special cases.
 * The coding in XLogInsertAllowed() depends on the first two of these states
 * being numerically the same as bool true and false.
 */
static int  LocalXLogInsertAllowed = -1;

/* Are we recovering using offline XLOG archives? */
static bool InArchiveRecovery = false;

/* Was the last xlog file restored from archive, or local? */
static bool restoredFromArchive = false;

/* options taken from recovery.conf */
static char *recoveryRestoreCommand = NULL;
static char *recoveryEndCommand = NULL;
static bool recoveryTarget = false;
static bool recoveryTargetExact = false;
static bool recoveryTargetInclusive = true;
static TransactionId recoveryTargetXid;
static TimestampTz recoveryTargetTime;
static TimestampTz recoveryLastXTime = 0;

/* if recoveryStopsHere returns true, it saves actual stop xid/time here */
static TransactionId recoveryStopXid;
static TimestampTz recoveryStopTime;
static bool recoveryStopAfter;

/*
 * During normal operation, the only timeline we care about is ThisTimeLineID.
 * During recovery, however, things are more complicated.  To simplify life
 * for rmgr code, we keep ThisTimeLineID set to the "current" timeline as we
 * scan through the WAL history (that is, it is the line that was active when
 * the currently-scanned WAL record was generated).  We also need these
 * timeline values:
 *
 * recoveryTargetTLI: the desired timeline that we want to end in.
 *
 * expectedTLIs: an integer list of recoveryTargetTLI and the TLIs of
 * its known parents, newest first (so recoveryTargetTLI is always the
 * first list member).  Only these TLIs are expected to be seen in the WAL
 * segments we read, and indeed only these TLIs will be considered as
 * candidate WAL files to open at all.
 *
 * curFileTLI: the TLI appearing in the name of the current input WAL file.
 * (This is not necessarily the same as ThisTimeLineID, because we could
 * be scanning data that was copied from an ancestor timeline when the current
 * file was created.)  During a sequential scan we do not allow this value
 * to decrease.
 */
static TimeLineID recoveryTargetTLI;
static List *expectedTLIs;
static TimeLineID curFileTLI;

/*
 * ProcLastRecPtr points to the start of the last XLOG record inserted by the
 * current backend.  It is updated for all inserts.  XactLastRecEnd points to
 * end+1 of the last record, and is reset when we end a top-level transaction,
 * or start a new one; so it can be used to tell if the current transaction has
 * created any XLOG records.
 */
static XLogRecPtr ProcLastRecPtr = {0, 0};

XLogRecPtr  XactLastRecEnd = {0, 0};

/*
 * RedoRecPtr is this backend's local copy of the REDO record pointer
 * (which is almost but not quite the same as a pointer to the most recent
 * CHECKPOINT record).  We update this from the shared-memory copy,
 * XLogCtl->Insert.RedoRecPtr, whenever we can safely do so (ie, when we
 * hold the Insert lock).  See XLogInsert for details.      We are also allowed
 * to update from XLogCtl->Insert.RedoRecPtr if we hold the info_lck;
 * see GetRedoRecPtr.  A freshly spawned backend obtains the value during
 * InitXLOGAccess.
 */
static XLogRecPtr RedoRecPtr;

/*----------
 * Shared-memory data structures for XLOG control
 *
 * LogwrtRqst indicates a byte position that we need to write and/or fsync
 * the log up to (all records before that point must be written or fsynced).
 * LogwrtResult indicates the byte positions we have already written/fsynced.
 * These structs are identical but are declared separately to indicate their
 * slightly different functions.
 *
 * We do a lot of pushups to minimize the amount of access to lockable
 * shared memory values.  There are actually three shared-memory copies of
 * LogwrtResult, plus one unshared copy in each backend.  Here's how it works:
 *          XLogCtl->LogwrtResult is protected by info_lck
 *          XLogCtl->Write.LogwrtResult is protected by WALWriteLock
 *          XLogCtl->Insert.LogwrtResult is protected by WALInsertLock
 * One must hold the associated lock to read or write any of these, but
 * of course no lock is needed to read/write the unshared LogwrtResult.
 *
 * XLogCtl->LogwrtResult and XLogCtl->Write.LogwrtResult are both "always
 * right", since both are updated by a write or flush operation before
 * it releases WALWriteLock.  The point of keeping XLogCtl->Write.LogwrtResult
 * is that it can be examined/modified by code that already holds WALWriteLock
 * without needing to grab info_lck as well.
 *
 * XLogCtl->Insert.LogwrtResult may lag behind the reality of the other two,
 * but is updated when convenient.  Again, it exists for the convenience of
 * code that is already holding WALInsertLock but not the other locks.
 *
 * The unshared LogwrtResult may lag behind any or all of these, and again
 * is updated when convenient.
 *
 * The request bookkeeping is simpler: there is a shared XLogCtl->LogwrtRqst
 * (protected by info_lck), but we don't need to cache any copies of it.
 *
 * Note that this all works because the request and result positions can only
 * advance forward, never back up, and so we can easily determine which of two
 * values is "more up to date".
 *
 * info_lck is only held long enough to read/update the protected variables,
 * so it's a plain spinlock.  The other locks are held longer (potentially
 * over I/O operations), so we use LWLocks for them.  These locks are:
 *
 * WALInsertLock: must be held to insert a record into the WAL buffers.
 *
 * WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
 * XLogFlush).
 *
 * ControlFileLock: must be held to read/update control file or create
 * new log file.
 *
 * CheckpointLock: must be held to do a checkpoint or restartpoint (ensures
 * only one checkpointer at a time; currently, with all checkpoints done by
 * the bgwriter, this is just pro forma).
 *
 *----------
 */

typedef struct XLogwrtRqst
{
      XLogRecPtr  Write;                  /* last byte + 1 to write out */
      XLogRecPtr  Flush;                  /* last byte + 1 to flush */
} XLogwrtRqst;

typedef struct XLogwrtResult
{
      XLogRecPtr  Write;                  /* last byte + 1 written out */
      XLogRecPtr  Flush;                  /* last byte + 1 flushed */
} XLogwrtResult;

/*
 * Shared state data for XLogInsert.
 */
typedef struct XLogCtlInsert
{
      XLogwrtResult LogwrtResult; /* a recent value of LogwrtResult */
      XLogRecPtr  PrevRecord;       /* start of previously-inserted record */
      int               curridx;          /* current block index in cache */
      XLogPageHeader currpage;      /* points to header of block in cache */
      char     *currpos;            /* current insertion point in cache */
      XLogRecPtr  RedoRecPtr;       /* current redo point for insertions */
      bool        forcePageWrites;  /* forcing full-page writes for PITR? */
} XLogCtlInsert;

/*
 * Shared state data for XLogWrite/XLogFlush.
 */
typedef struct XLogCtlWrite
{
      XLogwrtResult LogwrtResult; /* current value of LogwrtResult */
      int               curridx;          /* cache index of next block to write */
      pg_time_t   lastSegSwitchTime;            /* time of last xlog segment switch */
} XLogCtlWrite;

/*
 * Total shared-memory state for XLOG.
 */
typedef struct XLogCtlData
{
      /* Protected by WALInsertLock: */
      XLogCtlInsert Insert;

      /* Protected by info_lck: */
      XLogwrtRqst LogwrtRqst;
      XLogwrtResult LogwrtResult;
      uint32            ckptXidEpoch;     /* nextXID & epoch of latest checkpoint */
      TransactionId ckptXid;
      XLogRecPtr  asyncCommitLSN; /* LSN of newest async commit */

      /* Protected by WALWriteLock: */
      XLogCtlWrite Write;

      /*
       * These values do not change after startup, although the pointed-to pages
       * and xlblocks values certainly do.  Permission to read/write the pages
       * and xlblocks values depends on WALInsertLock and WALWriteLock.
       */
      char     *pages;              /* buffers for unwritten XLOG pages */
      XLogRecPtr *xlblocks;         /* 1st byte ptr-s + XLOG_BLCKSZ */
      int               XLogCacheBlck;    /* highest allocated xlog buffer index */
      TimeLineID  ThisTimeLineID;

      /*
       * SharedRecoveryInProgress indicates if we're still in crash or archive
       * recovery.  Protected by info_lck.
       */
      bool        SharedRecoveryInProgress;

      /*
       * During recovery, we keep a copy of the latest checkpoint record here.
       * Used by the background writer when it wants to create a restartpoint.
       *
       * Protected by info_lck.
       */
      XLogRecPtr  lastCheckPointRecPtr;
      CheckPoint  lastCheckPoint;

      /* end+1 of the last record replayed (or being replayed) */
      XLogRecPtr  replayEndRecPtr;

      slock_t           info_lck;         /* locks shared variables shown above */
} XLogCtlData;

static XLogCtlData *XLogCtl = NULL;

/*
 * We maintain an image of pg_control in shared memory.
 */
static ControlFileData *ControlFile = NULL;

/*
 * Macros for managing XLogInsert state.  In most cases, the calling routine
 * has local copies of XLogCtl->Insert and/or XLogCtl->Insert->curridx,
 * so these are passed as parameters instead of being fetched via XLogCtl.
 */

/* Free space remaining in the current xlog page buffer */
#define INSERT_FREESPACE(Insert)  \
      (XLOG_BLCKSZ - ((Insert)->currpos - (char *) (Insert)->currpage))

/* Construct XLogRecPtr value for current insertion point */
#define INSERT_RECPTR(recptr,Insert,curridx)  \
      ( \
        (recptr).xlogid = XLogCtl->xlblocks[curridx].xlogid, \
        (recptr).xrecoff = \
            XLogCtl->xlblocks[curridx].xrecoff - INSERT_FREESPACE(Insert) \
      )

#define PrevBufIdx(idx)       \
            (((idx) == 0) ? XLogCtl->XLogCacheBlck : ((idx) - 1))

#define NextBufIdx(idx)       \
            (((idx) == XLogCtl->XLogCacheBlck) ? 0 : ((idx) + 1))

/*
 * Private, possibly out-of-date copy of shared LogwrtResult.
 * See discussion above.
 */
static XLogwrtResult LogwrtResult = {{0, 0}, {0, 0}};

/*
 * openLogFile is -1 or a kernel FD for an open log file segment.
 * When it's open, openLogOff is the current seek offset in the file.
 * openLogId/openLogSeg identify the segment.  These variables are only
 * used to write the XLOG, and so will normally refer to the active segment.
 */
static int  openLogFile = -1;
static uint32 openLogId = 0;
static uint32 openLogSeg = 0;
static uint32 openLogOff = 0;

/*
 * These variables are used similarly to the ones above, but for reading
 * the XLOG.  Note, however, that readOff generally represents the offset
 * of the page just read, not the seek position of the FD itself, which
 * will be just past that page.
 */
static int  readFile = -1;
static uint32 readId = 0;
static uint32 readSeg = 0;
static uint32 readOff = 0;

/* Buffer for currently read page (XLOG_BLCKSZ bytes) */
static char *readBuf = NULL;

/* Buffer for current ReadRecord result (expandable) */
static char *readRecordBuf = NULL;
static uint32 readRecordBufSize = 0;

/* State information for XLOG reading */
static XLogRecPtr ReadRecPtr; /* start of last record read */
static XLogRecPtr EndRecPtr;  /* end+1 of last record read */
static XLogRecord *nextRecord = NULL;
static TimeLineID lastPageTLI = 0;

static XLogRecPtr minRecoveryPoint;       /* local copy of
                                                             * ControlFile->minRecoveryPoint */
static bool updateMinRecoveryPoint = true;

static bool InRedo = false;

/*
 * Flags set by interrupt handlers for later service in the redo loop.
 */
static volatile sig_atomic_t got_SIGHUP = false;
static volatile sig_atomic_t shutdown_requested = false;

/*
 * Flag set when executing a restore command, to tell SIGTERM signal handler
 * that it's safe to just proc_exit.
 */
static volatile sig_atomic_t in_restore_command = false;


static void XLogArchiveNotify(const char *xlog);
static void XLogArchiveNotifySeg(uint32 log, uint32 seg);
static bool XLogArchiveCheckDone(const char *xlog);
static bool XLogArchiveIsBusy(const char *xlog);
static void XLogArchiveCleanup(const char *xlog);
static void readRecoveryCommandFile(void);
static void exitArchiveRecovery(TimeLineID endTLI,
                              uint32 endLogId, uint32 endLogSeg);
static bool recoveryStopsHere(XLogRecord *record, bool *includeThis);
static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags);

static bool XLogCheckBuffer(XLogRecData *rdata, bool doPageWrites,
                        XLogRecPtr *lsn, BkpBlock *bkpb);
static bool AdvanceXLInsertBuffer(bool new_segment);
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch);
static int XLogFileInit(uint32 log, uint32 seg,
                   bool *use_existent, bool use_lock);
static bool InstallXLogFileSegment(uint32 *log, uint32 *seg, char *tmppath,
                                 bool find_free, int *max_advance,
                                 bool use_lock);
static int  XLogFileOpen(uint32 log, uint32 seg);
static int  XLogFileRead(uint32 log, uint32 seg, int emode);
static void XLogFileClose(void);
static bool RestoreArchivedFile(char *path, const char *xlogfname,
                              const char *recovername, off_t expectedSize);
static void ExecuteRecoveryEndCommand(void);
static void PreallocXlogFiles(XLogRecPtr endptr);
static void RemoveOldXlogFiles(uint32 log, uint32 seg, XLogRecPtr endptr);
static void ValidateXLOGDirectoryStructure(void);
static void CleanupBackupHistory(void);
static void UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force);
static XLogRecord *ReadRecord(XLogRecPtr *RecPtr, int emode);
static bool ValidXLOGHeader(XLogPageHeader hdr, int emode);
static XLogRecord *ReadCheckpointRecord(XLogRecPtr RecPtr, int whichChkpt);
static List *readTimeLineHistory(TimeLineID targetTLI);
static bool existsTimeLineHistory(TimeLineID probeTLI);
static TimeLineID findNewestTimeLine(TimeLineID startTLI);
static void writeTimeLineHistory(TimeLineID newTLI, TimeLineID parentTLI,
                               TimeLineID endTLI,
                               uint32 endLogId, uint32 endLogSeg);
static void WriteControlFile(void);
static void ReadControlFile(void);
static char *str_time(pg_time_t tnow);

#ifdef WAL_DEBUG
static void xlog_outrec(StringInfo buf, XLogRecord *record);
#endif
static void issue_xlog_fsync(void);
static void pg_start_backup_callback(int code, Datum arg);
static bool read_backup_label(XLogRecPtr *checkPointLoc,
                          XLogRecPtr *minRecoveryLoc);
static void rm_redo_error_callback(void *arg);
static int  get_sync_bit(int method);


/*
 * Insert an XLOG record having the specified RMID and info bytes,
 * with the body of the record being the data chunk(s) described by
 * the rdata chain (see xlog.h for notes about rdata).
 *
 * Returns XLOG pointer to end of record (beginning of next record).
 * This can be used as LSN for data pages affected by the logged action.
 * (LSN is the XLOG point up to which the XLOG must be flushed to disk
 * before the data page can be written out.  This implements the basic
 * WAL rule "write the log before the data".)
 *
 * NB: this routine feels free to scribble on the XLogRecData structs,
 * though not on the data they reference.  This is OK since the XLogRecData
 * structs are always just temporaries in the calling code.
 */
XLogRecPtr
XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
{
      XLogCtlInsert *Insert = &XLogCtl->Insert;
      XLogRecord *record;
      XLogContRecord *contrecord;
      XLogRecPtr  RecPtr;
      XLogRecPtr  WriteRqst;
      uint32            freespace;
      int               curridx;
      XLogRecData *rdt;
      Buffer            dtbuf[XLR_MAX_BKP_BLOCKS];
      bool        dtbuf_bkp[XLR_MAX_BKP_BLOCKS];
      BkpBlock    dtbuf_xlg[XLR_MAX_BKP_BLOCKS];
      XLogRecPtr  dtbuf_lsn[XLR_MAX_BKP_BLOCKS];
      XLogRecData dtbuf_rdt1[XLR_MAX_BKP_BLOCKS];
      XLogRecData dtbuf_rdt2[XLR_MAX_BKP_BLOCKS];
      XLogRecData dtbuf_rdt3[XLR_MAX_BKP_BLOCKS];
      pg_crc32    rdata_crc;
      uint32            len,
                        write_len;
      unsigned    i;
      bool        updrqst;
      bool        doPageWrites;
      bool        isLogSwitch = (rmid == RM_XLOG_ID && info == XLOG_SWITCH);

      /* cross-check on whether we should be here or not */
      if (!XLogInsertAllowed())
            elog(ERROR, "cannot make new WAL entries during recovery");

      /* info's high bits are reserved for use by me */
      if (info & XLR_INFO_MASK)
            elog(PANIC, "invalid xlog info mask %02X", info);

      TRACE_POSTGRESQL_XLOG_INSERT(rmid, info);

      /*
       * In bootstrap mode, we don't actually log anything but XLOG resources;
       * return a phony record pointer.
       */
      if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
      {
            RecPtr.xlogid = 0;
            RecPtr.xrecoff = SizeOfXLogLongPHD;       /* start of 1st chkpt record */
            return RecPtr;
      }

      /*
       * Here we scan the rdata chain, determine which buffers must be backed
       * up, and compute the CRC values for the data.  Note that the record
       * header isn't added into the CRC initially since we don't know the final
       * length or info bits quite yet.  Thus, the CRC will represent the CRC of
       * the whole record in the order "rdata, then backup blocks, then record
       * header".
       *
       * We may have to loop back to here if a race condition is detected below.
       * We could prevent the race by doing all this work while holding the
       * insert lock, but it seems better to avoid doing CRC calculations while
       * holding the lock.  This means we have to be careful about modifying the
       * rdata chain until we know we aren't going to loop back again.  The only
       * change we allow ourselves to make earlier is to set rdt->data = NULL in
       * chain items we have decided we will have to back up the whole buffer
       * for.  This is OK because we will certainly decide the same thing again
       * for those items if we do it over; doing it here saves an extra pass
       * over the chain later.
       */
begin:;
      for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
      {
            dtbuf[i] = InvalidBuffer;
            dtbuf_bkp[i] = false;
      }

      /*
       * Decide if we need to do full-page writes in this XLOG record: true if
       * full_page_writes is on or we have a PITR request for it.  Since we
       * don't yet have the insert lock, forcePageWrites could change under us,
       * but we'll recheck it once we have the lock.
       */
      doPageWrites = fullPageWrites || Insert->forcePageWrites;

      INIT_CRC32(rdata_crc);
      len = 0;
      for (rdt = rdata;;)
      {
            if (rdt->buffer == InvalidBuffer)
            {
                  /* Simple data, just include it */
                  len += rdt->len;
                  COMP_CRC32(rdata_crc, rdt->data, rdt->len);
            }
            else
            {
                  /* Find info for buffer */
                  for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
                  {
                        if (rdt->buffer == dtbuf[i])
                        {
                              /* Buffer already referenced by earlier chain item */
                              if (dtbuf_bkp[i])
                                    rdt->data = NULL;
                              else if (rdt->data)
                              {
                                    len += rdt->len;
                                    COMP_CRC32(rdata_crc, rdt->data, rdt->len);
                              }
                              break;
                        }
                        if (dtbuf[i] == InvalidBuffer)
                        {
                              /* OK, put it in this slot */
                              dtbuf[i] = rdt->buffer;
                              if (XLogCheckBuffer(rdt, doPageWrites,
                                                            &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
                              {
                                    dtbuf_bkp[i] = true;
                                    rdt->data = NULL;
                              }
                              else if (rdt->data)
                              {
                                    len += rdt->len;
                                    COMP_CRC32(rdata_crc, rdt->data, rdt->len);
                              }
                              break;
                        }
                  }
                  if (i >= XLR_MAX_BKP_BLOCKS)
                        elog(PANIC, "can backup at most %d blocks per xlog record",
                               XLR_MAX_BKP_BLOCKS);
            }
            /* Break out of loop when rdt points to last chain item */
            if (rdt->next == NULL)
                  break;
            rdt = rdt->next;
      }

      /*
       * Now add the backup block headers and data into the CRC
       */
      for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
      {
            if (dtbuf_bkp[i])
            {
                  BkpBlock   *bkpb = &(dtbuf_xlg[i]);
                  char     *page;

                  COMP_CRC32(rdata_crc,
                                 (char *) bkpb,
                                 sizeof(BkpBlock));
                  page = (char *) BufferGetBlock(dtbuf[i]);
                  if (bkpb->hole_length == 0)
                  {
                        COMP_CRC32(rdata_crc,
                                       page,
                                       BLCKSZ);
                  }
                  else
                  {
                        /* must skip the hole */
                        COMP_CRC32(rdata_crc,
                                       page,
                                       bkpb->hole_offset);
                        COMP_CRC32(rdata_crc,
                                       page + (bkpb->hole_offset + bkpb->hole_length),
                                       BLCKSZ - (bkpb->hole_offset + bkpb->hole_length));
                  }
            }
      }

      /*
       * NOTE: We disallow len == 0 because it provides a useful bit of extra
       * error checking in ReadRecord.  This means that all callers of
       * XLogInsert must supply at least some not-in-a-buffer data.  However, we
       * make an exception for XLOG SWITCH records because we don't want them to
       * ever cross a segment boundary.
       */
      if (len == 0 && !isLogSwitch)
            elog(PANIC, "invalid xlog record length %u", len);

      START_CRIT_SECTION();

      /* Now wait to get insert lock */
      LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);

      /*
       * Check to see if my RedoRecPtr is out of date.  If so, may have to go
       * back and recompute everything.  This can only happen just after a
       * checkpoint, so it's better to be slow in this case and fast otherwise.
       *
       * If we aren't doing full-page writes then RedoRecPtr doesn't actually
       * affect the contents of the XLOG record, so we'll update our local copy
       * but not force a recomputation.
       */
      if (!XLByteEQ(RedoRecPtr, Insert->RedoRecPtr))
      {
            Assert(XLByteLT(RedoRecPtr, Insert->RedoRecPtr));
            RedoRecPtr = Insert->RedoRecPtr;

            if (doPageWrites)
            {
                  for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
                  {
                        if (dtbuf[i] == InvalidBuffer)
                              continue;
                        if (dtbuf_bkp[i] == false &&
                              XLByteLE(dtbuf_lsn[i], RedoRecPtr))
                        {
                              /*
                               * Oops, this buffer now needs to be backed up, but we
                               * didn't think so above.  Start over.
                               */
                              LWLockRelease(WALInsertLock);
                              END_CRIT_SECTION();
                              goto begin;
                        }
                  }
            }
      }

      /*
       * Also check to see if forcePageWrites was just turned on; if we weren't
       * already doing full-page writes then go back and recompute. (If it was
       * just turned off, we could recompute the record without full pages, but
       * we choose not to bother.)
       */
      if (Insert->forcePageWrites && !doPageWrites)
      {
            /* Oops, must redo it with full-page data */
            LWLockRelease(WALInsertLock);
            END_CRIT_SECTION();
            goto begin;
      }

      /*
       * Make additional rdata chain entries for the backup blocks, so that we
       * don't need to special-case them in the write loop.  Note that we have
       * now irrevocably changed the input rdata chain.  At the exit of this
       * loop, write_len includes the backup block data.
       *
       * Also set the appropriate info bits to show which buffers were backed
       * up. The i'th XLR_SET_BKP_BLOCK bit corresponds to the i'th distinct
       * buffer value (ignoring InvalidBuffer) appearing in the rdata chain.
       */
      write_len = len;
      for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
      {
            BkpBlock   *bkpb;
            char     *page;

            if (!dtbuf_bkp[i])
                  continue;

            info |= XLR_SET_BKP_BLOCK(i);

            bkpb = &(dtbuf_xlg[i]);
            page = (char *) BufferGetBlock(dtbuf[i]);

            rdt->next = &(dtbuf_rdt1[i]);
            rdt = rdt->next;

            rdt->data = (char *) bkpb;
            rdt->len = sizeof(BkpBlock);
            write_len += sizeof(BkpBlock);

            rdt->next = &(dtbuf_rdt2[i]);
            rdt = rdt->next;

            if (bkpb->hole_length == 0)
            {
                  rdt->data = page;
                  rdt->len = BLCKSZ;
                  write_len += BLCKSZ;
                  rdt->next = NULL;
            }
            else
            {
                  /* must skip the hole */
                  rdt->data = page;
                  rdt->len = bkpb->hole_offset;
                  write_len += bkpb->hole_offset;

                  rdt->next = &(dtbuf_rdt3[i]);
                  rdt = rdt->next;

                  rdt->data = page + (bkpb->hole_offset + bkpb->hole_length);
                  rdt->len = BLCKSZ - (bkpb->hole_offset + bkpb->hole_length);
                  write_len += rdt->len;
                  rdt->next = NULL;
            }
      }

      /*
       * If we backed up any full blocks and online backup is not in progress,
       * mark the backup blocks as removable.  This allows the WAL archiver to
       * know whether it is safe to compress archived WAL data by transforming
       * full-block records into the non-full-block format.
       *
       * Note: we could just set the flag whenever !forcePageWrites, but
       * defining it like this leaves the info bit free for some potential other
       * use in records without any backup blocks.
       */
      if ((info & XLR_BKP_BLOCK_MASK) && !Insert->forcePageWrites)
            info |= XLR_BKP_REMOVABLE;

      /*
       * If there isn't enough space on the current XLOG page for a record
       * header, advance to the next page (leaving the unused space as zeroes).
       */
      updrqst = false;
      freespace = INSERT_FREESPACE(Insert);
      if (freespace < SizeOfXLogRecord)
      {
            updrqst = AdvanceXLInsertBuffer(false);
            freespace = INSERT_FREESPACE(Insert);
      }

      /* Compute record's XLOG location */
      curridx = Insert->curridx;
      INSERT_RECPTR(RecPtr, Insert, curridx);

      /*
       * If the record is an XLOG_SWITCH, and we are exactly at the start of a
       * segment, we need not insert it (and don't want to because we'd like
       * consecutive switch requests to be no-ops).  Instead, make sure
       * everything is written and flushed through the end of the prior segment,
       * and return the prior segment's end address.
       */
      if (isLogSwitch &&
            (RecPtr.xrecoff % XLogSegSize) == SizeOfXLogLongPHD)
      {
            /* We can release insert lock immediately */
            LWLockRelease(WALInsertLock);

            RecPtr.xrecoff -= SizeOfXLogLongPHD;
            if (RecPtr.xrecoff == 0)
            {
                  /* crossing a logid boundary */
                  RecPtr.xlogid -= 1;
                  RecPtr.xrecoff = XLogFileSize;
            }

            LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
            LogwrtResult = XLogCtl->Write.LogwrtResult;
            if (!XLByteLE(RecPtr, LogwrtResult.Flush))
            {
                  XLogwrtRqst FlushRqst;

                  FlushRqst.Write = RecPtr;
                  FlushRqst.Flush = RecPtr;
                  XLogWrite(FlushRqst, false, false);
            }
            LWLockRelease(WALWriteLock);

            END_CRIT_SECTION();

            return RecPtr;
      }

      /* Insert record header */

      record = (XLogRecord *) Insert->currpos;
      record->xl_prev = Insert->PrevRecord;
      record->xl_xid = GetCurrentTransactionIdIfAny();
      record->xl_tot_len = SizeOfXLogRecord + write_len;
      record->xl_len = len;         /* doesn't include backup blocks */
      record->xl_info = info;
      record->xl_rmid = rmid;

      /* Now we can finish computing the record's CRC */
      COMP_CRC32(rdata_crc, (char *) record + sizeof(pg_crc32),
                     SizeOfXLogRecord - sizeof(pg_crc32));
      FIN_CRC32(rdata_crc);
      record->xl_crc = rdata_crc;

#ifdef WAL_DEBUG
      if (XLOG_DEBUG)
      {
            StringInfoData buf;

            initStringInfo(&buf);
            appendStringInfo(&buf, "INSERT @ %X/%X: ",
                                     RecPtr.xlogid, RecPtr.xrecoff);
            xlog_outrec(&buf, record);
            if (rdata->data != NULL)
            {
                  appendStringInfo(&buf, " - ");
                  RmgrTable[record->xl_rmid].rm_desc(&buf, record->xl_info, rdata->data);
            }
            elog(LOG, "%s", buf.data);
            pfree(buf.data);
      }
#endif

      /* Record begin of record in appropriate places */
      ProcLastRecPtr = RecPtr;
      Insert->PrevRecord = RecPtr;

      Insert->currpos += SizeOfXLogRecord;
      freespace -= SizeOfXLogRecord;

      /*
       * Append the data, including backup blocks if any
       */
      while (write_len)
      {
            while (rdata->data == NULL)
                  rdata = rdata->next;

            if (freespace > 0)
            {
                  if (rdata->len > freespace)
                  {
                        memcpy(Insert->currpos, rdata->data, freespace);
                        rdata->data += freespace;
                        rdata->len -= freespace;
                        write_len -= freespace;
                  }
                  else
                  {
                        memcpy(Insert->currpos, rdata->data, rdata->len);
                        freespace -= rdata->len;
                        write_len -= rdata->len;
                        Insert->currpos += rdata->len;
                        rdata = rdata->next;
                        continue;
                  }
            }

            /* Use next buffer */
            updrqst = AdvanceXLInsertBuffer(false);
            curridx = Insert->curridx;
            /* Insert cont-record header */
            Insert->currpage->xlp_info |= XLP_FIRST_IS_CONTRECORD;
            contrecord = (XLogContRecord *) Insert->currpos;
            contrecord->xl_rem_len = write_len;
            Insert->currpos += SizeOfXLogContRecord;
            freespace = INSERT_FREESPACE(Insert);
      }

      /* Ensure next record will be properly aligned */
      Insert->currpos = (char *) Insert->currpage +
            MAXALIGN(Insert->currpos - (char *) Insert->currpage);
      freespace = INSERT_FREESPACE(Insert);

      /*
       * The recptr I return is the beginning of the *next* record. This will be
       * stored as LSN for changed data pages...
       */
      INSERT_RECPTR(RecPtr, Insert, curridx);

      /*
       * If the record is an XLOG_SWITCH, we must now write and flush all the
       * existing data, and then forcibly advance to the start of the next
       * segment.  It's not good to do this I/O while holding the insert lock,
       * but there seems too much risk of confusion if we try to release the
       * lock sooner.  Fortunately xlog switch needn't be a high-performance
       * operation anyway...
       */
      if (isLogSwitch)
      {
            XLogCtlWrite *Write = &XLogCtl->Write;
            XLogwrtRqst FlushRqst;
            XLogRecPtr  OldSegEnd;

            TRACE_POSTGRESQL_XLOG_SWITCH();

            LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);

            /*
             * Flush through the end of the page containing XLOG_SWITCH, and
             * perform end-of-segment actions (eg, notifying archiver).
             */
            WriteRqst = XLogCtl->xlblocks[curridx];
            FlushRqst.Write = WriteRqst;
            FlushRqst.Flush = WriteRqst;
            XLogWrite(FlushRqst, false, true);

            /* Set up the next buffer as first page of next segment */
            /* Note: AdvanceXLInsertBuffer cannot need to do I/O here */
            (void) AdvanceXLInsertBuffer(true);

            /* There should be no unwritten data */
            curridx = Insert->curridx;
            Assert(curridx == Write->curridx);

            /* Compute end address of old segment */
            OldSegEnd = XLogCtl->xlblocks[curridx];
            OldSegEnd.xrecoff -= XLOG_BLCKSZ;
            if (OldSegEnd.xrecoff == 0)
            {
                  /* crossing a logid boundary */
                  OldSegEnd.xlogid -= 1;
                  OldSegEnd.xrecoff = XLogFileSize;
            }

            /* Make it look like we've written and synced all of old segment */
            LogwrtResult.Write = OldSegEnd;
            LogwrtResult.Flush = OldSegEnd;

            /*
             * Update shared-memory status --- this code should match XLogWrite
             */
            {
                  /* use volatile pointer to prevent code rearrangement */
                  volatile XLogCtlData *xlogctl = XLogCtl;

                  SpinLockAcquire(&xlogctl->info_lck);
                  xlogctl->LogwrtResult = LogwrtResult;
                  if (XLByteLT(xlogctl->LogwrtRqst.Write, LogwrtResult.Write))
                        xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
                  if (XLByteLT(xlogctl->LogwrtRqst.Flush, LogwrtResult.Flush))
                        xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
                  SpinLockRelease(&xlogctl->info_lck);
            }

            Write->LogwrtResult = LogwrtResult;

            LWLockRelease(WALWriteLock);

            updrqst = false;        /* done already */
      }
      else
      {
            /* normal case, ie not xlog switch */

            /* Need to update shared LogwrtRqst if some block was filled up */
            if (freespace < SizeOfXLogRecord)
            {
                  /* curridx is filled and available for writing out */
                  updrqst = true;
            }
            else
            {
                  /* if updrqst already set, write through end of previous buf */
                  curridx = PrevBufIdx(curridx);
            }
            WriteRqst = XLogCtl->xlblocks[curridx];
      }

      LWLockRelease(WALInsertLock);

      if (updrqst)
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            /* advance global request to include new block(s) */
            if (XLByteLT(xlogctl->LogwrtRqst.Write, WriteRqst))
                  xlogctl->LogwrtRqst.Write = WriteRqst;
            /* update local result copy while I have the chance */
            LogwrtResult = xlogctl->LogwrtResult;
            SpinLockRelease(&xlogctl->info_lck);
      }

      XactLastRecEnd = RecPtr;

      END_CRIT_SECTION();

      return RecPtr;
}

/*
 * Determine whether the buffer referenced by an XLogRecData item has to
 * be backed up, and if so fill a BkpBlock struct for it.  In any case
 * save the buffer's LSN at *lsn.
 */
static bool
XLogCheckBuffer(XLogRecData *rdata, bool doPageWrites,
                        XLogRecPtr *lsn, BkpBlock *bkpb)
{
      Page        page;

      page = BufferGetPage(rdata->buffer);

      /*
       * XXX We assume page LSN is first data on *every* page that can be passed
       * to XLogInsert, whether it otherwise has the standard page layout or
       * not.
       */
      *lsn = PageGetLSN(page);

      if (doPageWrites &&
            XLByteLE(PageGetLSN(page), RedoRecPtr))
      {
            /*
             * The page needs to be backed up, so set up *bkpb
             */
            BufferGetTag(rdata->buffer, &bkpb->node, &bkpb->fork, &bkpb->block);

            if (rdata->buffer_std)
            {
                  /* Assume we can omit data between pd_lower and pd_upper */
                  uint16            lower = ((PageHeader) page)->pd_lower;
                  uint16            upper = ((PageHeader) page)->pd_upper;

                  if (lower >= SizeOfPageHeaderData &&
                        upper > lower &&
                        upper <= BLCKSZ)
                  {
                        bkpb->hole_offset = lower;
                        bkpb->hole_length = upper - lower;
                  }
                  else
                  {
                        /* No "hole" to compress out */
                        bkpb->hole_offset = 0;
                        bkpb->hole_length = 0;
                  }
            }
            else
            {
                  /* Not a standard page header, don't try to eliminate "hole" */
                  bkpb->hole_offset = 0;
                  bkpb->hole_length = 0;
            }

            return true;                  /* buffer requires backup */
      }

      return false;                       /* buffer does not need to be backed up */
}

/*
 * XLogArchiveNotify
 *
 * Create an archive notification file
 *
 * The name of the notification file is the message that will be picked up
 * by the archiver, e.g. we write 0000000100000001000000C6.ready
 * and the archiver then knows to archive XLOGDIR/0000000100000001000000C6,
 * then when complete, rename it to 0000000100000001000000C6.done
 */
static void
XLogArchiveNotify(const char *xlog)
{
      char        archiveStatusPath[MAXPGPATH];
      FILE     *fd;

      /* insert an otherwise empty file called <XLOG>.ready */
      StatusFilePath(archiveStatusPath, xlog, ".ready");
      fd = AllocateFile(archiveStatusPath, "w");
      if (fd == NULL)
      {
            ereport(LOG,
                        (errcode_for_file_access(),
                         errmsg("could not create archive status file \"%s\": %m",
                                    archiveStatusPath)));
            return;
      }
      if (FreeFile(fd))
      {
            ereport(LOG,
                        (errcode_for_file_access(),
                         errmsg("could not write archive status file \"%s\": %m",
                                    archiveStatusPath)));
            return;
      }

      /* Notify archiver that it's got something to do */
      if (IsUnderPostmaster)
            SendPostmasterSignal(PMSIGNAL_WAKEN_ARCHIVER);
}

/*
 * Convenience routine to notify using log/seg representation of filename
 */
static void
XLogArchiveNotifySeg(uint32 log, uint32 seg)
{
      char        xlog[MAXFNAMELEN];

      XLogFileName(xlog, ThisTimeLineID, log, seg);
      XLogArchiveNotify(xlog);
}

/*
 * XLogArchiveCheckDone
 *
 * This is called when we are ready to delete or recycle an old XLOG segment
 * file or backup history file.  If it is okay to delete it then return true.
 * If it is not time to delete it, make sure a .ready file exists, and return
 * false.
 *
 * If <XLOG>.done exists, then return true; else if <XLOG>.ready exists,
 * then return false; else create <XLOG>.ready and return false.
 *
 * The reason we do things this way is so that if the original attempt to
 * create <XLOG>.ready fails, we'll retry during subsequent checkpoints.
 */
static bool
XLogArchiveCheckDone(const char *xlog)
{
      char        archiveStatusPath[MAXPGPATH];
      struct stat stat_buf;

      /* Always deletable if archiving is off */
      if (!XLogArchivingActive())
            return true;

      /* First check for .done --- this means archiver is done with it */
      StatusFilePath(archiveStatusPath, xlog, ".done");
      if (stat(archiveStatusPath, &stat_buf) == 0)
            return true;

      /* check for .ready --- this means archiver is still busy with it */
      StatusFilePath(archiveStatusPath, xlog, ".ready");
      if (stat(archiveStatusPath, &stat_buf) == 0)
            return false;

      /* Race condition --- maybe archiver just finished, so recheck */
      StatusFilePath(archiveStatusPath, xlog, ".done");
      if (stat(archiveStatusPath, &stat_buf) == 0)
            return true;

      /* Retry creation of the .ready file */
      XLogArchiveNotify(xlog);
      return false;
}

/*
 * XLogArchiveIsBusy
 *
 * Check to see if an XLOG segment file is still unarchived.
 * This is almost but not quite the inverse of XLogArchiveCheckDone: in
 * the first place we aren't chartered to recreate the .ready file, and
 * in the second place we should consider that if the file is already gone
 * then it's not busy.  (This check is needed to handle the race condition
 * that a checkpoint already deleted the no-longer-needed file.)
 */
static bool
XLogArchiveIsBusy(const char *xlog)
{
      char        archiveStatusPath[MAXPGPATH];
      struct stat stat_buf;

      /* First check for .done --- this means archiver is done with it */
      StatusFilePath(archiveStatusPath, xlog, ".done");
      if (stat(archiveStatusPath, &stat_buf) == 0)
            return false;

      /* check for .ready --- this means archiver is still busy with it */
      StatusFilePath(archiveStatusPath, xlog, ".ready");
      if (stat(archiveStatusPath, &stat_buf) == 0)
            return true;

      /* Race condition --- maybe archiver just finished, so recheck */
      StatusFilePath(archiveStatusPath, xlog, ".done");
      if (stat(archiveStatusPath, &stat_buf) == 0)
            return false;

      /*
       * Check to see if the WAL file has been removed by checkpoint, which
       * implies it has already been archived, and explains why we can't see a
       * status file for it.
       */
      snprintf(archiveStatusPath, MAXPGPATH, XLOGDIR "/%s", xlog);
      if (stat(archiveStatusPath, &stat_buf) != 0 &&
            errno == ENOENT)
            return false;

      return true;
}

/*
 * XLogArchiveCleanup
 *
 * Cleanup archive notification file(s) for a particular xlog segment
 */
static void
XLogArchiveCleanup(const char *xlog)
{
      char        archiveStatusPath[MAXPGPATH];

      /* Remove the .done file */
      StatusFilePath(archiveStatusPath, xlog, ".done");
      unlink(archiveStatusPath);
      /* should we complain about failure? */

      /* Remove the .ready file if present --- normally it shouldn't be */
      StatusFilePath(archiveStatusPath, xlog, ".ready");
      unlink(archiveStatusPath);
      /* should we complain about failure? */
}

/*
 * Advance the Insert state to the next buffer page, writing out the next
 * buffer if it still contains unwritten data.
 *
 * If new_segment is TRUE then we set up the next buffer page as the first
 * page of the next xlog segment file, possibly but not usually the next
 * consecutive file page.
 *
 * The global LogwrtRqst.Write pointer needs to be advanced to include the
 * just-filled page.  If we can do this for free (without an extra lock),
 * we do so here.  Otherwise the caller must do it.  We return TRUE if the
 * request update still needs to be done, FALSE if we did it internally.
 *
 * Must be called with WALInsertLock held.
 */
static bool
AdvanceXLInsertBuffer(bool new_segment)
{
      XLogCtlInsert *Insert = &XLogCtl->Insert;
      XLogCtlWrite *Write = &XLogCtl->Write;
      int               nextidx = NextBufIdx(Insert->curridx);
      bool        update_needed = true;
      XLogRecPtr  OldPageRqstPtr;
      XLogwrtRqst WriteRqst;
      XLogRecPtr  NewPageEndPtr;
      XLogPageHeader NewPage;

      /* Use Insert->LogwrtResult copy if it's more fresh */
      if (XLByteLT(LogwrtResult.Write, Insert->LogwrtResult.Write))
            LogwrtResult = Insert->LogwrtResult;

      /*
       * Get ending-offset of the buffer page we need to replace (this may be
       * zero if the buffer hasn't been used yet).  Fall through if it's already
       * written out.
       */
      OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
      if (!XLByteLE(OldPageRqstPtr, LogwrtResult.Write))
      {
            /* nope, got work to do... */
            XLogRecPtr  FinishedPageRqstPtr;

            FinishedPageRqstPtr = XLogCtl->xlblocks[Insert->curridx];

            /* Before waiting, get info_lck and update LogwrtResult */
            {
                  /* use volatile pointer to prevent code rearrangement */
                  volatile XLogCtlData *xlogctl = XLogCtl;

                  SpinLockAcquire(&xlogctl->info_lck);
                  if (XLByteLT(xlogctl->LogwrtRqst.Write, FinishedPageRqstPtr))
                        xlogctl->LogwrtRqst.Write = FinishedPageRqstPtr;
                  LogwrtResult = xlogctl->LogwrtResult;
                  SpinLockRelease(&xlogctl->info_lck);
            }

            update_needed = false;  /* Did the shared-request update */

            if (XLByteLE(OldPageRqstPtr, LogwrtResult.Write))
            {
                  /* OK, someone wrote it already */
                  Insert->LogwrtResult = LogwrtResult;
            }
            else
            {
                  /* Must acquire write lock */
                  LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
                  LogwrtResult = Write->LogwrtResult;
                  if (XLByteLE(OldPageRqstPtr, LogwrtResult.Write))
                  {
                        /* OK, someone wrote it already */
                        LWLockRelease(WALWriteLock);
                        Insert->LogwrtResult = LogwrtResult;
                  }
                  else
                  {
                        /*
                         * Have to write buffers while holding insert lock. This is
                         * not good, so only write as much as we absolutely must.
                         */
                        TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_START();
                        WriteRqst.Write = OldPageRqstPtr;
                        WriteRqst.Flush.xlogid = 0;
                        WriteRqst.Flush.xrecoff = 0;
                        XLogWrite(WriteRqst, false, false);
                        LWLockRelease(WALWriteLock);
                        Insert->LogwrtResult = LogwrtResult;
                        TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
                  }
            }
      }

      /*
       * Now the next buffer slot is free and we can set it up to be the next
       * output page.
       */
      NewPageEndPtr = XLogCtl->xlblocks[Insert->curridx];

      if (new_segment)
      {
            /* force it to a segment start point */
            NewPageEndPtr.xrecoff += XLogSegSize - 1;
            NewPageEndPtr.xrecoff -= NewPageEndPtr.xrecoff % XLogSegSize;
      }

      if (NewPageEndPtr.xrecoff >= XLogFileSize)
      {
            /* crossing a logid boundary */
            NewPageEndPtr.xlogid += 1;
            NewPageEndPtr.xrecoff = XLOG_BLCKSZ;
      }
      else
            NewPageEndPtr.xrecoff += XLOG_BLCKSZ;
      XLogCtl->xlblocks[nextidx] = NewPageEndPtr;
      NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);

      Insert->curridx = nextidx;
      Insert->currpage = NewPage;

      Insert->currpos = ((char *) NewPage) +SizeOfXLogShortPHD;

      /*
       * Be sure to re-zero the buffer so that bytes beyond what we've written
       * will look like zeroes and not valid XLOG records...
       */
      MemSet((char *) NewPage, 0, XLOG_BLCKSZ);

      /*
       * Fill the new page's header
       */
      NewPage   ->xlp_magic = XLOG_PAGE_MAGIC;

      /* NewPage->xlp_info = 0; */  /* done by memset */
      NewPage   ->xlp_tli = ThisTimeLineID;
      NewPage   ->xlp_pageaddr.xlogid = NewPageEndPtr.xlogid;
      NewPage   ->xlp_pageaddr.xrecoff = NewPageEndPtr.xrecoff - XLOG_BLCKSZ;

      /*
       * If first page of an XLOG segment file, make it a long header.
       */
      if ((NewPage->xlp_pageaddr.xrecoff % XLogSegSize) == 0)
      {
            XLogLongPageHeader NewLongPage = (XLogLongPageHeader) NewPage;

            NewLongPage->xlp_sysid = ControlFile->system_identifier;
            NewLongPage->xlp_seg_size = XLogSegSize;
            NewLongPage->xlp_xlog_blcksz = XLOG_BLCKSZ;
            NewPage   ->xlp_info |= XLP_LONG_HEADER;

            Insert->currpos = ((char *) NewPage) +SizeOfXLogLongPHD;
      }

      return update_needed;
}

/*
 * Check whether we've consumed enough xlog space that a checkpoint is needed.
 *
 * Caller must have just finished filling the open log file (so that
 * openLogId/openLogSeg are valid).  We measure the distance from RedoRecPtr
 * to the open log file and see if that exceeds CheckPointSegments.
 *
 * Note: it is caller's responsibility that RedoRecPtr is up-to-date.
 */
static bool
XLogCheckpointNeeded(void)
{
      /*
       * A straight computation of segment number could overflow 32 bits. Rather
       * than assuming we have working 64-bit arithmetic, we compare the
       * highest-order bits separately, and force a checkpoint immediately when
       * they change.
       */
      uint32            old_segno,
                        new_segno;
      uint32            old_highbits,
                        new_highbits;

      old_segno = (RedoRecPtr.xlogid % XLogSegSize) * XLogSegsPerFile +
            (RedoRecPtr.xrecoff / XLogSegSize);
      old_highbits = RedoRecPtr.xlogid / XLogSegSize;
      new_segno = (openLogId % XLogSegSize) * XLogSegsPerFile + openLogSeg;
      new_highbits = openLogId / XLogSegSize;
      if (new_highbits != old_highbits ||
            new_segno >= old_segno + (uint32) (CheckPointSegments - 1))
            return true;
      return false;
}

/*
 * Write and/or fsync the log at least as far as WriteRqst indicates.
 *
 * If flexible == TRUE, we don't have to write as far as WriteRqst, but
 * may stop at any convenient boundary (such as a cache or logfile boundary).
 * This option allows us to avoid uselessly issuing multiple writes when a
 * single one would do.
 *
 * If xlog_switch == TRUE, we are intending an xlog segment switch, so
 * perform end-of-segment actions after writing the last page, even if
 * it's not physically the end of its segment.  (NB: this will work properly
 * only if caller specifies WriteRqst == page-end and flexible == false,
 * and there is some data to write.)
 *
 * Must be called with WALWriteLock held.
 */
static void
XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
{
      XLogCtlWrite *Write = &XLogCtl->Write;
      bool        ispartialpage;
      bool        last_iteration;
      bool        finishing_seg;
      bool        use_existent;
      int               curridx;
      int               npages;
      int               startidx;
      uint32            startoffset;

      /* We should always be inside a critical section here */
      Assert(CritSectionCount > 0);

      /*
       * Update local LogwrtResult (caller probably did this already, but...)
       */
      LogwrtResult = Write->LogwrtResult;

      /*
       * Since successive pages in the xlog cache are consecutively allocated,
       * we can usually gather multiple pages together and issue just one
       * write() call.  npages is the number of pages we have determined can be
       * written together; startidx is the cache block index of the first one,
       * and startoffset is the file offset at which it should go. The latter
       * two variables are only valid when npages > 0, but we must initialize
       * all of them to keep the compiler quiet.
       */
      npages = 0;
      startidx = 0;
      startoffset = 0;

      /*
       * Within the loop, curridx is the cache block index of the page to
       * consider writing.  We advance Write->curridx only after successfully
       * writing pages.  (Right now, this refinement is useless since we are
       * going to PANIC if any error occurs anyway; but someday it may come in
       * useful.)
       */
      curridx = Write->curridx;

      while (XLByteLT(LogwrtResult.Write, WriteRqst.Write))
      {
            /*
             * Make sure we're not ahead of the insert process.  This could happen
             * if we're passed a bogus WriteRqst.Write that is past the end of the
             * last page that's been initialized by AdvanceXLInsertBuffer.
             */
            if (!XLByteLT(LogwrtResult.Write, XLogCtl->xlblocks[curridx]))
                  elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
                         LogwrtResult.Write.xlogid, LogwrtResult.Write.xrecoff,
                         XLogCtl->xlblocks[curridx].xlogid,
                         XLogCtl->xlblocks[curridx].xrecoff);

            /* Advance LogwrtResult.Write to end of current buffer page */
            LogwrtResult.Write = XLogCtl->xlblocks[curridx];
            ispartialpage = XLByteLT(WriteRqst.Write, LogwrtResult.Write);

            if (!XLByteInPrevSeg(LogwrtResult.Write, openLogId, openLogSeg))
            {
                  /*
                   * Switch to new logfile segment.  We cannot have any pending
                   * pages here (since we dump what we have at segment end).
                   */
                  Assert(npages == 0);
                  if (openLogFile >= 0)
                        XLogFileClose();
                  XLByteToPrevSeg(LogwrtResult.Write, openLogId, openLogSeg);

                  /* create/use new log file */
                  use_existent = true;
                  openLogFile = XLogFileInit(openLogId, openLogSeg,
                                                         &use_existent, true);
                  openLogOff = 0;
            }

            /* Make sure we have the current logfile open */
            if (openLogFile < 0)
            {
                  XLByteToPrevSeg(LogwrtResult.Write, openLogId, openLogSeg);
                  openLogFile = XLogFileOpen(openLogId, openLogSeg);
                  openLogOff = 0;
            }

            /* Add current page to the set of pending pages-to-dump */
            if (npages == 0)
            {
                  /* first of group */
                  startidx = curridx;
                  startoffset = (LogwrtResult.Write.xrecoff - XLOG_BLCKSZ) % XLogSegSize;
            }
            npages++;

            /*
             * Dump the set if this will be the last loop iteration, or if we are
             * at the last page of the cache area (since the next page won't be
             * contiguous in memory), or if we are at the end of the logfile
             * segment.
             */
            last_iteration = !XLByteLT(LogwrtResult.Write, WriteRqst.Write);

            finishing_seg = !ispartialpage &&
                  (startoffset + npages * XLOG_BLCKSZ) >= XLogSegSize;

            if (last_iteration ||
                  curridx == XLogCtl->XLogCacheBlck ||
                  finishing_seg)
            {
                  char     *from;
                  Size        nbytes;

                  /* Need to seek in the file? */
                  if (openLogOff != startoffset)
                  {
                        if (lseek(openLogFile, (off_t) startoffset, SEEK_SET) < 0)
                              ereport(PANIC,
                                          (errcode_for_file_access(),
                                           errmsg("could not seek in log file %u, "
                                                      "segment %u to offset %u: %m",
                                                      openLogId, openLogSeg, startoffset)));
                        openLogOff = startoffset;
                  }

                  /* OK to write the page(s) */
                  from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
                  nbytes = npages * (Size) XLOG_BLCKSZ;
                  errno = 0;
                  if (write(openLogFile, from, nbytes) != nbytes)
                  {
                        /* if write didn't set errno, assume no disk space */
                        if (errno == 0)
                              errno = ENOSPC;
                        ereport(PANIC,
                                    (errcode_for_file_access(),
                                     errmsg("could not write to log file %u, segment %u "
                                                "at offset %u, length %lu: %m",
                                                openLogId, openLogSeg,
                                                openLogOff, (unsigned long) nbytes)));
                  }

                  /* Update state for write */
                  openLogOff += nbytes;
                  Write->curridx = ispartialpage ? curridx : NextBufIdx(curridx);
                  npages = 0;

                  /*
                   * If we just wrote the whole last page of a logfile segment,
                   * fsync the segment immediately.  This avoids having to go back
                   * and re-open prior segments when an fsync request comes along
                   * later. Doing it here ensures that one and only one backend will
                   * perform this fsync.
                   *
                   * We also do this if this is the last page written for an xlog
                   * switch.
                   *
                   * This is also the right place to notify the Archiver that the
                   * segment is ready to copy to archival storage, and to update the
                   * timer for archive_timeout, and to signal for a checkpoint if
                   * too many logfile segments have been used since the last
                   * checkpoint.
                   */
                  if (finishing_seg || (xlog_switch && last_iteration))
                  {
                        issue_xlog_fsync();
                        LogwrtResult.Flush = LogwrtResult.Write;        /* end of page */

                        if (XLogArchivingActive())
                              XLogArchiveNotifySeg(openLogId, openLogSeg);

                        Write->lastSegSwitchTime = (pg_time_t) time(NULL);

                        /*
                         * Signal bgwriter to start a checkpoint if we've consumed too
                         * much xlog since the last one.  For speed, we first check
                         * using the local copy of RedoRecPtr, which might be out of
                         * date; if it looks like a checkpoint is needed, forcibly
                         * update RedoRecPtr and recheck.
                         */
                        if (IsUnderPostmaster &&
                              XLogCheckpointNeeded())
                        {
                              (void) GetRedoRecPtr();
                              if (XLogCheckpointNeeded())
                                    RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
                        }
                  }
            }

            if (ispartialpage)
            {
                  /* Only asked to write a partial page */
                  LogwrtResult.Write = WriteRqst.Write;
                  break;
            }
            curridx = NextBufIdx(curridx);

            /* If flexible, break out of loop as soon as we wrote something */
            if (flexible && npages == 0)
                  break;
      }

      Assert(npages == 0);
      Assert(curridx == Write->curridx);

      /*
       * If asked to flush, do so
       */
      if (XLByteLT(LogwrtResult.Flush, WriteRqst.Flush) &&
            XLByteLT(LogwrtResult.Flush, LogwrtResult.Write))
      {
            /*
             * Could get here without iterating above loop, in which case we might
             * have no open file or the wrong one.    However, we do not need to
             * fsync more than one file.
             */
            if (sync_method != SYNC_METHOD_OPEN &&
                  sync_method != SYNC_METHOD_OPEN_DSYNC)
            {
                  if (openLogFile >= 0 &&
                        !XLByteInPrevSeg(LogwrtResult.Write, openLogId, openLogSeg))
                        XLogFileClose();
                  if (openLogFile < 0)
                  {
                        XLByteToPrevSeg(LogwrtResult.Write, openLogId, openLogSeg);
                        openLogFile = XLogFileOpen(openLogId, openLogSeg);
                        openLogOff = 0;
                  }
                  issue_xlog_fsync();
            }
            LogwrtResult.Flush = LogwrtResult.Write;
      }

      /*
       * Update shared-memory status
       *
       * We make sure that the shared 'request' values do not fall behind the
       * 'result' values.  This is not absolutely essential, but it saves some
       * code in a couple of places.
       */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            xlogctl->LogwrtResult = LogwrtResult;
            if (XLByteLT(xlogctl->LogwrtRqst.Write, LogwrtResult.Write))
                  xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
            if (XLByteLT(xlogctl->LogwrtRqst.Flush, LogwrtResult.Flush))
                  xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
            SpinLockRelease(&xlogctl->info_lck);
      }

      Write->LogwrtResult = LogwrtResult;
}

/*
 * Record the LSN for an asynchronous transaction commit.
 * (This should not be called for aborts, nor for synchronous commits.)
 */
void
XLogSetAsyncCommitLSN(XLogRecPtr asyncCommitLSN)
{
      /* use volatile pointer to prevent code rearrangement */
      volatile XLogCtlData *xlogctl = XLogCtl;

      SpinLockAcquire(&xlogctl->info_lck);
      if (XLByteLT(xlogctl->asyncCommitLSN, asyncCommitLSN))
            xlogctl->asyncCommitLSN = asyncCommitLSN;
      SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Advance minRecoveryPoint in control file.
 *
 * If we crash during recovery, we must reach this point again before the
 * database is consistent.
 *
 * If 'force' is true, 'lsn' argument is ignored. Otherwise, minRecoveryPoint
 * is only updated if it's not already greater than or equal to 'lsn'.
 */
static void
UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force)
{
      /* Quick check using our local copy of the variable */
      if (!updateMinRecoveryPoint || (!force && XLByteLE(lsn, minRecoveryPoint)))
            return;

      LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

      /* update local copy */
      minRecoveryPoint = ControlFile->minRecoveryPoint;

      /*
       * An invalid minRecoveryPoint means that we need to recover all the WAL,
       * i.e., we're doing crash recovery.  We never modify the control file's
       * value in that case, so we can short-circuit future checks here too.
       */
      if (minRecoveryPoint.xlogid == 0 && minRecoveryPoint.xrecoff == 0)
            updateMinRecoveryPoint = false;
      else if (force || XLByteLT(minRecoveryPoint, lsn))
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;
            XLogRecPtr  newMinRecoveryPoint;

            /*
             * To avoid having to update the control file too often, we update it
             * all the way to the last record being replayed, even though 'lsn'
             * would suffice for correctness.  This also allows the 'force' case
             * to not need a valid 'lsn' value.
             *
             * Another important reason for doing it this way is that the passed
             * 'lsn' value could be bogus, i.e., past the end of available WAL,
             * if the caller got it from a corrupted heap page.  Accepting such
             * a value as the min recovery point would prevent us from coming up
             * at all.  Instead, we just log a warning and continue with recovery.
             * (See also the comments about corrupt LSNs in XLogFlush.)
             */
            SpinLockAcquire(&xlogctl->info_lck);
            newMinRecoveryPoint = xlogctl->replayEndRecPtr;
            SpinLockRelease(&xlogctl->info_lck);

            if (!force && XLByteLT(newMinRecoveryPoint, lsn))
                  elog(WARNING,
                         "xlog min recovery request %X/%X is past current point %X/%X",
                         lsn.xlogid, lsn.xrecoff,
                         newMinRecoveryPoint.xlogid, newMinRecoveryPoint.xrecoff);

            /* update control file */
            if (XLByteLT(ControlFile->minRecoveryPoint, newMinRecoveryPoint))
            {
                  ControlFile->minRecoveryPoint = newMinRecoveryPoint;
                  UpdateControlFile();
                  minRecoveryPoint = newMinRecoveryPoint;

                  ereport(DEBUG2,
                              (errmsg("updated min recovery point to %X/%X",
                                    minRecoveryPoint.xlogid, minRecoveryPoint.xrecoff)));
            }
      }
      LWLockRelease(ControlFileLock);
}

/*
 * Ensure that all XLOG data through the given position is flushed to disk.
 *
 * NOTE: this differs from XLogWrite mainly in that the WALWriteLock is not
 * already held, and we try to avoid acquiring it if possible.
 */
void
XLogFlush(XLogRecPtr record)
{
      XLogRecPtr  WriteRqstPtr;
      XLogwrtRqst WriteRqst;

      /*
       * During REDO, we are reading not writing WAL.  Therefore, instead of
       * trying to flush the WAL, we should update minRecoveryPoint instead.
       * We test XLogInsertAllowed(), not InRecovery, because we need the
       * bgwriter to act this way too, and because when the bgwriter tries
       * to write the end-of-recovery checkpoint, it should indeed flush.
       */
      if (!XLogInsertAllowed())
      {
            UpdateMinRecoveryPoint(record, false);
            return;
      }

      /* Quick exit if already known flushed */
      if (XLByteLE(record, LogwrtResult.Flush))
            return;

#ifdef WAL_DEBUG
      if (XLOG_DEBUG)
            elog(LOG, "xlog flush request %X/%X; write %X/%X; flush %X/%X",
                   record.xlogid, record.xrecoff,
                   LogwrtResult.Write.xlogid, LogwrtResult.Write.xrecoff,
                   LogwrtResult.Flush.xlogid, LogwrtResult.Flush.xrecoff);
#endif

      START_CRIT_SECTION();

      /*
       * Since fsync is usually a horribly expensive operation, we try to
       * piggyback as much data as we can on each fsync: if we see any more data
       * entered into the xlog buffer, we'll write and fsync that too, so that
       * the final value of LogwrtResult.Flush is as large as possible. This
       * gives us some chance of avoiding another fsync immediately after.
       */

      /* initialize to given target; may increase below */
      WriteRqstPtr = record;

      /* read LogwrtResult and update local state */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            if (XLByteLT(WriteRqstPtr, xlogctl->LogwrtRqst.Write))
                  WriteRqstPtr = xlogctl->LogwrtRqst.Write;
            LogwrtResult = xlogctl->LogwrtResult;
            SpinLockRelease(&xlogctl->info_lck);
      }

      /* done already? */
      if (!XLByteLE(record, LogwrtResult.Flush))
      {
            /* now wait for the write lock */
            LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
            LogwrtResult = XLogCtl->Write.LogwrtResult;
            if (!XLByteLE(record, LogwrtResult.Flush))
            {
                  /* try to write/flush later additions to XLOG as well */
                  if (LWLockConditionalAcquire(WALInsertLock, LW_EXCLUSIVE))
                  {
                        XLogCtlInsert *Insert = &XLogCtl->Insert;
                        uint32            freespace = INSERT_FREESPACE(Insert);

                        if (freespace < SizeOfXLogRecord)         /* buffer is full */
                              WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
                        else
                        {
                              WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
                              WriteRqstPtr.xrecoff -= freespace;
                        }
                        LWLockRelease(WALInsertLock);
                        WriteRqst.Write = WriteRqstPtr;
                        WriteRqst.Flush = WriteRqstPtr;
                  }
                  else
                  {
                        WriteRqst.Write = WriteRqstPtr;
                        WriteRqst.Flush = record;
                  }
                  XLogWrite(WriteRqst, false, false);
            }
            LWLockRelease(WALWriteLock);
      }

      END_CRIT_SECTION();

      /*
       * If we still haven't flushed to the request point then we have a
       * problem; most likely, the requested flush point is past end of XLOG.
       * This has been seen to occur when a disk page has a corrupted LSN.
       *
       * Formerly we treated this as a PANIC condition, but that hurts the
       * system's robustness rather than helping it: we do not want to take down
       * the whole system due to corruption on one data page.  In particular, if
       * the bad page is encountered again during recovery then we would be
       * unable to restart the database at all!  (This scenario actually
       * happened in the field several times with 7.1 releases.)  As of 8.4,
       * bad LSNs encountered during recovery are UpdateMinRecoveryPoint's
       * problem; the only time we can reach here during recovery is while
       * flushing the end-of-recovery checkpoint record, and we don't expect
       * that to have a bad LSN.
       *
       * Note that for calls from xact.c, the ERROR will
       * be promoted to PANIC since xact.c calls this routine inside a critical
       * section.  However, calls from bufmgr.c are not within critical sections
       * and so we will not force a restart for a bad LSN on a data page.
       */
      if (XLByteLT(LogwrtResult.Flush, record))
            elog(ERROR,
            "xlog flush request %X/%X is not satisfied --- flushed only to %X/%X",
                   record.xlogid, record.xrecoff,
                   LogwrtResult.Flush.xlogid, LogwrtResult.Flush.xrecoff);
}

/*
 * Flush xlog, but without specifying exactly where to flush to.
 *
 * We normally flush only completed blocks; but if there is nothing to do on
 * that basis, we check for unflushed async commits in the current incomplete
 * block, and flush through the latest one of those.  Thus, if async commits
 * are not being used, we will flush complete blocks only.  We can guarantee
 * that async commits reach disk after at most three cycles; normally only
 * one or two.    (We allow XLogWrite to write "flexibly", meaning it can stop
 * at the end of the buffer ring; this makes a difference only with very high
 * load or long wal_writer_delay, but imposes one extra cycle for the worst
 * case for async commits.)
 *
 * This routine is invoked periodically by the background walwriter process.
 */
void
XLogBackgroundFlush(void)
{
      XLogRecPtr  WriteRqstPtr;
      bool        flexible = true;

      /* XLOG doesn't need flushing during recovery */
      if (RecoveryInProgress())
            return;

      /* read LogwrtResult and update local state */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            LogwrtResult = xlogctl->LogwrtResult;
            WriteRqstPtr = xlogctl->LogwrtRqst.Write;
            SpinLockRelease(&xlogctl->info_lck);
      }

      /* back off to last completed page boundary */
      WriteRqstPtr.xrecoff -= WriteRqstPtr.xrecoff % XLOG_BLCKSZ;

      /* if we have already flushed that far, consider async commit records */
      if (XLByteLE(WriteRqstPtr, LogwrtResult.Flush))
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            WriteRqstPtr = xlogctl->asyncCommitLSN;
            SpinLockRelease(&xlogctl->info_lck);
            flexible = false;       /* ensure it all gets written */
      }

      /* Done if already known flushed */
      if (XLByteLE(WriteRqstPtr, LogwrtResult.Flush))
            return;

#ifdef WAL_DEBUG
      if (XLOG_DEBUG)
            elog(LOG, "xlog bg flush request %X/%X; write %X/%X; flush %X/%X",
                   WriteRqstPtr.xlogid, WriteRqstPtr.xrecoff,
                   LogwrtResult.Write.xlogid, LogwrtResult.Write.xrecoff,
                   LogwrtResult.Flush.xlogid, LogwrtResult.Flush.xrecoff);
#endif

      START_CRIT_SECTION();

      /* now wait for the write lock */
      LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
      LogwrtResult = XLogCtl->Write.LogwrtResult;
      if (!XLByteLE(WriteRqstPtr, LogwrtResult.Flush))
      {
            XLogwrtRqst WriteRqst;

            WriteRqst.Write = WriteRqstPtr;
            WriteRqst.Flush = WriteRqstPtr;
            XLogWrite(WriteRqst, flexible, false);
      }
      LWLockRelease(WALWriteLock);

      END_CRIT_SECTION();
}

/*
 * Flush any previous asynchronously-committed transactions' commit records.
 *
 * NOTE: it is unwise to assume that this provides any strong guarantees.
 * In particular, because of the inexact LSN bookkeeping used by clog.c,
 * we cannot assume that hint bits will be settable for these transactions.
 */
void
XLogAsyncCommitFlush(void)
{
      XLogRecPtr  WriteRqstPtr;

      /* use volatile pointer to prevent code rearrangement */
      volatile XLogCtlData *xlogctl = XLogCtl;

      /* There's no asynchronously committed transactions during recovery */
      if (RecoveryInProgress())
            return;

      SpinLockAcquire(&xlogctl->info_lck);
      WriteRqstPtr = xlogctl->asyncCommitLSN;
      SpinLockRelease(&xlogctl->info_lck);

      XLogFlush(WriteRqstPtr);
}

/*
 * Test whether XLOG data has been flushed up to (at least) the given position.
 *
 * Returns true if a flush is still needed.  (It may be that someone else
 * is already in process of flushing that far, however.)
 */
bool
XLogNeedsFlush(XLogRecPtr record)
{
      /* XLOG doesn't need flushing during recovery */
      if (RecoveryInProgress())
            return false;

      /* Quick exit if already known flushed */
      if (XLByteLE(record, LogwrtResult.Flush))
            return false;

      /* read LogwrtResult and update local state */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            LogwrtResult = xlogctl->LogwrtResult;
            SpinLockRelease(&xlogctl->info_lck);
      }

      /* check again */
      if (XLByteLE(record, LogwrtResult.Flush))
            return false;

      return true;
}

/*
 * Create a new XLOG file segment, or open a pre-existing one.
 *
 * log, seg: identify segment to be created/opened.
 *
 * *use_existent: if TRUE, OK to use a pre-existing file (else, any
 * pre-existing file will be deleted).    On return, TRUE if a pre-existing
 * file was used.
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns FD of opened file.
 *
 * Note: errors here are ERROR not PANIC because we might or might not be
 * inside a critical section (eg, during checkpoint there is no reason to
 * take down the system on failure).  They will promote to PANIC if we are
 * in a critical section.
 */
static int
XLogFileInit(uint32 log, uint32 seg,
                   bool *use_existent, bool use_lock)
{
      char        path[MAXPGPATH];
      char        tmppath[MAXPGPATH];
      char     *zbuffer;
      uint32            installed_log;
      uint32            installed_seg;
      int               max_advance;
      int               fd;
      int               nbytes;

      XLogFilePath(path, ThisTimeLineID, log, seg);

      /*
       * Try to use existent file (checkpoint maker may have created it already)
       */
      if (*use_existent)
      {
            fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                                       S_IRUSR | S_IWUSR);
            if (fd < 0)
            {
                  if (errno != ENOENT)
                        ereport(ERROR,
                                    (errcode_for_file_access(),
                                     errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
                                                path, log, seg)));
            }
            else
                  return fd;
      }

      /*
       * Initialize an empty (all zeroes) segment.  NOTE: it is possible that
       * another process is doing the same thing.  If so, we will end up
       * pre-creating an extra log segment.  That seems OK, and better than
       * holding the lock throughout this lengthy process.
       */
      elog(DEBUG2, "creating and filling new WAL file");

      snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

      unlink(tmppath);

      /* do not use get_sync_bit() here --- want to fsync only at end of fill */
      fd = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                                 S_IRUSR | S_IWUSR);
      if (fd < 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not create file \"%s\": %m", tmppath)));

      /*
       * Zero-fill the file.  We have to do this the hard way to ensure that all
       * the file space has really been allocated --- on platforms that allow
       * "holes" in files, just seeking to the end doesn't allocate intermediate
       * space.  This way, we know that we have all the space and (after the
       * fsync below) that all the indirect blocks are down on disk.    Therefore,
       * fdatasync(2) or O_DSYNC will be sufficient to sync future writes to the
       * log file.
       *
       * Note: palloc zbuffer, instead of just using a local char array, to
       * ensure it is reasonably well-aligned; this may save a few cycles
       * transferring data to the kernel.
       */
      zbuffer = (char *) palloc0(XLOG_BLCKSZ);
      for (nbytes = 0; nbytes < XLogSegSize; nbytes += XLOG_BLCKSZ)
      {
            errno = 0;
            if ((int) write(fd, zbuffer, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
            {
                  int               save_errno = errno;

                  /*
                   * If we fail to make the file, delete it to release disk space
                   */
                  unlink(tmppath);
                  /* if write didn't set errno, assume problem is no disk space */
                  errno = save_errno ? save_errno : ENOSPC;

                  ereport(ERROR,
                              (errcode_for_file_access(),
                               errmsg("could not write to file \"%s\": %m", tmppath)));
            }
      }
      pfree(zbuffer);

      if (pg_fsync(fd) != 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not fsync file \"%s\": %m", tmppath)));

      if (close(fd))
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not close file \"%s\": %m", tmppath)));

      /*
       * Now move the segment into place with its final name.
       *
       * If caller didn't want to use a pre-existing file, get rid of any
       * pre-existing file.  Otherwise, cope with possibility that someone else
       * has created the file while we were filling ours: if so, use ours to
       * pre-create a future log segment.
       */
      installed_log = log;
      installed_seg = seg;
      max_advance = XLOGfileslop;
      if (!InstallXLogFileSegment(&installed_log, &installed_seg, tmppath,
                                                *use_existent, &max_advance,
                                                use_lock))
      {
            /* No need for any more future segments... */
            unlink(tmppath);
      }

      elog(DEBUG2, "done creating and filling new WAL file");

      /* Set flag to tell caller there was no existent file */
      *use_existent = false;

      /* Now open original target segment (might not be file I just made) */
      fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                                 S_IRUSR | S_IWUSR);
      if (fd < 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
               errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
                          path, log, seg)));

      return fd;
}

/*
 * Create a new XLOG file segment by copying a pre-existing one.
 *
 * log, seg: identify segment to be created.
 *
 * srcTLI, srclog, srcseg: identify segment to be copied (could be from
 *          a different timeline)
 *
 * Currently this is only used during recovery, and so there are no locking
 * considerations.      But we should be just as tense as XLogFileInit to avoid
 * emplacing a bogus file.
 */
static void
XLogFileCopy(uint32 log, uint32 seg,
                   TimeLineID srcTLI, uint32 srclog, uint32 srcseg)
{
      char        path[MAXPGPATH];
      char        tmppath[MAXPGPATH];
      char        buffer[XLOG_BLCKSZ];
      int               srcfd;
      int               fd;
      int               nbytes;

      /*
       * Open the source file
       */
      XLogFilePath(path, srcTLI, srclog, srcseg);
      srcfd = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
      if (srcfd < 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not open file \"%s\": %m", path)));

      /*
       * Copy into a temp file name.
       */
      snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

      unlink(tmppath);

      /* do not use get_sync_bit() here --- want to fsync only at end of fill */
      fd = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                                 S_IRUSR | S_IWUSR);
      if (fd < 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not create file \"%s\": %m", tmppath)));

      /*
       * Do the data copying.
       */
      for (nbytes = 0; nbytes < XLogSegSize; nbytes += sizeof(buffer))
      {
            errno = 0;
            if ((int) read(srcfd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
            {
                  if (errno != 0)
                        ereport(ERROR,
                                    (errcode_for_file_access(),
                                     errmsg("could not read file \"%s\": %m", path)));
                  else
                        ereport(ERROR,
                                    (errmsg("not enough data in file \"%s\"", path)));
            }
            errno = 0;
            if ((int) write(fd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
            {
                  int               save_errno = errno;

                  /*
                   * If we fail to make the file, delete it to release disk space
                   */
                  unlink(tmppath);
                  /* if write didn't set errno, assume problem is no disk space */
                  errno = save_errno ? save_errno : ENOSPC;

                  ereport(ERROR,
                              (errcode_for_file_access(),
                               errmsg("could not write to file \"%s\": %m", tmppath)));
            }
      }

      if (pg_fsync(fd) != 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not fsync file \"%s\": %m", tmppath)));

      if (close(fd))
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not close file \"%s\": %m", tmppath)));

      close(srcfd);

      /*
       * Now move the segment into place with its final name.
       */
      if (!InstallXLogFileSegment(&log, &seg, tmppath, false, NULL, false))
            elog(ERROR, "InstallXLogFileSegment should not have failed");
}

/*
 * Install a new XLOG segment file as a current or future log segment.
 *
 * This is used both to install a newly-created segment (which has a temp
 * filename while it's being created) and to recycle an old segment.
 *
 * *log, *seg: identify segment to install as (or first possible target).
 * When find_free is TRUE, these are modified on return to indicate the
 * actual installation location or last segment searched.
 *
 * tmppath: initial name of file to install.  It will be renamed into place.
 *
 * find_free: if TRUE, install the new segment at the first empty log/seg
 * number at or after the passed numbers.  If FALSE, install the new segment
 * exactly where specified, deleting any existing segment file there.
 *
 * *max_advance: maximum number of log/seg slots to advance past the starting
 * point.  Fail if no free slot is found in this range.  On return, reduced
 * by the number of slots skipped over.  (Irrelevant, and may be NULL,
 * when find_free is FALSE.)
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns TRUE if file installed, FALSE if not installed because of
 * exceeding max_advance limit.  On Windows, we also return FALSE if we
 * can't rename the file into place because someone's got it open.
 * (Any other kind of failure causes ereport().)
 */
static bool
InstallXLogFileSegment(uint32 *log, uint32 *seg, char *tmppath,
                                 bool find_free, int *max_advance,
                                 bool use_lock)
{
      char        path[MAXPGPATH];
      struct stat stat_buf;

      XLogFilePath(path, ThisTimeLineID, *log, *seg);

      /*
       * We want to be sure that only one process does this at a time.
       */
      if (use_lock)
            LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

      if (!find_free)
      {
            /* Force installation: get rid of any pre-existing segment file */
            unlink(path);
      }
      else
      {
            /* Find a free slot to put it in */
            while (stat(path, &stat_buf) == 0)
            {
                  if (*max_advance <= 0)
                  {
                        /* Failed to find a free slot within specified range */
                        if (use_lock)
                              LWLockRelease(ControlFileLock);
                        return false;
                  }
                  NextLogSeg(*log, *seg);
                  (*max_advance)--;
                  XLogFilePath(path, ThisTimeLineID, *log, *seg);
            }
      }

      /*
       * Prefer link() to rename() here just to be really sure that we don't
       * overwrite an existing logfile.  However, there shouldn't be one, so
       * rename() is an acceptable substitute except for the truly paranoid.
       */
#if HAVE_WORKING_LINK
      if (link(tmppath, path) < 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not link file \"%s\" to \"%s\" (initialization of log file %u, segment %u): %m",
                                    tmppath, path, *log, *seg)));
      unlink(tmppath);
#else
      if (rename(tmppath, path) < 0)
      {
#ifdef WIN32
#if !defined(__CYGWIN__)
            if (GetLastError() == ERROR_ACCESS_DENIED)
#else
            if (errno == EACCES)
#endif
            {
                  if (use_lock)
                        LWLockRelease(ControlFileLock);
                  return false;
            }
#endif   /* WIN32 */

            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not rename file \"%s\" to \"%s\" (initialization of log file %u, segment %u): %m",
                                    tmppath, path, *log, *seg)));
      }
#endif

      if (use_lock)
            LWLockRelease(ControlFileLock);

      return true;
}

/*
 * Open a pre-existing logfile segment for writing.
 */
static int
XLogFileOpen(uint32 log, uint32 seg)
{
      char        path[MAXPGPATH];
      int               fd;

      XLogFilePath(path, ThisTimeLineID, log, seg);

      fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                                 S_IRUSR | S_IWUSR);
      if (fd < 0)
            ereport(PANIC,
                        (errcode_for_file_access(),
               errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
                          path, log, seg)));

      return fd;
}

/*
 * Open a logfile segment for reading (during recovery).
 */
static int
XLogFileRead(uint32 log, uint32 seg, int emode)
{
      char        path[MAXPGPATH];
      char        xlogfname[MAXFNAMELEN];
      char        activitymsg[MAXFNAMELEN + 16];
      ListCell   *cell;
      int               fd;

      /*
       * Loop looking for a suitable timeline ID: we might need to read any of
       * the timelines listed in expectedTLIs.
       *
       * We expect curFileTLI on entry to be the TLI of the preceding file in
       * sequence, or 0 if there was no predecessor.  We do not allow curFileTLI
       * to go backwards; this prevents us from picking up the wrong file when a
       * parent timeline extends to higher segment numbers than the child we
       * want to read.
       */
      foreach(cell, expectedTLIs)
      {
            TimeLineID  tli = (TimeLineID) lfirst_int(cell);

            if (tli < curFileTLI)
                  break;                        /* don't bother looking at too-old TLIs */

            XLogFileName(xlogfname, tli, log, seg);

            if (InArchiveRecovery)
            {
                  /* Report recovery progress in PS display */
                  snprintf(activitymsg, sizeof(activitymsg), "waiting for %s",
                               xlogfname);
                  set_ps_display(activitymsg, false);

                  restoredFromArchive = RestoreArchivedFile(path, xlogfname,
                                                                                "RECOVERYXLOG",
                                                                                XLogSegSize);
            }
            else
                  XLogFilePath(path, tli, log, seg);

            fd = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
            if (fd >= 0)
            {
                  /* Success! */
                  curFileTLI = tli;

                  /* Report recovery progress in PS display */
                  snprintf(activitymsg, sizeof(activitymsg), "recovering %s",
                               xlogfname);
                  set_ps_display(activitymsg, false);

                  return fd;
            }
            if (errno != ENOENT)    /* unexpected failure? */
                  ereport(PANIC,
                              (errcode_for_file_access(),
                  errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
                           path, log, seg)));
      }

      /* Couldn't find it.  For simplicity, complain about front timeline */
      XLogFilePath(path, recoveryTargetTLI, log, seg);
      errno = ENOENT;
      ereport(emode,
                  (errcode_for_file_access(),
               errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
                          path, log, seg)));
      return -1;
}

/*
 * Close the current logfile segment for writing.
 */
static void
XLogFileClose(void)
{
      Assert(openLogFile >= 0);

      /*
       * WAL segment files will not be re-read in normal operation, so we advise
       * the OS to release any cached pages.    But do not do so if WAL archiving
       * is active, because archiver process could use the cache to read the WAL
       * segment.  Also, don't bother with it if we are using O_DIRECT, since
       * the kernel is presumably not caching in that case.
       */
#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
      if (!XLogArchivingActive() &&
            (get_sync_bit(sync_method) & PG_O_DIRECT) == 0)
            (void) posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
#endif

      if (close(openLogFile))
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not close log file %u, segment %u: %m",
                                    openLogId, openLogSeg)));
      openLogFile = -1;
}

/*
 * Attempt to retrieve the specified file from off-line archival storage.
 * If successful, fill "path" with its complete path (note that this will be
 * a temp file name that doesn't follow the normal naming convention), and
 * return TRUE.
 *
 * If not successful, fill "path" with the name of the normal on-line file
 * (which may or may not actually exist, but we'll try to use it), and return
 * FALSE.
 *
 * For fixed-size files, the caller may pass the expected size as an
 * additional crosscheck on successful recovery.  If the file size is not
 * known, set expectedSize = 0.
 */
static bool
RestoreArchivedFile(char *path, const char *xlogfname,
                              const char *recovername, off_t expectedSize)
{
      char        xlogpath[MAXPGPATH];
      char        xlogRestoreCmd[MAXPGPATH];
      char        lastRestartPointFname[MAXPGPATH];
      char     *dp;
      char     *endp;
      const char *sp;
      int               rc;
      bool        signaled;
      struct stat stat_buf;
      uint32            restartLog;
      uint32            restartSeg;

      /*
       * When doing archive recovery, we always prefer an archived log file even
       * if a file of the same name exists in XLOGDIR.  The reason is that the
       * file in XLOGDIR could be an old, un-filled or partly-filled version
       * that was copied and restored as part of backing up $PGDATA.
       *
       * We could try to optimize this slightly by checking the local copy
       * lastchange timestamp against the archived copy, but we have no API to
       * do this, nor can we guarantee that the lastchange timestamp was
       * preserved correctly when we copied to archive. Our aim is robustness,
       * so we elect not to do this.
       *
       * If we cannot obtain the log file from the archive, however, we will try
       * to use the XLOGDIR file if it exists.  This is so that we can make use
       * of log segments that weren't yet transferred to the archive.
       *
       * Notice that we don't actually overwrite any files when we copy back
       * from archive because the recoveryRestoreCommand may inadvertently
       * restore inappropriate xlogs, or they may be corrupt, so we may wish to
       * fallback to the segments remaining in current XLOGDIR later. The
       * copy-from-archive filename is always the same, ensuring that we don't
       * run out of disk space on long recoveries.
       */
      snprintf(xlogpath, MAXPGPATH, XLOGDIR "/%s", recovername);

      /*
       * Make sure there is no existing file named recovername.
       */
      if (stat(xlogpath, &stat_buf) != 0)
      {
            if (errno != ENOENT)
                  ereport(FATAL,
                              (errcode_for_file_access(),
                               errmsg("could not stat file \"%s\": %m",
                                          xlogpath)));
      }
      else
      {
            if (unlink(xlogpath) != 0)
                  ereport(FATAL,
                              (errcode_for_file_access(),
                               errmsg("could not remove file \"%s\": %m",
                                          xlogpath)));
      }

      /*
       * Calculate the archive file cutoff point for use during log shipping
       * replication. All files earlier than this point can be deleted from the
       * archive, though there is no requirement to do so.
       *
       * We initialise this with the filename of an InvalidXLogRecPtr, which
       * will prevent the deletion of any WAL files from the archive because of
       * the alphabetic sorting property of WAL filenames.
       *
       * Once we have successfully located the redo pointer of the checkpoint
       * from which we start recovery we never request a file prior to the redo
       * pointer of the last restartpoint. When redo begins we know that we have
       * successfully located it, so there is no need for additional status
       * flags to signify the point when we can begin deleting WAL files from
       * the archive.
       */
      if (InRedo)
      {
            XLByteToSeg(ControlFile->checkPointCopy.redo,
                              restartLog, restartSeg);
            XLogFileName(lastRestartPointFname,
                               ControlFile->checkPointCopy.ThisTimeLineID,
                               restartLog, restartSeg);
            /* we shouldn't need anything earlier than last restart point */
            Assert(strcmp(lastRestartPointFname, xlogfname) <= 0);
      }
      else
            XLogFileName(lastRestartPointFname, 0, 0, 0);

      /*
       * construct the command to be executed
       */
      dp = xlogRestoreCmd;
      endp = xlogRestoreCmd + MAXPGPATH - 1;
      *endp = '\0';

      for (sp = recoveryRestoreCommand; *sp; sp++)
      {
            if (*sp == '%')
            {
                  switch (sp[1])
                  {
                        case 'p':
                              /* %p: relative path of target file */
                              sp++;
                              StrNCpy(dp, xlogpath, endp - dp);
                              make_native_path(dp);
                              dp += strlen(dp);
                              break;
                        case 'f':
                              /* %f: filename of desired file */
                              sp++;
                              StrNCpy(dp, xlogfname, endp - dp);
                              dp += strlen(dp);
                              break;
                        case 'r':
                              /* %r: filename of last restartpoint */
                              sp++;
                              StrNCpy(dp, lastRestartPointFname, endp - dp);
                              dp += strlen(dp);
                              break;
                        case '%':
                              /* convert %% to a single % */
                              sp++;
                              if (dp < endp)
                                    *dp++ = *sp;
                              break;
                        default:
                              /* otherwise treat the % as not special */
                              if (dp < endp)
                                    *dp++ = *sp;
                              break;
                  }
            }
            else
            {
                  if (dp < endp)
                        *dp++ = *sp;
            }
      }
      *dp = '\0';

      ereport(DEBUG3,
                  (errmsg_internal("executing restore command \"%s\"",
                                           xlogRestoreCmd)));

      /*
       * Set in_restore_command to tell the signal handler that we should exit
       * right away on SIGTERM. We know that we're at a safe point to do that.
       * Check if we had already received the signal, so that we don't miss a
       * shutdown request received just before this.
       */
      in_restore_command = true;
      if (shutdown_requested)
            proc_exit(1);

      /*
       * Copy xlog from archival storage to XLOGDIR
       */
      rc = system(xlogRestoreCmd);

      in_restore_command = false;

      if (rc == 0)
      {
            /*
             * command apparently succeeded, but let's make sure the file is
             * really there now and has the correct size.
             *
             * XXX I made wrong-size a fatal error to ensure the DBA would notice
             * it, but is that too strong?      We could try to plow ahead with a
             * local copy of the file ... but the problem is that there probably
             * isn't one, and we'd incorrectly conclude we've reached the end of
             * WAL and we're done recovering ...
             */
            if (stat(xlogpath, &stat_buf) == 0)
            {
                  if (expectedSize > 0 && stat_buf.st_size != expectedSize)
                        ereport(FATAL,
                                    (errmsg("archive file \"%s\" has wrong size: %lu instead of %lu",
                                                xlogfname,
                                                (unsigned long) stat_buf.st_size,
                                                (unsigned long) expectedSize)));
                  else
                  {
                        ereport(LOG,
                                    (errmsg("restored log file \"%s\" from archive",
                                                xlogfname)));
                        strcpy(path, xlogpath);
                        return true;
                  }
            }
            else
            {
                  /* stat failed */
                  if (errno != ENOENT)
                        ereport(FATAL,
                                    (errcode_for_file_access(),
                                     errmsg("could not stat file \"%s\": %m",
                                                xlogpath)));
            }
      }

      /*
       * Remember, we rollforward UNTIL the restore fails so failure here is
       * just part of the process... that makes it difficult to determine
       * whether the restore failed because there isn't an archive to restore,
       * or because the administrator has specified the restore program
       * incorrectly.  We have to assume the former.
       *
       * However, if the failure was due to any sort of signal, it's best to
       * punt and abort recovery.  (If we "return false" here, upper levels will
       * assume that recovery is complete and start up the database!) It's
       * essential to abort on child SIGINT and SIGQUIT, because per spec
       * system() ignores SIGINT and SIGQUIT while waiting; if we see one of
       * those it's a good bet we should have gotten it too.
       *
       * On SIGTERM, assume we have received a fast shutdown request, and exit
       * cleanly. It's pure chance whether we receive the SIGTERM first, or the
       * child process. If we receive it first, the signal handler will call
       * proc_exit, otherwise we do it here. If we or the child process received
       * SIGTERM for any other reason than a fast shutdown request, postmaster
       * will perform an immediate shutdown when it sees us exiting
       * unexpectedly.
       *
       * Per the Single Unix Spec, shells report exit status > 128 when a called
       * command died on a signal.  Also, 126 and 127 are used to report
       * problems such as an unfindable command; treat those as fatal errors
       * too.
       */
      if (WIFSIGNALED(rc) && WTERMSIG(rc) == SIGTERM)
            proc_exit(1);

      signaled = WIFSIGNALED(rc) || WEXITSTATUS(rc) > 125;

      ereport(signaled ? FATAL : DEBUG2,
            (errmsg("could not restore file \"%s\" from archive: return code %d",
                        xlogfname, rc)));

      /*
       * if an archived file is not available, there might still be a version of
       * this file in XLOGDIR, so return that as the filename to open.
       *
       * In many recovery scenarios we expect this to fail also, but if so that
       * just means we've reached the end of WAL.
       */
      snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlogfname);
      return false;
}

/*
 * Attempt to execute the recovery_end_command.
 */
static void
ExecuteRecoveryEndCommand(void)
{
      char        xlogRecoveryEndCmd[MAXPGPATH];
      char        lastRestartPointFname[MAXPGPATH];
      char     *dp;
      char     *endp;
      const char *sp;
      int               rc;
      bool        signaled;
      uint32            restartLog;
      uint32            restartSeg;

      Assert(recoveryEndCommand);

      /*
       * Calculate the archive file cutoff point for use during log shipping
       * replication. All files earlier than this point can be deleted from the
       * archive, though there is no requirement to do so.
       *
       * We initialise this with the filename of an InvalidXLogRecPtr, which
       * will prevent the deletion of any WAL files from the archive because of
       * the alphabetic sorting property of WAL filenames.
       *
       * Once we have successfully located the redo pointer of the checkpoint
       * from which we start recovery we never request a file prior to the redo
       * pointer of the last restartpoint. When redo begins we know that we have
       * successfully located it, so there is no need for additional status
       * flags to signify the point when we can begin deleting WAL files from
       * the archive.
       */
      if (InRedo)
      {
            XLByteToSeg(ControlFile->checkPointCopy.redo,
                              restartLog, restartSeg);
            XLogFileName(lastRestartPointFname,
                               ControlFile->checkPointCopy.ThisTimeLineID,
                               restartLog, restartSeg);
      }
      else
            XLogFileName(lastRestartPointFname, 0, 0, 0);

      /*
       * construct the command to be executed
       */
      dp = xlogRecoveryEndCmd;
      endp = xlogRecoveryEndCmd + MAXPGPATH - 1;
      *endp = '\0';

      for (sp = recoveryEndCommand; *sp; sp++)
      {
            if (*sp == '%')
            {
                  switch (sp[1])
                  {
                        case 'r':
                              /* %r: filename of last restartpoint */
                              sp++;
                              StrNCpy(dp, lastRestartPointFname, endp - dp);
                              dp += strlen(dp);
                              break;
                        case '%':
                              /* convert %% to a single % */
                              sp++;
                              if (dp < endp)
                                    *dp++ = *sp;
                              break;
                        default:
                              /* otherwise treat the % as not special */
                              if (dp < endp)
                                    *dp++ = *sp;
                              break;
                  }
            }
            else
            {
                  if (dp < endp)
                        *dp++ = *sp;
            }
      }
      *dp = '\0';

      ereport(DEBUG3,
                  (errmsg_internal("executing recovery end command \"%s\"",
                                           xlogRecoveryEndCmd)));

      /*
       * execute the constructed command
       */
      rc = system(xlogRecoveryEndCmd);
      if (rc != 0)
      {
            /*
             * If the failure was due to any sort of signal, it's best to punt and
             * abort recovery. See also detailed comments on signals in
             * RestoreArchivedFile().
             */
            signaled = WIFSIGNALED(rc) || WEXITSTATUS(rc) > 125;

            ereport(signaled ? FATAL : WARNING,
                        (errmsg("recovery_end_command \"%s\": return code %d",
                                    xlogRecoveryEndCmd, rc)));
      }
}

/*
 * Preallocate log files beyond the specified log endpoint.
 *
 * XXX this is currently extremely conservative, since it forces only one
 * future log segment to exist, and even that only if we are 75% done with
 * the current one.  This is only appropriate for very low-WAL-volume systems.
 * High-volume systems will be OK once they've built up a sufficient set of
 * recycled log segments, but the startup transient is likely to include
 * a lot of segment creations by foreground processes, which is not so good.
 */
static void
PreallocXlogFiles(XLogRecPtr endptr)
{
      uint32            _logId;
      uint32            _logSeg;
      int               lf;
      bool        use_existent;

      XLByteToPrevSeg(endptr, _logId, _logSeg);
      if ((endptr.xrecoff - 1) % XLogSegSize >=
            (uint32) (0.75 * XLogSegSize))
      {
            NextLogSeg(_logId, _logSeg);
            use_existent = true;
            lf = XLogFileInit(_logId, _logSeg, &use_existent, true);
            close(lf);
            if (!use_existent)
                  CheckpointStats.ckpt_segs_added++;
      }
}

/*
 * Recycle or remove all log files older or equal to passed log/seg#
 *
 * endptr is current (or recent) end of xlog; this is used to determine
 * whether we want to recycle rather than delete no-longer-wanted log files.
 */
static void
RemoveOldXlogFiles(uint32 log, uint32 seg, XLogRecPtr endptr)
{
      uint32            endlogId;
      uint32            endlogSeg;
      int               max_advance;
      DIR            *xldir;
      struct dirent *xlde;
      char        lastoff[MAXFNAMELEN];
      char        path[MAXPGPATH];
      struct stat statbuf;

      /*
       * Initialize info about where to try to recycle to.  We allow recycling
       * segments up to XLOGfileslop segments beyond the current XLOG location.
       */
      XLByteToPrevSeg(endptr, endlogId, endlogSeg);
      max_advance = XLOGfileslop;

      xldir = AllocateDir(XLOGDIR);
      if (xldir == NULL)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not open transaction log directory \"%s\": %m",
                                    XLOGDIR)));

      XLogFileName(lastoff, ThisTimeLineID, log, seg);

      while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
      {
            /*
             * We ignore the timeline part of the XLOG segment identifiers in
             * deciding whether a segment is still needed.  This ensures that we
             * won't prematurely remove a segment from a parent timeline. We could
             * probably be a little more proactive about removing segments of
             * non-parent timelines, but that would be a whole lot more
             * complicated.
             *
             * We use the alphanumeric sorting property of the filenames to decide
             * which ones are earlier than the lastoff segment.
             */
            if (strlen(xlde->d_name) == 24 &&
                  strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
                  strcmp(xlde->d_name + 8, lastoff + 8) <= 0)
            {
                  if (XLogArchiveCheckDone(xlde->d_name))
                  {
                        snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);

                        /*
                         * Before deleting the file, see if it can be recycled as a
                         * future log segment. Only recycle normal files, pg_standby
                         * for example can create symbolic links pointing to a
                         * separate archive directory.
                         */
                        if (lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
                              InstallXLogFileSegment(&endlogId, &endlogSeg, path,
                                                               true, &max_advance, true))
                        {
                              ereport(DEBUG2,
                                          (errmsg("recycled transaction log file \"%s\"",
                                                      xlde->d_name)));
                              CheckpointStats.ckpt_segs_recycled++;
                              /* Needn't recheck that slot on future iterations */
                              if (max_advance > 0)
                              {
                                    NextLogSeg(endlogId, endlogSeg);
                                    max_advance--;
                              }
                        }
                        else
                        {
                              /* No need for any more future segments... */
                              ereport(DEBUG2,
                                          (errmsg("removing transaction log file \"%s\"",
                                                      xlde->d_name)));
                              unlink(path);
                              CheckpointStats.ckpt_segs_removed++;
                        }

                        XLogArchiveCleanup(xlde->d_name);
                  }
            }
      }

      FreeDir(xldir);
}

/*
 * Verify whether pg_xlog and pg_xlog/archive_status exist.
 * If the latter does not exist, recreate it.
 *
 * It is not the goal of this function to verify the contents of these
 * directories, but to help in cases where someone has performed a cluster
 * copy for PITR purposes but omitted pg_xlog from the copy.
 *
 * We could also recreate pg_xlog if it doesn't exist, but a deliberate
 * policy decision was made not to.  It is fairly common for pg_xlog to be
 * a symlink, and if that was the DBA's intent then automatically making a
 * plain directory would result in degraded performance with no notice.
 */
static void
ValidateXLOGDirectoryStructure(void)
{
      char        path[MAXPGPATH];
      struct stat stat_buf;

      /* Check for pg_xlog; if it doesn't exist, error out */
      if (stat(XLOGDIR, &stat_buf) != 0 ||
            !S_ISDIR(stat_buf.st_mode))
            ereport(FATAL,
                        (errmsg("required WAL directory \"%s\" does not exist",
                                    XLOGDIR)));

      /* Check for archive_status */
      snprintf(path, MAXPGPATH, XLOGDIR "/archive_status");
      if (stat(path, &stat_buf) == 0)
      {
            /* Check for weird cases where it exists but isn't a directory */
            if (!S_ISDIR(stat_buf.st_mode))
                  ereport(FATAL,
                              (errmsg("required WAL directory \"%s\" does not exist",
                                          path)));
      }
      else
      {
            ereport(LOG,
                        (errmsg("creating missing WAL directory \"%s\"", path)));
            if (mkdir(path, 0700) < 0)
                  ereport(FATAL,
                              (errmsg("could not create missing directory \"%s\": %m",
                                          path)));
      }
}

/*
 * Remove previous backup history files.  This also retries creation of
 * .ready files for any backup history files for which XLogArchiveNotify
 * failed earlier.
 */
static void
CleanupBackupHistory(void)
{
      DIR            *xldir;
      struct dirent *xlde;
      char        path[MAXPGPATH];

      xldir = AllocateDir(XLOGDIR);
      if (xldir == NULL)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not open transaction log directory \"%s\": %m",
                                    XLOGDIR)));

      while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
      {
            if (strlen(xlde->d_name) > 24 &&
                  strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
                  strcmp(xlde->d_name + strlen(xlde->d_name) - strlen(".backup"),
                           ".backup") == 0)
            {
                  if (XLogArchiveCheckDone(xlde->d_name))
                  {
                        ereport(DEBUG2,
                        (errmsg("removing transaction log backup history file \"%s\"",
                                    xlde->d_name)));
                        snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);
                        unlink(path);
                        XLogArchiveCleanup(xlde->d_name);
                  }
            }
      }

      FreeDir(xldir);
}

/*
 * Restore the backup blocks present in an XLOG record, if any.
 *
 * We assume all of the record has been read into memory at *record.
 *
 * Note: when a backup block is available in XLOG, we restore it
 * unconditionally, even if the page in the database appears newer.
 * This is to protect ourselves against database pages that were partially
 * or incorrectly written during a crash.  We assume that the XLOG data
 * must be good because it has passed a CRC check, while the database
 * page might not be.  This will force us to replay all subsequent
 * modifications of the page that appear in XLOG, rather than possibly
 * ignoring them as already applied, but that's not a huge drawback.
 *
 * If 'cleanup' is true, a cleanup lock is used when restoring blocks.
 * Otherwise, a normal exclusive lock is used.  At the moment, that's just
 * pro forma, because there can't be any regular backends in the system
 * during recovery.  The 'cleanup' argument applies to all backup blocks
 * in the WAL record, that suffices for now.
 */
void
RestoreBkpBlocks(XLogRecPtr lsn, XLogRecord *record, bool cleanup)
{
      Buffer            buffer;
      Page        page;
      BkpBlock    bkpb;
      char     *blk;
      int               i;

      if (!(record->xl_info & XLR_BKP_BLOCK_MASK))
            return;

      blk = (char *) XLogRecGetData(record) + record->xl_len;
      for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
      {
            if (!(record->xl_info & XLR_SET_BKP_BLOCK(i)))
                  continue;

            memcpy(&bkpb, blk, sizeof(BkpBlock));
            blk += sizeof(BkpBlock);

            buffer = XLogReadBufferExtended(bkpb.node, bkpb.fork, bkpb.block,
                                                            RBM_ZERO);
            Assert(BufferIsValid(buffer));
            if (cleanup)
                  LockBufferForCleanup(buffer);
            else
                  LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

            page = (Page) BufferGetPage(buffer);

            if (bkpb.hole_length == 0)
            {
                  memcpy((char *) page, blk, BLCKSZ);
            }
            else
            {
                  /* must zero-fill the hole */
                  MemSet((char *) page, 0, BLCKSZ);
                  memcpy((char *) page, blk, bkpb.hole_offset);
                  memcpy((char *) page + (bkpb.hole_offset + bkpb.hole_length),
                           blk + bkpb.hole_offset,
                           BLCKSZ - (bkpb.hole_offset + bkpb.hole_length));
            }

            PageSetLSN(page, lsn);
            PageSetTLI(page, ThisTimeLineID);
            MarkBufferDirty(buffer);
            UnlockReleaseBuffer(buffer);

            blk += BLCKSZ - bkpb.hole_length;
      }
}

/*
 * CRC-check an XLOG record.  We do not believe the contents of an XLOG
 * record (other than to the minimal extent of computing the amount of
 * data to read in) until we've checked the CRCs.
 *
 * We assume all of the record has been read into memory at *record.
 */
static bool
RecordIsValid(XLogRecord *record, XLogRecPtr recptr, int emode)
{
      pg_crc32    crc;
      int               i;
      uint32            len = record->xl_len;
      BkpBlock    bkpb;
      char     *blk;

      /* First the rmgr data */
      INIT_CRC32(crc);
      COMP_CRC32(crc, XLogRecGetData(record), len);

      /* Add in the backup blocks, if any */
      blk = (char *) XLogRecGetData(record) + len;
      for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
      {
            uint32            blen;

            if (!(record->xl_info & XLR_SET_BKP_BLOCK(i)))
                  continue;

            memcpy(&bkpb, blk, sizeof(BkpBlock));
            if (bkpb.hole_offset + bkpb.hole_length > BLCKSZ)
            {
                  ereport(emode,
                              (errmsg("incorrect hole size in record at %X/%X",
                                          recptr.xlogid, recptr.xrecoff)));
                  return false;
            }
            blen = sizeof(BkpBlock) + BLCKSZ - bkpb.hole_length;
            COMP_CRC32(crc, blk, blen);
            blk += blen;
      }

      /* Check that xl_tot_len agrees with our calculation */
      if (blk != (char *) record + record->xl_tot_len)
      {
            ereport(emode,
                        (errmsg("incorrect total length in record at %X/%X",
                                    recptr.xlogid, recptr.xrecoff)));
            return false;
      }

      /* Finally include the record header */
      COMP_CRC32(crc, (char *) record + sizeof(pg_crc32),
                     SizeOfXLogRecord - sizeof(pg_crc32));
      FIN_CRC32(crc);

      if (!EQ_CRC32(record->xl_crc, crc))
      {
            ereport(emode,
            (errmsg("incorrect resource manager data checksum in record at %X/%X",
                        recptr.xlogid, recptr.xrecoff)));
            return false;
      }

      return true;
}

/*
 * Attempt to read an XLOG record.
 *
 * If RecPtr is not NULL, try to read a record at that position.  Otherwise
 * try to read a record just after the last one previously read.
 *
 * If no valid record is available, returns NULL, or fails if emode is PANIC.
 * (emode must be either PANIC or LOG.)
 *
 * The record is copied into readRecordBuf, so that on successful return,
 * the returned record pointer always points there.
 */
static XLogRecord *
ReadRecord(XLogRecPtr *RecPtr, int emode)
{
      XLogRecord *record;
      char     *buffer;
      XLogRecPtr  tmpRecPtr = EndRecPtr;
      bool        randAccess = false;
      uint32            len,
                        total_len;
      uint32            targetPageOff;
      uint32            targetRecOff;
      uint32            pageHeaderSize;

      if (readBuf == NULL)
      {
            /*
             * First time through, permanently allocate readBuf.  We do it this
             * way, rather than just making a static array, for two reasons: (1)
             * no need to waste the storage in most instantiations of the backend;
             * (2) a static char array isn't guaranteed to have any particular
             * alignment, whereas malloc() will provide MAXALIGN'd storage.
             */
            readBuf = (char *) malloc(XLOG_BLCKSZ);
            Assert(readBuf != NULL);
      }

      if (RecPtr == NULL)
      {
            RecPtr = &tmpRecPtr;
            /* fast case if next record is on same page */
            if (nextRecord != NULL)
            {
                  record = nextRecord;
                  goto got_record;
            }
            /* align old recptr to next page */
            if (tmpRecPtr.xrecoff % XLOG_BLCKSZ != 0)
                  tmpRecPtr.xrecoff += (XLOG_BLCKSZ - tmpRecPtr.xrecoff % XLOG_BLCKSZ);
            if (tmpRecPtr.xrecoff >= XLogFileSize)
            {
                  (tmpRecPtr.xlogid)++;
                  tmpRecPtr.xrecoff = 0;
            }
            /* We will account for page header size below */
      }
      else
      {
            if (!XRecOffIsValid(RecPtr->xrecoff))
                  ereport(PANIC,
                              (errmsg("invalid record offset at %X/%X",
                                          RecPtr->xlogid, RecPtr->xrecoff)));

            /*
             * Since we are going to a random position in WAL, forget any prior
             * state about what timeline we were in, and allow it to be any
             * timeline in expectedTLIs.  We also set a flag to allow curFileTLI
             * to go backwards (but we can't reset that variable right here, since
             * we might not change files at all).
             */
            lastPageTLI = 0;        /* see comment in ValidXLOGHeader */
            randAccess = true;            /* allow curFileTLI to go backwards too */
      }

      if (readFile >= 0 && !XLByteInSeg(*RecPtr, readId, readSeg))
      {
            close(readFile);
            readFile = -1;
      }
      XLByteToSeg(*RecPtr, readId, readSeg);
      if (readFile < 0)
      {
            /* Now it's okay to reset curFileTLI if random fetch */
            if (randAccess)
                  curFileTLI = 0;

            readFile = XLogFileRead(readId, readSeg, emode);
            if (readFile < 0)
                  goto next_record_is_invalid;

            /*
             * Whenever switching to a new WAL segment, we read the first page of
             * the file and validate its header, even if that's not where the
             * target record is.  This is so that we can check the additional
             * identification info that is present in the first page's "long"
             * header.
             */
            readOff = 0;
            if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
            {
                  ereport(emode,
                              (errcode_for_file_access(),
                               errmsg("could not read from log file %u, segment %u, offset %u: %m",
                                          readId, readSeg, readOff)));
                  goto next_record_is_invalid;
            }
            if (!ValidXLOGHeader((XLogPageHeader) readBuf, emode))
                  goto next_record_is_invalid;
      }

      targetPageOff = ((RecPtr->xrecoff % XLogSegSize) / XLOG_BLCKSZ) * XLOG_BLCKSZ;
      if (readOff != targetPageOff)
      {
            readOff = targetPageOff;
            if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0)
            {
                  ereport(emode,
                              (errcode_for_file_access(),
                               errmsg("could not seek in log file %u, segment %u to offset %u: %m",
                                          readId, readSeg, readOff)));
                  goto next_record_is_invalid;
            }
            if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
            {
                  ereport(emode,
                              (errcode_for_file_access(),
                               errmsg("could not read from log file %u, segment %u, offset %u: %m",
                                          readId, readSeg, readOff)));
                  goto next_record_is_invalid;
            }
            if (!ValidXLOGHeader((XLogPageHeader) readBuf, emode))
                  goto next_record_is_invalid;
      }
      pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) readBuf);
      targetRecOff = RecPtr->xrecoff % XLOG_BLCKSZ;
      if (targetRecOff == 0)
      {
            /*
             * Can only get here in the continuing-from-prev-page case, because
             * XRecOffIsValid eliminated the zero-page-offset case otherwise. Need
             * to skip over the new page's header.
             */
            tmpRecPtr.xrecoff += pageHeaderSize;
            targetRecOff = pageHeaderSize;
      }
      else if (targetRecOff < pageHeaderSize)
      {
            ereport(emode,
                        (errmsg("invalid record offset at %X/%X",
                                    RecPtr->xlogid, RecPtr->xrecoff)));
            goto next_record_is_invalid;
      }
      if ((((XLogPageHeader) readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) &&
            targetRecOff == pageHeaderSize)
      {
            ereport(emode,
                        (errmsg("contrecord is requested by %X/%X",
                                    RecPtr->xlogid, RecPtr->xrecoff)));
            goto next_record_is_invalid;
      }
      record = (XLogRecord *) ((char *) readBuf + RecPtr->xrecoff % XLOG_BLCKSZ);

got_record:;

      /*
       * xl_len == 0 is bad data for everything except XLOG SWITCH, where it is
       * required.
       */
      if (record->xl_rmid == RM_XLOG_ID && record->xl_info == XLOG_SWITCH)
      {
            if (record->xl_len != 0)
            {
                  ereport(emode,
                              (errmsg("invalid xlog switch record at %X/%X",
                                          RecPtr->xlogid, RecPtr->xrecoff)));
                  goto next_record_is_invalid;
            }
      }
      else if (record->xl_len == 0)
      {
            ereport(emode,
                        (errmsg("record with zero length at %X/%X",
                                    RecPtr->xlogid, RecPtr->xrecoff)));
            goto next_record_is_invalid;
      }
      if (record->xl_tot_len < SizeOfXLogRecord + record->xl_len ||
            record->xl_tot_len > SizeOfXLogRecord + record->xl_len +
            XLR_MAX_BKP_BLOCKS * (sizeof(BkpBlock) + BLCKSZ))
      {
            ereport(emode,
                        (errmsg("invalid record length at %X/%X",
                                    RecPtr->xlogid, RecPtr->xrecoff)));
            goto next_record_is_invalid;
      }
      if (record->xl_rmid > RM_MAX_ID)
      {
            ereport(emode,
                        (errmsg("invalid resource manager ID %u at %X/%X",
                                    record->xl_rmid, RecPtr->xlogid, RecPtr->xrecoff)));
            goto next_record_is_invalid;
      }
      if (randAccess)
      {
            /*
             * We can't exactly verify the prev-link, but surely it should be less
             * than the record's own address.
             */
            if (!XLByteLT(record->xl_prev, *RecPtr))
            {
                  ereport(emode,
                              (errmsg("record with incorrect prev-link %X/%X at %X/%X",
                                          record->xl_prev.xlogid, record->xl_prev.xrecoff,
                                          RecPtr->xlogid, RecPtr->xrecoff)));
                  goto next_record_is_invalid;
            }
      }
      else
      {
            /*
             * Record's prev-link should exactly match our previous location. This
             * check guards against torn WAL pages where a stale but valid-looking
             * WAL record starts on a sector boundary.
             */
            if (!XLByteEQ(record->xl_prev, ReadRecPtr))
            {
                  ereport(emode,
                              (errmsg("record with incorrect prev-link %X/%X at %X/%X",
                                          record->xl_prev.xlogid, record->xl_prev.xrecoff,
                                          RecPtr->xlogid, RecPtr->xrecoff)));
                  goto next_record_is_invalid;
            }
      }

      /*
       * Allocate or enlarge readRecordBuf as needed.  To avoid useless small
       * increases, round its size to a multiple of XLOG_BLCKSZ, and make sure
       * it's at least 4*Max(BLCKSZ, XLOG_BLCKSZ) to start with.  (That is
       * enough for all "normal" records, but very large commit or abort records
       * might need more space.)
       */
      total_len = record->xl_tot_len;
      if (total_len > readRecordBufSize)
      {
            uint32            newSize = total_len;

            newSize += XLOG_BLCKSZ - (newSize % XLOG_BLCKSZ);
            newSize = Max(newSize, 4 * Max(BLCKSZ, XLOG_BLCKSZ));
            if (readRecordBuf)
                  free(readRecordBuf);
            readRecordBuf = (char *) malloc(newSize);
            if (!readRecordBuf)
            {
                  readRecordBufSize = 0;
                  /* We treat this as a "bogus data" condition */
                  ereport(emode,
                              (errmsg("record length %u at %X/%X too long",
                                          total_len, RecPtr->xlogid, RecPtr->xrecoff)));
                  goto next_record_is_invalid;
            }
            readRecordBufSize = newSize;
      }

      buffer = readRecordBuf;
      nextRecord = NULL;
      len = XLOG_BLCKSZ - RecPtr->xrecoff % XLOG_BLCKSZ;
      if (total_len > len)
      {
            /* Need to reassemble record */
            XLogContRecord *contrecord;
            uint32            gotlen = len;

            memcpy(buffer, record, len);
            record = (XLogRecord *) buffer;
            buffer += len;
            for (;;)
            {
                  readOff += XLOG_BLCKSZ;
                  if (readOff >= XLogSegSize)
                  {
                        close(readFile);
                        readFile = -1;
                        NextLogSeg(readId, readSeg);
                        readFile = XLogFileRead(readId, readSeg, emode);
                        if (readFile < 0)
                              goto next_record_is_invalid;
                        readOff = 0;
                  }
                  if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
                  {
                        ereport(emode,
                                    (errcode_for_file_access(),
                                     errmsg("could not read from log file %u, segment %u, offset %u: %m",
                                                readId, readSeg, readOff)));
                        goto next_record_is_invalid;
                  }
                  if (!ValidXLOGHeader((XLogPageHeader) readBuf, emode))
                        goto next_record_is_invalid;
                  if (!(((XLogPageHeader) readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD))
                  {
                        ereport(emode,
                                    (errmsg("there is no contrecord flag in log file %u, segment %u, offset %u",
                                                readId, readSeg, readOff)));
                        goto next_record_is_invalid;
                  }
                  pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) readBuf);
                  contrecord = (XLogContRecord *) ((char *) readBuf + pageHeaderSize);
                  if (contrecord->xl_rem_len == 0 ||
                        total_len != (contrecord->xl_rem_len + gotlen))
                  {
                        ereport(emode,
                                    (errmsg("invalid contrecord length %u in log file %u, segment %u, offset %u",
                                                contrecord->xl_rem_len,
                                                readId, readSeg, readOff)));
                        goto next_record_is_invalid;
                  }
                  len = XLOG_BLCKSZ - pageHeaderSize - SizeOfXLogContRecord;
                  if (contrecord->xl_rem_len > len)
                  {
                        memcpy(buffer, (char *) contrecord + SizeOfXLogContRecord, len);
                        gotlen += len;
                        buffer += len;
                        continue;
                  }
                  memcpy(buffer, (char *) contrecord + SizeOfXLogContRecord,
                           contrecord->xl_rem_len);
                  break;
            }
            if (!RecordIsValid(record, *RecPtr, emode))
                  goto next_record_is_invalid;
            pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) readBuf);
            if (XLOG_BLCKSZ - SizeOfXLogRecord >= pageHeaderSize +
                  MAXALIGN(SizeOfXLogContRecord + contrecord->xl_rem_len))
            {
                  nextRecord = (XLogRecord *) ((char *) contrecord +
                              MAXALIGN(SizeOfXLogContRecord + contrecord->xl_rem_len));
            }
            EndRecPtr.xlogid = readId;
            EndRecPtr.xrecoff = readSeg * XLogSegSize + readOff +
                  pageHeaderSize +
                  MAXALIGN(SizeOfXLogContRecord + contrecord->xl_rem_len);
            ReadRecPtr = *RecPtr;
            /* needn't worry about XLOG SWITCH, it can't cross page boundaries */
            return record;
      }

      /* Record does not cross a page boundary */
      if (!RecordIsValid(record, *RecPtr, emode))
            goto next_record_is_invalid;
      if (XLOG_BLCKSZ - SizeOfXLogRecord >= RecPtr->xrecoff % XLOG_BLCKSZ +
            MAXALIGN(total_len))
            nextRecord = (XLogRecord *) ((char *) record + MAXALIGN(total_len));
      EndRecPtr.xlogid = RecPtr->xlogid;
      EndRecPtr.xrecoff = RecPtr->xrecoff + MAXALIGN(total_len);
      ReadRecPtr = *RecPtr;
      memcpy(buffer, record, total_len);

      /*
       * Special processing if it's an XLOG SWITCH record
       */
      if (record->xl_rmid == RM_XLOG_ID && record->xl_info == XLOG_SWITCH)
      {
            /* Pretend it extends to end of segment */
            EndRecPtr.xrecoff += XLogSegSize - 1;
            EndRecPtr.xrecoff -= EndRecPtr.xrecoff % XLogSegSize;
            nextRecord = NULL;            /* definitely not on same page */

            /*
             * Pretend that readBuf contains the last page of the segment. This is
             * just to avoid Assert failure in StartupXLOG if XLOG ends with this
             * segment.
             */
            readOff = XLogSegSize - XLOG_BLCKSZ;
      }
      return (XLogRecord *) buffer;

next_record_is_invalid:;
      if (readFile >= 0)
      {
            close(readFile);
            readFile = -1;
      }
      nextRecord = NULL;
      return NULL;
}

/*
 * Check whether the xlog header of a page just read in looks valid.
 *
 * This is just a convenience subroutine to avoid duplicated code in
 * ReadRecord.    It's not intended for use from anywhere else.
 */
static bool
ValidXLOGHeader(XLogPageHeader hdr, int emode)
{
      XLogRecPtr  recaddr;

      if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
      {
            ereport(emode,
                        (errmsg("invalid magic number %04X in log file %u, segment %u, offset %u",
                                    hdr->xlp_magic, readId, readSeg, readOff)));
            return false;
      }
      if ((hdr->xlp_info & ~XLP_ALL_FLAGS) != 0)
      {
            ereport(emode,
                        (errmsg("invalid info bits %04X in log file %u, segment %u, offset %u",
                                    hdr->xlp_info, readId, readSeg, readOff)));
            return false;
      }
      if (hdr->xlp_info & XLP_LONG_HEADER)
      {
            XLogLongPageHeader longhdr = (XLogLongPageHeader) hdr;

            if (longhdr->xlp_sysid != ControlFile->system_identifier)
            {
                  char        fhdrident_str[32];
                  char        sysident_str[32];

                  /*
                   * Format sysids separately to keep platform-dependent format code
                   * out of the translatable message string.
                   */
                  snprintf(fhdrident_str, sizeof(fhdrident_str), UINT64_FORMAT,
                               longhdr->xlp_sysid);
                  snprintf(sysident_str, sizeof(sysident_str), UINT64_FORMAT,
                               ControlFile->system_identifier);
                  ereport(emode,
                              (errmsg("WAL file is from different system"),
                               errdetail("WAL file SYSID is %s, pg_control SYSID is %s",
                                             fhdrident_str, sysident_str)));
                  return false;
            }
            if (longhdr->xlp_seg_size != XLogSegSize)
            {
                  ereport(emode,
                              (errmsg("WAL file is from different system"),
                               errdetail("Incorrect XLOG_SEG_SIZE in page header.")));
                  return false;
            }
            if (longhdr->xlp_xlog_blcksz != XLOG_BLCKSZ)
            {
                  ereport(emode,
                              (errmsg("WAL file is from different system"),
                               errdetail("Incorrect XLOG_BLCKSZ in page header.")));
                  return false;
            }
      }
      else if (readOff == 0)
      {
            /* hmm, first page of file doesn't have a long header? */
            ereport(emode,
                        (errmsg("invalid info bits %04X in log file %u, segment %u, offset %u",
                                    hdr->xlp_info, readId, readSeg, readOff)));
            return false;
      }

      recaddr.xlogid = readId;
      recaddr.xrecoff = readSeg * XLogSegSize + readOff;
      if (!XLByteEQ(hdr->xlp_pageaddr, recaddr))
      {
            ereport(emode,
                        (errmsg("unexpected pageaddr %X/%X in log file %u, segment %u, offset %u",
                                    hdr->xlp_pageaddr.xlogid, hdr->xlp_pageaddr.xrecoff,
                                    readId, readSeg, readOff)));
            return false;
      }

      /*
       * Check page TLI is one of the expected values.
       */
      if (!list_member_int(expectedTLIs, (int) hdr->xlp_tli))
      {
            ereport(emode,
                        (errmsg("unexpected timeline ID %u in log file %u, segment %u, offset %u",
                                    hdr->xlp_tli,
                                    readId, readSeg, readOff)));
            return false;
      }

      /*
       * Since child timelines are always assigned a TLI greater than their
       * immediate parent's TLI, we should never see TLI go backwards across
       * successive pages of a consistent WAL sequence.
       *
       * Of course this check should only be applied when advancing sequentially
       * across pages; therefore ReadRecord resets lastPageTLI to zero when
       * going to a random page.
       */
      if (hdr->xlp_tli < lastPageTLI)
      {
            ereport(emode,
                        (errmsg("out-of-sequence timeline ID %u (after %u) in log file %u, segment %u, offset %u",
                                    hdr->xlp_tli, lastPageTLI,
                                    readId, readSeg, readOff)));
            return false;
      }
      lastPageTLI = hdr->xlp_tli;
      return true;
}

/*
 * Try to read a timeline's history file.
 *
 * If successful, return the list of component TLIs (the given TLI followed by
 * its ancestor TLIs).  If we can't find the history file, assume that the
 * timeline has no parents, and return a list of just the specified timeline
 * ID.
 */
static List *
readTimeLineHistory(TimeLineID targetTLI)
{
      List     *result;
      char        path[MAXPGPATH];
      char        histfname[MAXFNAMELEN];
      char        fline[MAXPGPATH];
      FILE     *fd;

      if (InArchiveRecovery)
      {
            TLHistoryFileName(histfname, targetTLI);
            RestoreArchivedFile(path, histfname, "RECOVERYHISTORY", 0);
      }
      else
            TLHistoryFilePath(path, targetTLI);

      fd = AllocateFile(path, "r");
      if (fd == NULL)
      {
            if (errno != ENOENT)
                  ereport(FATAL,
                              (errcode_for_file_access(),
                               errmsg("could not open file \"%s\": %m", path)));
            /* Not there, so assume no parents */
            return list_make1_int((int) targetTLI);
      }

      result = NIL;

      /*
       * Parse the file...
       */
      while (fgets(fline, sizeof(fline), fd) != NULL)
      {
            /* skip leading whitespace and check for # comment */
            char     *ptr;
            char     *endptr;
            TimeLineID  tli;

            for (ptr = fline; *ptr; ptr++)
            {
                  if (!isspace((unsigned char) *ptr))
                        break;
            }
            if (*ptr == '\0' || *ptr == '#')
                  continue;

            /* expect a numeric timeline ID as first field of line */
            tli = (TimeLineID) strtoul(ptr, &endptr, 0);
            if (endptr == ptr)
                  ereport(FATAL,
                              (errmsg("syntax error in history file: %s", fline),
                               errhint("Expected a numeric timeline ID.")));

            if (result &&
                  tli <= (TimeLineID) linitial_int(result))
                  ereport(FATAL,
                              (errmsg("invalid data in history file: %s", fline),
                           errhint("Timeline IDs must be in increasing sequence.")));

            /* Build list with newest item first */
            result = lcons_int((int) tli, result);

            /* we ignore the remainder of each line */
      }

      FreeFile(fd);

      if (result &&
            targetTLI <= (TimeLineID) linitial_int(result))
            ereport(FATAL,
                        (errmsg("invalid data in history file \"%s\"", path),
                  errhint("Timeline IDs must be less than child timeline's ID.")));

      result = lcons_int((int) targetTLI, result);

      ereport(DEBUG3,
                  (errmsg_internal("history of timeline %u is %s",
                                           targetTLI, nodeToString(result))));

      return result;
}

/*
 * Probe whether a timeline history file exists for the given timeline ID
 */
static bool
existsTimeLineHistory(TimeLineID probeTLI)
{
      char        path[MAXPGPATH];
      char        histfname[MAXFNAMELEN];
      FILE     *fd;

      if (InArchiveRecovery)
      {
            TLHistoryFileName(histfname, probeTLI);
            RestoreArchivedFile(path, histfname, "RECOVERYHISTORY", 0);
      }
      else
            TLHistoryFilePath(path, probeTLI);

      fd = AllocateFile(path, "r");
      if (fd != NULL)
      {
            FreeFile(fd);
            return true;
      }
      else
      {
            if (errno != ENOENT)
                  ereport(FATAL,
                              (errcode_for_file_access(),
                               errmsg("could not open file \"%s\": %m", path)));
            return false;
      }
}

/*
 * Find the newest existing timeline, assuming that startTLI exists.
 *
 * Note: while this is somewhat heuristic, it does positively guarantee
 * that (result + 1) is not a known timeline, and therefore it should
 * be safe to assign that ID to a new timeline.
 */
static TimeLineID
findNewestTimeLine(TimeLineID startTLI)
{
      TimeLineID  newestTLI;
      TimeLineID  probeTLI;

      /*
       * The algorithm is just to probe for the existence of timeline history
       * files.  XXX is it useful to allow gaps in the sequence?
       */
      newestTLI = startTLI;

      for (probeTLI = startTLI + 1;; probeTLI++)
      {
            if (existsTimeLineHistory(probeTLI))
            {
                  newestTLI = probeTLI;         /* probeTLI exists */
            }
            else
            {
                  /* doesn't exist, assume we're done */
                  break;
            }
      }

      return newestTLI;
}

/*
 * Create a new timeline history file.
 *
 *    newTLI: ID of the new timeline
 *    parentTLI: ID of its immediate parent
 *    endTLI et al: ID of the last used WAL file, for annotation purposes
 *
 * Currently this is only used during recovery, and so there are no locking
 * considerations.      But we should be just as tense as XLogFileInit to avoid
 * emplacing a bogus file.
 */
static void
writeTimeLineHistory(TimeLineID newTLI, TimeLineID parentTLI,
                               TimeLineID endTLI, uint32 endLogId, uint32 endLogSeg)
{
      char        path[MAXPGPATH];
      char        tmppath[MAXPGPATH];
      char        histfname[MAXFNAMELEN];
      char        xlogfname[MAXFNAMELEN];
      char        buffer[BLCKSZ];
      int               srcfd;
      int               fd;
      int               nbytes;

      Assert(newTLI > parentTLI); /* else bad selection of newTLI */

      /*
       * Write into a temp file name.
       */
      snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

      unlink(tmppath);

      /* do not use get_sync_bit() here --- want to fsync only at end of fill */
      fd = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL,
                                 S_IRUSR | S_IWUSR);
      if (fd < 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not create file \"%s\": %m", tmppath)));

      /*
       * If a history file exists for the parent, copy it verbatim
       */
      if (InArchiveRecovery)
      {
            TLHistoryFileName(histfname, parentTLI);
            RestoreArchivedFile(path, histfname, "RECOVERYHISTORY", 0);
      }
      else
            TLHistoryFilePath(path, parentTLI);

      srcfd = BasicOpenFile(path, O_RDONLY, 0);
      if (srcfd < 0)
      {
            if (errno != ENOENT)
                  ereport(ERROR,
                              (errcode_for_file_access(),
                               errmsg("could not open file \"%s\": %m", path)));
            /* Not there, so assume parent has no parents */
      }
      else
      {
            for (;;)
            {
                  errno = 0;
                  nbytes = (int) read(srcfd, buffer, sizeof(buffer));
                  if (nbytes < 0 || errno != 0)
                        ereport(ERROR,
                                    (errcode_for_file_access(),
                                     errmsg("could not read file \"%s\": %m", path)));
                  if (nbytes == 0)
                        break;
                  errno = 0;
                  if ((int) write(fd, buffer, nbytes) != nbytes)
                  {
                        int               save_errno = errno;

                        /*
                         * If we fail to make the file, delete it to release disk
                         * space
                         */
                        unlink(tmppath);

                        /*
                         * if write didn't set errno, assume problem is no disk space
                         */
                        errno = save_errno ? save_errno : ENOSPC;

                        ereport(ERROR,
                                    (errcode_for_file_access(),
                               errmsg("could not write to file \"%s\": %m", tmppath)));
                  }
            }
            close(srcfd);
      }

      /*
       * Append one line with the details of this timeline split.
       *
       * If we did have a parent file, insert an extra newline just in case the
       * parent file failed to end with one.
       */
      XLogFileName(xlogfname, endTLI, endLogId, endLogSeg);

      snprintf(buffer, sizeof(buffer),
                   "%s%u\t%s\t%s transaction %u at %s\n",
                   (srcfd < 0) ? "" : "\n",
                   parentTLI,
                   xlogfname,
                   recoveryStopAfter ? "after" : "before",
                   recoveryStopXid,
                   timestamptz_to_str(recoveryStopTime));

      nbytes = strlen(buffer);
      errno = 0;
      if ((int) write(fd, buffer, nbytes) != nbytes)
      {
            int               save_errno = errno;

            /*
             * If we fail to make the file, delete it to release disk space
             */
            unlink(tmppath);
            /* if write didn't set errno, assume problem is no disk space */
            errno = save_errno ? save_errno : ENOSPC;

            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not write to file \"%s\": %m", tmppath)));
      }

      if (pg_fsync(fd) != 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not fsync file \"%s\": %m", tmppath)));

      if (close(fd))
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not close file \"%s\": %m", tmppath)));


      /*
       * Now move the completed history file into place with its final name.
       */
      TLHistoryFilePath(path, newTLI);

      /*
       * Prefer link() to rename() here just to be really sure that we don't
       * overwrite an existing logfile.  However, there shouldn't be one, so
       * rename() is an acceptable substitute except for the truly paranoid.
       */
#if HAVE_WORKING_LINK
      if (link(tmppath, path) < 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not link file \"%s\" to \"%s\": %m",
                                    tmppath, path)));
      unlink(tmppath);
#else
      if (rename(tmppath, path) < 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not rename file \"%s\" to \"%s\": %m",
                                    tmppath, path)));
#endif

      /* The history file can be archived immediately. */
      TLHistoryFileName(histfname, newTLI);
      XLogArchiveNotify(histfname);
}

/*
 * I/O routines for pg_control
 *
 * *ControlFile is a buffer in shared memory that holds an image of the
 * contents of pg_control.    WriteControlFile() initializes pg_control
 * given a preloaded buffer, ReadControlFile() loads the buffer from
 * the pg_control file (during postmaster or standalone-backend startup),
 * and UpdateControlFile() rewrites pg_control after we modify xlog state.
 *
 * For simplicity, WriteControlFile() initializes the fields of pg_control
 * that are related to checking backend/database compatibility, and
 * ReadControlFile() verifies they are correct.  We could split out the
 * I/O and compatibility-check functions, but there seems no need currently.
 */
static void
WriteControlFile(void)
{
      int               fd;
      char        buffer[PG_CONTROL_SIZE];            /* need not be aligned */

      /*
       * Initialize version and compatibility-check fields
       */
      ControlFile->pg_control_version = PG_CONTROL_VERSION;
      ControlFile->catalog_version_no = CATALOG_VERSION_NO;

      ControlFile->maxAlign = MAXIMUM_ALIGNOF;
      ControlFile->floatFormat = FLOATFORMAT_VALUE;

      ControlFile->blcksz = BLCKSZ;
      ControlFile->relseg_size = RELSEG_SIZE;
      ControlFile->xlog_blcksz = XLOG_BLCKSZ;
      ControlFile->xlog_seg_size = XLOG_SEG_SIZE;

      ControlFile->nameDataLen = NAMEDATALEN;
      ControlFile->indexMaxKeys = INDEX_MAX_KEYS;

      ControlFile->toast_max_chunk_size = TOAST_MAX_CHUNK_SIZE;

#ifdef HAVE_INT64_TIMESTAMP
      ControlFile->enableIntTimes = true;
#else
      ControlFile->enableIntTimes = false;
#endif
      ControlFile->float4ByVal = FLOAT4PASSBYVAL;
      ControlFile->float8ByVal = FLOAT8PASSBYVAL;

      /* Contents are protected with a CRC */
      INIT_CRC32(ControlFile->crc);
      COMP_CRC32(ControlFile->crc,
                     (char *) ControlFile,
                     offsetof(ControlFileData, crc));
      FIN_CRC32(ControlFile->crc);

      /*
       * We write out PG_CONTROL_SIZE bytes into pg_control, zero-padding the
       * excess over sizeof(ControlFileData).  This reduces the odds of
       * premature-EOF errors when reading pg_control.  We'll still fail when we
       * check the contents of the file, but hopefully with a more specific
       * error than "couldn't read pg_control".
       */
      if (sizeof(ControlFileData) > PG_CONTROL_SIZE)
            elog(PANIC, "sizeof(ControlFileData) is larger than PG_CONTROL_SIZE; fix either one");

      memset(buffer, 0, PG_CONTROL_SIZE);
      memcpy(buffer, ControlFile, sizeof(ControlFileData));

      fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                 O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                                 S_IRUSR | S_IWUSR);
      if (fd < 0)
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not create control file \"%s\": %m",
                                    XLOG_CONTROL_FILE)));

      errno = 0;
      if (write(fd, buffer, PG_CONTROL_SIZE) != PG_CONTROL_SIZE)
      {
            /* if write didn't set errno, assume problem is no disk space */
            if (errno == 0)
                  errno = ENOSPC;
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not write to control file: %m")));
      }

      if (pg_fsync(fd) != 0)
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not fsync control file: %m")));

      if (close(fd))
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not close control file: %m")));
}

static void
ReadControlFile(void)
{
      pg_crc32    crc;
      int               fd;

      /*
       * Read data...
       */
      fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                 O_RDWR | PG_BINARY,
                                 S_IRUSR | S_IWUSR);
      if (fd < 0)
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not open control file \"%s\": %m",
                                    XLOG_CONTROL_FILE)));

      if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not read from control file: %m")));

      close(fd);

      /*
       * Check for expected pg_control format version.  If this is wrong, the
       * CRC check will likely fail because we'll be checking the wrong number
       * of bytes.  Complaining about wrong version will probably be more
       * enlightening than complaining about wrong CRC.
       */

      if (ControlFile->pg_control_version != PG_CONTROL_VERSION && ControlFile->pg_control_version % 65536 == 0 && ControlFile->pg_control_version / 65536 != 0)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d (0x%08x),"
             " but the server was compiled with PG_CONTROL_VERSION %d (0x%08x).",
                  ControlFile->pg_control_version, ControlFile->pg_control_version,
                                       PG_CONTROL_VERSION, PG_CONTROL_VERSION),
                         errhint("This could be a problem of mismatched byte ordering.  It looks like you need to initdb.")));

      if (ControlFile->pg_control_version != PG_CONTROL_VERSION)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d,"
                          " but the server was compiled with PG_CONTROL_VERSION %d.",
                                    ControlFile->pg_control_version, PG_CONTROL_VERSION),
                         errhint("It looks like you need to initdb.")));

      /* Now check the CRC. */
      INIT_CRC32(crc);
      COMP_CRC32(crc,
                     (char *) ControlFile,
                     offsetof(ControlFileData, crc));
      FIN_CRC32(crc);

      if (!EQ_CRC32(crc, ControlFile->crc))
            ereport(FATAL,
                        (errmsg("incorrect checksum in control file")));

      /*
       * Do compatibility checking immediately.  If the database isn't
       * compatible with the backend executable, we want to abort before we can
       * possibly do any damage.
       */
      if (ControlFile->catalog_version_no != CATALOG_VERSION_NO)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized with CATALOG_VERSION_NO %d,"
                          " but the server was compiled with CATALOG_VERSION_NO %d.",
                                    ControlFile->catalog_version_no, CATALOG_VERSION_NO),
                         errhint("It looks like you need to initdb.")));
      if (ControlFile->maxAlign != MAXIMUM_ALIGNOF)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
               errdetail("The database cluster was initialized with MAXALIGN %d,"
                               " but the server was compiled with MAXALIGN %d.",
                               ControlFile->maxAlign, MAXIMUM_ALIGNOF),
                         errhint("It looks like you need to initdb.")));
      if (ControlFile->floatFormat != FLOATFORMAT_VALUE)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster appears to use a different floating-point number format than the server executable."),
                         errhint("It looks like you need to initdb.")));
      if (ControlFile->blcksz != BLCKSZ)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                   errdetail("The database cluster was initialized with BLCKSZ %d,"
                                 " but the server was compiled with BLCKSZ %d.",
                                 ControlFile->blcksz, BLCKSZ),
                         errhint("It looks like you need to recompile or initdb.")));
      if (ControlFile->relseg_size != RELSEG_SIZE)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
            errdetail("The database cluster was initialized with RELSEG_SIZE %d,"
                          " but the server was compiled with RELSEG_SIZE %d.",
                          ControlFile->relseg_size, RELSEG_SIZE),
                         errhint("It looks like you need to recompile or initdb.")));
      if (ControlFile->xlog_blcksz != XLOG_BLCKSZ)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
            errdetail("The database cluster was initialized with XLOG_BLCKSZ %d,"
                          " but the server was compiled with XLOG_BLCKSZ %d.",
                          ControlFile->xlog_blcksz, XLOG_BLCKSZ),
                         errhint("It looks like you need to recompile or initdb.")));
      if (ControlFile->xlog_seg_size != XLOG_SEG_SIZE)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized with XLOG_SEG_SIZE %d,"
                                 " but the server was compiled with XLOG_SEG_SIZE %d.",
                                       ControlFile->xlog_seg_size, XLOG_SEG_SIZE),
                         errhint("It looks like you need to recompile or initdb.")));
      if (ControlFile->nameDataLen != NAMEDATALEN)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
            errdetail("The database cluster was initialized with NAMEDATALEN %d,"
                          " but the server was compiled with NAMEDATALEN %d.",
                          ControlFile->nameDataLen, NAMEDATALEN),
                         errhint("It looks like you need to recompile or initdb.")));
      if (ControlFile->indexMaxKeys != INDEX_MAX_KEYS)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized with INDEX_MAX_KEYS %d,"
                                " but the server was compiled with INDEX_MAX_KEYS %d.",
                                       ControlFile->indexMaxKeys, INDEX_MAX_KEYS),
                         errhint("It looks like you need to recompile or initdb.")));
      if (ControlFile->toast_max_chunk_size != TOAST_MAX_CHUNK_SIZE)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized with TOAST_MAX_CHUNK_SIZE %d,"
                        " but the server was compiled with TOAST_MAX_CHUNK_SIZE %d.",
                    ControlFile->toast_max_chunk_size, (int) TOAST_MAX_CHUNK_SIZE),
                         errhint("It looks like you need to recompile or initdb.")));

#ifdef HAVE_INT64_TIMESTAMP
      if (ControlFile->enableIntTimes != true)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized without HAVE_INT64_TIMESTAMP"
                          " but the server was compiled with HAVE_INT64_TIMESTAMP."),
                         errhint("It looks like you need to recompile or initdb.")));
#else
      if (ControlFile->enableIntTimes != false)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized with HAVE_INT64_TIMESTAMP"
                     " but the server was compiled without HAVE_INT64_TIMESTAMP."),
                         errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT4_BYVAL
      if (ControlFile->float4ByVal != true)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized without USE_FLOAT4_BYVAL"
                                " but the server was compiled with USE_FLOAT4_BYVAL."),
                         errhint("It looks like you need to recompile or initdb.")));
#else
      if (ControlFile->float4ByVal != false)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
            errdetail("The database cluster was initialized with USE_FLOAT4_BYVAL"
                          " but the server was compiled without USE_FLOAT4_BYVAL."),
                         errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT8_BYVAL
      if (ControlFile->float8ByVal != true)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
                         errdetail("The database cluster was initialized without USE_FLOAT8_BYVAL"
                                " but the server was compiled with USE_FLOAT8_BYVAL."),
                         errhint("It looks like you need to recompile or initdb.")));
#else
      if (ControlFile->float8ByVal != false)
            ereport(FATAL,
                        (errmsg("database files are incompatible with server"),
            errdetail("The database cluster was initialized with USE_FLOAT8_BYVAL"
                          " but the server was compiled without USE_FLOAT8_BYVAL."),
                         errhint("It looks like you need to recompile or initdb.")));
#endif
}

void
UpdateControlFile(void)
{
      int               fd;

      INIT_CRC32(ControlFile->crc);
      COMP_CRC32(ControlFile->crc,
                     (char *) ControlFile,
                     offsetof(ControlFileData, crc));
      FIN_CRC32(ControlFile->crc);

      fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                 O_RDWR | PG_BINARY,
                                 S_IRUSR | S_IWUSR);
      if (fd < 0)
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not open control file \"%s\": %m",
                                    XLOG_CONTROL_FILE)));

      errno = 0;
      if (write(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
      {
            /* if write didn't set errno, assume problem is no disk space */
            if (errno == 0)
                  errno = ENOSPC;
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not write to control file: %m")));
      }

      if (pg_fsync(fd) != 0)
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not fsync control file: %m")));

      if (close(fd))
            ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not close control file: %m")));
}

/*
 * Initialization of shared memory for XLOG
 */
Size
XLOGShmemSize(void)
{
      Size        size;

      /* XLogCtl */
      size = sizeof(XLogCtlData);
      /* xlblocks array */
      size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
      /* extra alignment padding for XLOG I/O buffers */
      size = add_size(size, ALIGNOF_XLOG_BUFFER);
      /* and the buffers themselves */
      size = add_size(size, mul_size(XLOG_BLCKSZ, XLOGbuffers));

      /*
       * Note: we don't count ControlFileData, it comes out of the "slop factor"
       * added by CreateSharedMemoryAndSemaphores.  This lets us use this
       * routine again below to compute the actual allocation size.
       */

      return size;
}

void
XLOGShmemInit(void)
{
      bool        foundCFile,
                        foundXLog;
      char     *allocptr;

      ControlFile = (ControlFileData *)
            ShmemInitStruct("Control File", sizeof(ControlFileData), &foundCFile);
      XLogCtl = (XLogCtlData *)
            ShmemInitStruct("XLOG Ctl", XLOGShmemSize(), &foundXLog);

      if (foundCFile || foundXLog)
      {
            /* both should be present or neither */
            Assert(foundCFile && foundXLog);
            return;
      }

      memset(XLogCtl, 0, sizeof(XLogCtlData));

      /*
       * Since XLogCtlData contains XLogRecPtr fields, its sizeof should be a
       * multiple of the alignment for same, so no extra alignment padding is
       * needed here.
       */
      allocptr = ((char *) XLogCtl) + sizeof(XLogCtlData);
      XLogCtl->xlblocks = (XLogRecPtr *) allocptr;
      memset(XLogCtl->xlblocks, 0, sizeof(XLogRecPtr) * XLOGbuffers);
      allocptr += sizeof(XLogRecPtr) * XLOGbuffers;

      /*
       * Align the start of the page buffers to an ALIGNOF_XLOG_BUFFER boundary.
       */
      allocptr = (char *) TYPEALIGN(ALIGNOF_XLOG_BUFFER, allocptr);
      XLogCtl->pages = allocptr;
      memset(XLogCtl->pages, 0, (Size) XLOG_BLCKSZ * XLOGbuffers);

      /*
       * Do basic initialization of XLogCtl shared data. (StartupXLOG will fill
       * in additional info.)
       */
      XLogCtl->XLogCacheBlck = XLOGbuffers - 1;
      XLogCtl->SharedRecoveryInProgress = true;
      XLogCtl->Insert.currpage = (XLogPageHeader) (XLogCtl->pages);
      SpinLockInit(&XLogCtl->info_lck);

      /*
       * If we are not in bootstrap mode, pg_control should already exist. Read
       * and validate it immediately (see comments in ReadControlFile() for the
       * reasons why).
       */
      if (!IsBootstrapProcessingMode())
            ReadControlFile();
}

/*
 * This func must be called ONCE on system install.  It creates pg_control
 * and the initial XLOG segment.
 */
void
BootStrapXLOG(void)
{
      CheckPoint  checkPoint;
      char     *buffer;
      XLogPageHeader page;
      XLogLongPageHeader longpage;
      XLogRecord *record;
      bool        use_existent;
      uint64            sysidentifier;
      struct timeval tv;
      pg_crc32    crc;

      /*
       * Select a hopefully-unique system identifier code for this installation.
       * We use the result of gettimeofday(), including the fractional seconds
       * field, as being about as unique as we can easily get.  (Think not to
       * use random(), since it hasn't been seeded and there's no portable way
       * to seed it other than the system clock value...)  The upper half of the
       * uint64 value is just the tv_sec part, while the lower half is the XOR
       * of tv_sec and tv_usec.  This is to ensure that we don't lose uniqueness
       * unnecessarily if "uint64" is really only 32 bits wide.  A person
       * knowing this encoding can determine the initialization time of the
       * installation, which could perhaps be useful sometimes.
       */
      gettimeofday(&tv, NULL);
      sysidentifier = ((uint64) tv.tv_sec) << 32;
      sysidentifier |= (uint32) (tv.tv_sec | tv.tv_usec);

      /* First timeline ID is always 1 */
      ThisTimeLineID = 1;

      /* page buffer must be aligned suitably for O_DIRECT */
      buffer = (char *) palloc(XLOG_BLCKSZ + ALIGNOF_XLOG_BUFFER);
      page = (XLogPageHeader) TYPEALIGN(ALIGNOF_XLOG_BUFFER, buffer);
      memset(page, 0, XLOG_BLCKSZ);

      /* Set up information for the initial checkpoint record */
      checkPoint.redo.xlogid = 0;
      checkPoint.redo.xrecoff = SizeOfXLogLongPHD;
      checkPoint.ThisTimeLineID = ThisTimeLineID;
      checkPoint.nextXidEpoch = 0;
      checkPoint.nextXid = FirstNormalTransactionId;
      checkPoint.nextOid = FirstBootstrapObjectId;
      checkPoint.nextMulti = FirstMultiXactId;
      checkPoint.nextMultiOffset = 0;
      checkPoint.time = (pg_time_t) time(NULL);

      ShmemVariableCache->nextXid = checkPoint.nextXid;
      ShmemVariableCache->nextOid = checkPoint.nextOid;
      ShmemVariableCache->oidCount = 0;
      MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);

      /* Set up the XLOG page header */
      page->xlp_magic = XLOG_PAGE_MAGIC;
      page->xlp_info = XLP_LONG_HEADER;
      page->xlp_tli = ThisTimeLineID;
      page->xlp_pageaddr.xlogid = 0;
      page->xlp_pageaddr.xrecoff = 0;
      longpage = (XLogLongPageHeader) page;
      longpage->xlp_sysid = sysidentifier;
      longpage->xlp_seg_size = XLogSegSize;
      longpage->xlp_xlog_blcksz = XLOG_BLCKSZ;

      /* Insert the initial checkpoint record */
      record = (XLogRecord *) ((char *) page + SizeOfXLogLongPHD);
      record->xl_prev.xlogid = 0;
      record->xl_prev.xrecoff = 0;
      record->xl_xid = InvalidTransactionId;
      record->xl_tot_len = SizeOfXLogRecord + sizeof(checkPoint);
      record->xl_len = sizeof(checkPoint);
      record->xl_info = XLOG_CHECKPOINT_SHUTDOWN;
      record->xl_rmid = RM_XLOG_ID;
      memcpy(XLogRecGetData(record), &checkPoint, sizeof(checkPoint));

      INIT_CRC32(crc);
      COMP_CRC32(crc, &checkPoint, sizeof(checkPoint));
      COMP_CRC32(crc, (char *) record + sizeof(pg_crc32),
                     SizeOfXLogRecord - sizeof(pg_crc32));
      FIN_CRC32(crc);
      record->xl_crc = crc;

      /* Create first XLOG segment file */
      use_existent = false;
      openLogFile = XLogFileInit(0, 0, &use_existent, false);

      /* Write the first page with the initial record */
      errno = 0;
      if (write(openLogFile, page, XLOG_BLCKSZ) != XLOG_BLCKSZ)
      {
            /* if write didn't set errno, assume problem is no disk space */
            if (errno == 0)
                  errno = ENOSPC;
            ereport(PANIC,
                        (errcode_for_file_access(),
                    errmsg("could not write bootstrap transaction log file: %m")));
      }

      if (pg_fsync(openLogFile) != 0)
            ereport(PANIC,
                        (errcode_for_file_access(),
                    errmsg("could not fsync bootstrap transaction log file: %m")));

      if (close(openLogFile))
            ereport(PANIC,
                        (errcode_for_file_access(),
                    errmsg("could not close bootstrap transaction log file: %m")));

      openLogFile = -1;

      /* Now create pg_control */

      memset(ControlFile, 0, sizeof(ControlFileData));
      /* Initialize pg_control status fields */
      ControlFile->system_identifier = sysidentifier;
      ControlFile->state = DB_SHUTDOWNED;
      ControlFile->time = checkPoint.time;
      ControlFile->checkPoint = checkPoint.redo;
      ControlFile->checkPointCopy = checkPoint;
      /* some additional ControlFile fields are set in WriteControlFile() */

      WriteControlFile();

      /* Bootstrap the commit log, too */
      BootStrapCLOG();
      BootStrapSUBTRANS();
      BootStrapMultiXact();

      pfree(buffer);
}

static char *
str_time(pg_time_t tnow)
{
      static char buf[128];

      pg_strftime(buf, sizeof(buf),
                        "%Y-%m-%d %H:%M:%S %Z",
                        pg_localtime(&tnow, log_timezone));

      return buf;
}

/*
 * See if there is a recovery command file (recovery.conf), and if so
 * read in parameters for archive recovery.
 *
 * XXX longer term intention is to expand this to
 * cater for additional parameters and controls
 * possibly use a flex lexer similar to the GUC one
 */
static void
readRecoveryCommandFile(void)
{
      FILE     *fd;
      char        cmdline[MAXPGPATH];
      TimeLineID  rtli = 0;
      bool        rtliGiven = false;
      bool        syntaxError = false;

      fd = AllocateFile(RECOVERY_COMMAND_FILE, "r");
      if (fd == NULL)
      {
            if (errno == ENOENT)
                  return;                       /* not there, so no archive recovery */
            ereport(FATAL,
                        (errcode_for_file_access(),
                         errmsg("could not open recovery command file \"%s\": %m",
                                    RECOVERY_COMMAND_FILE)));
      }

      ereport(LOG,
                  (errmsg("starting archive recovery")));

      /*
       * Parse the file...
       */
      while (fgets(cmdline, sizeof(cmdline), fd) != NULL)
      {
            /* skip leading whitespace and check for # comment */
            char     *ptr;
            char     *tok1;
            char     *tok2;

            for (ptr = cmdline; *ptr; ptr++)
            {
                  if (!isspace((unsigned char) *ptr))
                        break;
            }
            if (*ptr == '\0' || *ptr == '#')
                  continue;

            /* identify the quoted parameter value */
            tok1 = strtok(ptr, "'");
            if (!tok1)
            {
                  syntaxError = true;
                  break;
            }
            tok2 = strtok(NULL, "'");
            if (!tok2)
            {
                  syntaxError = true;
                  break;
            }
            /* reparse to get just the parameter name */
            tok1 = strtok(ptr, " \t=");
            if (!tok1)
            {
                  syntaxError = true;
                  break;
            }

            if (strcmp(tok1, "restore_command") == 0)
            {
                  recoveryRestoreCommand = pstrdup(tok2);
                  ereport(LOG,
                              (errmsg("restore_command = '%s'",
                                          recoveryRestoreCommand)));
            }
            else if (strcmp(tok1, "recovery_end_command") == 0)
            {
                  recoveryEndCommand = pstrdup(tok2);
                  ereport(LOG,
                              (errmsg("recovery_end_command = '%s'",
                                          recoveryEndCommand)));
            }
            else if (strcmp(tok1, "recovery_target_timeline") == 0)
            {
                  rtliGiven = true;
                  if (strcmp(tok2, "latest") == 0)
                        rtli = 0;
                  else
                  {
                        errno = 0;
                        rtli = (TimeLineID) strtoul(tok2, NULL, 0);
                        if (errno == EINVAL || errno == ERANGE)
                              ereport(FATAL,
                                          (errmsg("recovery_target_timeline is not a valid number: \"%s\"",
                                                      tok2)));
                  }
                  if (rtli)
                        ereport(LOG,
                                    (errmsg("recovery_target_timeline = %u", rtli)));
                  else
                        ereport(LOG,
                                    (errmsg("recovery_target_timeline = latest")));
            }
            else if (strcmp(tok1, "recovery_target_xid") == 0)
            {
                  errno = 0;
                  recoveryTargetXid = (TransactionId) strtoul(tok2, NULL, 0);
                  if (errno == EINVAL || errno == ERANGE)
                        ereport(FATAL,
                         (errmsg("recovery_target_xid is not a valid number: \"%s\"",
                                     tok2)));
                  ereport(LOG,
                              (errmsg("recovery_target_xid = %u",
                                          recoveryTargetXid)));
                  recoveryTarget = true;
                  recoveryTargetExact = true;
            }
            else if (strcmp(tok1, "recovery_target_time") == 0)
            {
                  /*
                   * if recovery_target_xid specified, then this overrides
                   * recovery_target_time
                   */
                  if (recoveryTargetExact)
                        continue;
                  recoveryTarget = true;
                  recoveryTargetExact = false;

                  /*
                   * Convert the time string given by the user to TimestampTz form.
                   */
                  recoveryTargetTime =
                        DatumGetTimestampTz(DirectFunctionCall3(timestamptz_in,
                                                                                    CStringGetDatum(tok2),
                                                                        ObjectIdGetDatum(InvalidOid),
                                                                                    Int32GetDatum(-1)));
                  ereport(LOG,
                              (errmsg("recovery_target_time = '%s'",
                                          timestamptz_to_str(recoveryTargetTime))));
            }
            else if (strcmp(tok1, "recovery_target_inclusive") == 0)
            {
                  /*
                   * does nothing if a recovery_target is not also set
                   */
                  if (!parse_bool(tok2, &recoveryTargetInclusive))
                        ereport(ERROR,
                                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                     errmsg("parameter \"recovery_target_inclusive\" requires a Boolean value")));
                  ereport(LOG,
                              (errmsg("recovery_target_inclusive = %s", tok2)));
            }
            else
                  ereport(FATAL,
                              (errmsg("unrecognized recovery parameter \"%s\"",
                                          tok1)));
      }

      FreeFile(fd);

      if (syntaxError)
            ereport(FATAL,
                        (errmsg("syntax error in recovery command file: %s",
                                    cmdline),
                    errhint("Lines should have the format parameter = 'value'.")));

      /* Check that required parameters were supplied */
      if (recoveryRestoreCommand == NULL)
            ereport(FATAL,
                        (errmsg("recovery command file \"%s\" did not specify restore_command",
                                    RECOVERY_COMMAND_FILE)));

      /* Enable fetching from archive recovery area */
      InArchiveRecovery = true;

      /*
       * If user specified recovery_target_timeline, validate it or compute the
       * "latest" value.      We can't do this until after we've gotten the restore
       * command and set InArchiveRecovery, because we need to fetch timeline
       * history files from the archive.
       */
      if (rtliGiven)
      {
            if (rtli)
            {
                  /* Timeline 1 does not have a history file, all else should */
                  if (rtli != 1 && !existsTimeLineHistory(rtli))
                        ereport(FATAL,
                                    (errmsg("recovery target timeline %u does not exist",
                                                rtli)));
                  recoveryTargetTLI = rtli;
            }
            else
            {
                  /* We start the "latest" search from pg_control's timeline */
                  recoveryTargetTLI = findNewestTimeLine(recoveryTargetTLI);
            }
      }
}

/*
 * Exit archive-recovery state
 */
static void
exitArchiveRecovery(TimeLineID endTLI, uint32 endLogId, uint32 endLogSeg)
{
      char        recoveryPath[MAXPGPATH];
      char        xlogpath[MAXPGPATH];
      XLogRecPtr  InvalidXLogRecPtr = {0, 0};

      /*
       * We are no longer in archive recovery state.
       */
      InArchiveRecovery = false;

      /*
       * Update min recovery point one last time.
       */
      UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);

      /*
       * We should have the ending log segment currently open.  Verify, and then
       * close it (to avoid problems on Windows with trying to rename or delete
       * an open file).
       */
      Assert(readFile >= 0);
      Assert(readId == endLogId);
      Assert(readSeg == endLogSeg);

      close(readFile);
      readFile = -1;

      /*
       * If the segment was fetched from archival storage, we want to replace
       * the existing xlog segment (if any) with the archival version.  This is
       * because whatever is in XLOGDIR is very possibly older than what we have
       * from the archives, since it could have come from restoring a PGDATA
       * backup.  In any case, the archival version certainly is more
       * descriptive of what our current database state is, because that is what
       * we replayed from.
       *
       * Note that if we are establishing a new timeline, ThisTimeLineID is
       * already set to the new value, and so we will create a new file instead
       * of overwriting any existing file.  (This is, in fact, always the case
       * at present.)
       */
      snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYXLOG");
      XLogFilePath(xlogpath, ThisTimeLineID, endLogId, endLogSeg);

      if (restoredFromArchive)
      {
            ereport(DEBUG3,
                        (errmsg_internal("moving last restored xlog to \"%s\"",
                                                 xlogpath)));
            unlink(xlogpath);       /* might or might not exist */
            if (rename(recoveryPath, xlogpath) != 0)
                  ereport(FATAL,
                              (errcode_for_file_access(),
                               errmsg("could not rename file \"%s\" to \"%s\": %m",
                                          recoveryPath, xlogpath)));
            /* XXX might we need to fix permissions on the file? */
      }
      else
      {
            /*
             * If the latest segment is not archival, but there's still a
             * RECOVERYXLOG laying about, get rid of it.
             */
            unlink(recoveryPath);   /* ignore any error */

            /*
             * If we are establishing a new timeline, we have to copy data from
             * the last WAL segment of the old timeline to create a starting WAL
             * segment for the new timeline.
             *
             * Notify the archiver that the last WAL segment of the old timeline
             * is ready to copy to archival storage. Otherwise, it is not archived
             * for a while.
             */
            if (endTLI != ThisTimeLineID)
            {
                  XLogFileCopy(endLogId, endLogSeg,
                                     endTLI, endLogId, endLogSeg);

                  if (XLogArchivingActive())
                  {
                        XLogFileName(xlogpath, endTLI, endLogId, endLogSeg);
                        XLogArchiveNotify(xlogpath);
                  }
            }
      }

      /*
       * Let's just make real sure there are not .ready or .done flags posted
       * for the new segment.
       */
      XLogFileName(xlogpath, ThisTimeLineID, endLogId, endLogSeg);
      XLogArchiveCleanup(xlogpath);

      /* Get rid of any remaining recovered timeline-history file, too */
      snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYHISTORY");
      unlink(recoveryPath);         /* ignore any error */

      /*
       * Rename the config file out of the way, so that we don't accidentally
       * re-enter archive recovery mode in a subsequent crash.
       */
      unlink(RECOVERY_COMMAND_DONE);
      if (rename(RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE) != 0)
            ereport(FATAL,
                        (errcode_for_file_access(),
                         errmsg("could not rename file \"%s\" to \"%s\": %m",
                                    RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE)));

      ereport(LOG,
                  (errmsg("archive recovery complete")));
}

/*
 * For point-in-time recovery, this function decides whether we want to
 * stop applying the XLOG at or after the current record.
 *
 * Returns TRUE if we are stopping, FALSE otherwise.  On TRUE return,
 * *includeThis is set TRUE if we should apply this record before stopping.
 *
 * We also track the timestamp of the latest applied COMMIT/ABORT record
 * in recoveryLastXTime, for logging purposes.
 * Also, some information is saved in recoveryStopXid et al for use in
 * annotating the new timeline's history file.
 */
static bool
recoveryStopsHere(XLogRecord *record, bool *includeThis)
{
      bool        stopsHere;
      uint8       record_info;
      TimestampTz recordXtime;

      /* We only consider stopping at COMMIT or ABORT records */
      if (record->xl_rmid != RM_XACT_ID)
            return false;
      record_info = record->xl_info & ~XLR_INFO_MASK;
      if (record_info == XLOG_XACT_COMMIT)
      {
            xl_xact_commit *recordXactCommitData;

            recordXactCommitData = (xl_xact_commit *) XLogRecGetData(record);
            recordXtime = recordXactCommitData->xact_time;
      }
      else if (record_info == XLOG_XACT_ABORT)
      {
            xl_xact_abort *recordXactAbortData;

            recordXactAbortData = (xl_xact_abort *) XLogRecGetData(record);
            recordXtime = recordXactAbortData->xact_time;
      }
      else
            return false;

      /* Do we have a PITR target at all? */
      if (!recoveryTarget)
      {
            recoveryLastXTime = recordXtime;
            return false;
      }

      if (recoveryTargetExact)
      {
            /*
             * there can be only one transaction end record with this exact
             * transactionid
             *
             * when testing for an xid, we MUST test for equality only, since
             * transactions are numbered in the order they start, not the order
             * they complete. A higher numbered xid will complete before you about
             * 50% of the time...
             */
            stopsHere = (record->xl_xid == recoveryTargetXid);
            if (stopsHere)
                  *includeThis = recoveryTargetInclusive;
      }
      else
      {
            /*
             * there can be many transactions that share the same commit time, so
             * we stop after the last one, if we are inclusive, or stop at the
             * first one if we are exclusive
             */
            if (recoveryTargetInclusive)
                  stopsHere = (recordXtime > recoveryTargetTime);
            else
                  stopsHere = (recordXtime >= recoveryTargetTime);
            if (stopsHere)
                  *includeThis = false;
      }

      if (stopsHere)
      {
            recoveryStopXid = record->xl_xid;
            recoveryStopTime = recordXtime;
            recoveryStopAfter = *includeThis;

            if (record_info == XLOG_XACT_COMMIT)
            {
                  if (recoveryStopAfter)
                        ereport(LOG,
                                    (errmsg("recovery stopping after commit of transaction %u, time %s",
                                                recoveryStopXid,
                                                timestamptz_to_str(recoveryStopTime))));
                  else
                        ereport(LOG,
                                    (errmsg("recovery stopping before commit of transaction %u, time %s",
                                                recoveryStopXid,
                                                timestamptz_to_str(recoveryStopTime))));
            }
            else
            {
                  if (recoveryStopAfter)
                        ereport(LOG,
                                    (errmsg("recovery stopping after abort of transaction %u, time %s",
                                                recoveryStopXid,
                                                timestamptz_to_str(recoveryStopTime))));
                  else
                        ereport(LOG,
                                    (errmsg("recovery stopping before abort of transaction %u, time %s",
                                                recoveryStopXid,
                                                timestamptz_to_str(recoveryStopTime))));
            }

            if (recoveryStopAfter)
                  recoveryLastXTime = recordXtime;
      }
      else
            recoveryLastXTime = recordXtime;

      return stopsHere;
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
      XLogCtlInsert *Insert;
      CheckPoint  checkPoint;
      bool        wasShutdown;
      bool        reachedStopPoint = false;
      bool        haveBackupLabel = false;
      XLogRecPtr  RecPtr,
                        LastRec,
                        checkPointLoc,
                        backupStopLoc,
                        EndOfLog;
      uint32            endLogId;
      uint32            endLogSeg;
      XLogRecord *record;
      uint32            freespace;
      TransactionId oldestActiveXID;
      bool        bgwriterLaunched = false;

      /*
       * Read control file and check XLOG status looks valid.
       *
       * Note: in most control paths, *ControlFile is already valid and we need
       * not do ReadControlFile() here, but might as well do it to be sure.
       */
      ReadControlFile();

      if (ControlFile->state < DB_SHUTDOWNED ||
            ControlFile->state > DB_IN_PRODUCTION ||
            !XRecOffIsValid(ControlFile->checkPoint.xrecoff))
            ereport(FATAL,
                        (errmsg("control file contains invalid data")));

      if (ControlFile->state == DB_SHUTDOWNED)
            ereport(LOG,
                        (errmsg("database system was shut down at %s",
                                    str_time(ControlFile->time))));
      else if (ControlFile->state == DB_SHUTDOWNING)
            ereport(LOG,
                        (errmsg("database system shutdown was interrupted; last known up at %s",
                                    str_time(ControlFile->time))));
      else if (ControlFile->state == DB_IN_CRASH_RECOVERY)
            ereport(LOG,
               (errmsg("database system was interrupted while in recovery at %s",
                           str_time(ControlFile->time)),
                  errhint("This probably means that some data is corrupted and"
                              " you will have to use the last backup for recovery.")));
      else if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY)
            ereport(LOG,
                        (errmsg("database system was interrupted while in recovery at log time %s",
                                    str_time(ControlFile->checkPointCopy.time)),
                         errhint("If this has occurred more than once some data might be corrupted"
                    " and you might need to choose an earlier recovery target.")));
      else if (ControlFile->state == DB_IN_PRODUCTION)
            ereport(LOG,
                    (errmsg("database system was interrupted; last known up at %s",
                                str_time(ControlFile->time))));

      /* This is just to allow attaching to startup process with a debugger */
#ifdef XLOG_REPLAY_DELAY
      if (ControlFile->state != DB_SHUTDOWNED)
            pg_usleep(60000000L);
#endif

      /*
       * Verify that pg_xlog and pg_xlog/archive_status exist.  In cases where
       * someone has performed a copy for PITR, these directories may have been
       * excluded and need to be re-created.
       */
      ValidateXLOGDirectoryStructure();

      /*
       * Initialize on the assumption we want to recover to the same timeline
       * that's active according to pg_control.
       */
      recoveryTargetTLI = ControlFile->checkPointCopy.ThisTimeLineID;

      /*
       * Check for recovery control file, and if so set up state for offline
       * recovery
       */
      readRecoveryCommandFile();

      /* Now we can determine the list of expected TLIs */
      expectedTLIs = readTimeLineHistory(recoveryTargetTLI);

      /*
       * If pg_control's timeline is not in expectedTLIs, then we cannot
       * proceed: the backup is not part of the history of the requested
       * timeline.
       */
      if (!list_member_int(expectedTLIs,
                                     (int) ControlFile->checkPointCopy.ThisTimeLineID))
            ereport(FATAL,
                        (errmsg("requested timeline %u is not a child of database system timeline %u",
                                    recoveryTargetTLI,
                                    ControlFile->checkPointCopy.ThisTimeLineID)));

      if (read_backup_label(&checkPointLoc, &backupStopLoc))
      {
            /*
             * When a backup_label file is present, we want to roll forward from
             * the checkpoint it identifies, rather than using pg_control.
             */
            record = ReadCheckpointRecord(checkPointLoc, 0);
            if (record != NULL)
            {
                  ereport(DEBUG1,
                              (errmsg("checkpoint record is at %X/%X",
                                          checkPointLoc.xlogid, checkPointLoc.xrecoff)));
                  InRecovery = true;      /* force recovery even if SHUTDOWNED */
            }
            else
            {
                  ereport(PANIC,
                              (errmsg("could not locate required checkpoint record"),
                               errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
            }
            /* set flag to delete it later */
            haveBackupLabel = true;
      }
      else
      {
            /*
             * Get the last valid checkpoint record.  If the latest one according
             * to pg_control is broken, try the next-to-last one.
             */
            checkPointLoc = ControlFile->checkPoint;
            record = ReadCheckpointRecord(checkPointLoc, 1);
            if (record != NULL)
            {
                  ereport(DEBUG1,
                              (errmsg("checkpoint record is at %X/%X",
                                          checkPointLoc.xlogid, checkPointLoc.xrecoff)));
            }
            else
            {
                  checkPointLoc = ControlFile->prevCheckPoint;
                  record = ReadCheckpointRecord(checkPointLoc, 2);
                  if (record != NULL)
                  {
                        ereport(LOG,
                                    (errmsg("using previous checkpoint record at %X/%X",
                                            checkPointLoc.xlogid, checkPointLoc.xrecoff)));
                        InRecovery = true;            /* force recovery even if SHUTDOWNED */
                  }
                  else
                        ereport(PANIC,
                               (errmsg("could not locate a valid checkpoint record")));
            }
      }

      LastRec = RecPtr = checkPointLoc;
      memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
      wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);

      ereport(DEBUG1,
                  (errmsg("redo record is at %X/%X; shutdown %s",
                              checkPoint.redo.xlogid, checkPoint.redo.xrecoff,
                              wasShutdown ? "TRUE" : "FALSE")));
      ereport(DEBUG1,
                  (errmsg("next transaction ID: %u/%u; next OID: %u",
                              checkPoint.nextXidEpoch, checkPoint.nextXid,
                              checkPoint.nextOid)));
      ereport(DEBUG1,
                  (errmsg("next MultiXactId: %u; next MultiXactOffset: %u",
                              checkPoint.nextMulti, checkPoint.nextMultiOffset)));
      if (!TransactionIdIsNormal(checkPoint.nextXid))
            ereport(PANIC,
                        (errmsg("invalid next transaction ID")));

      ShmemVariableCache->nextXid = checkPoint.nextXid;
      ShmemVariableCache->nextOid = checkPoint.nextOid;
      ShmemVariableCache->oidCount = 0;
      MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);

      /*
       * We must replay WAL entries using the same TimeLineID they were created
       * under, so temporarily adopt the TLI indicated by the checkpoint (see
       * also xlog_redo()).
       */
      ThisTimeLineID = checkPoint.ThisTimeLineID;

      RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;

      if (XLByteLT(RecPtr, checkPoint.redo))
            ereport(PANIC,
                        (errmsg("invalid redo in checkpoint record")));

      /*
       * Check whether we need to force recovery from WAL.  If it appears to
       * have been a clean shutdown and we did not have a recovery.conf file,
       * then assume no recovery needed.
       */
      if (XLByteLT(checkPoint.redo, RecPtr))
      {
            if (wasShutdown)
                  ereport(PANIC,
                              (errmsg("invalid redo record in shutdown checkpoint")));
            InRecovery = true;
      }
      else if (ControlFile->state != DB_SHUTDOWNED)
            InRecovery = true;
      else if (InArchiveRecovery)
      {
            /* force recovery due to presence of recovery.conf */
            InRecovery = true;
      }

      /* REDO */
      if (InRecovery)
      {
            int               rmid;

            /*
             * Update pg_control to show that we are recovering and to show the
             * selected checkpoint as the place we are starting from. We also mark
             * pg_control with any minimum recovery stop point obtained from a
             * backup history file.
             */
            if (InArchiveRecovery)
            {
                  ereport(LOG,
                              (errmsg("automatic recovery in progress")));
                  ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
            }
            else
            {
                  ereport(LOG,
                              (errmsg("database system was not properly shut down; "
                                          "automatic recovery in progress")));
                  ControlFile->state = DB_IN_CRASH_RECOVERY;
            }
            ControlFile->prevCheckPoint = ControlFile->checkPoint;
            ControlFile->checkPoint = checkPointLoc;
            ControlFile->checkPointCopy = checkPoint;
            if (backupStopLoc.xlogid != 0 || backupStopLoc.xrecoff != 0)
            {
                  if (XLByteLT(ControlFile->minRecoveryPoint, backupStopLoc))
                        ControlFile->minRecoveryPoint = backupStopLoc;
            }
            ControlFile->time = (pg_time_t) time(NULL);
            /* No need to hold ControlFileLock yet, we aren't up far enough */
            UpdateControlFile();

            /* initialize our local copy of minRecoveryPoint */
            minRecoveryPoint = ControlFile->minRecoveryPoint;

            /*
             * Reset pgstat data, because it may be invalid after recovery.
             */
            pgstat_reset_all();

            /*
             * If there was a backup label file, it's done its job and the info
             * has now been propagated into pg_control.  We must get rid of the
             * label file so that if we crash during recovery, we'll pick up at
             * the latest recovery restartpoint instead of going all the way back
             * to the backup start point.  It seems prudent though to just rename
             * the file out of the way rather than delete it completely.
             */
            if (haveBackupLabel)
            {
                  unlink(BACKUP_LABEL_OLD);
                  if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) != 0)
                        ereport(FATAL,
                                    (errcode_for_file_access(),
                                     errmsg("could not rename file \"%s\" to \"%s\": %m",
                                                BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
            }

            /* Initialize resource managers */
            for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
            {
                  if (RmgrTable[rmid].rm_startup != NULL)
                        RmgrTable[rmid].rm_startup();
            }

            /*
             * Find the first record that logically follows the checkpoint --- it
             * might physically precede it, though.
             */
            if (XLByteLT(checkPoint.redo, RecPtr))
            {
                  /* back up to find the record */
                  record = ReadRecord(&(checkPoint.redo), PANIC);
            }
            else
            {
                  /* just have to read next record after CheckPoint */
                  record = ReadRecord(NULL, LOG);
            }

            if (record != NULL)
            {
                  bool        recoveryContinue = true;
                  bool        recoveryApply = true;
                  bool        reachedMinRecoveryPoint = false;
                  ErrorContextCallback errcontext;

                  /* use volatile pointer to prevent code rearrangement */
                  volatile XLogCtlData *xlogctl = XLogCtl;

                  /* initialize shared replayEndRecPtr */
                  SpinLockAcquire(&xlogctl->info_lck);
                  xlogctl->replayEndRecPtr = ReadRecPtr;
                  SpinLockRelease(&xlogctl->info_lck);

                  InRedo = true;

                  if (minRecoveryPoint.xlogid == 0 && minRecoveryPoint.xrecoff == 0)
                        ereport(LOG,
                                    (errmsg("redo starts at %X/%X",
                                                ReadRecPtr.xlogid, ReadRecPtr.xrecoff)));
                  else
                        ereport(LOG,
                                    (errmsg("redo starts at %X/%X, consistency will be reached at %X/%X",
                                                ReadRecPtr.xlogid, ReadRecPtr.xrecoff,
                                    minRecoveryPoint.xlogid, minRecoveryPoint.xrecoff)));

                  /*
                   * Let postmaster know we've started redo now, so that it can
                   * launch bgwriter to perform restartpoints.  We don't bother
                   * during crash recovery as restartpoints can only be performed
                   * during archive recovery.  And we'd like to keep crash recovery
                   * simple, to avoid introducing bugs that could you from
                   * recovering after crash.
                   *
                   * After this point, we can no longer assume that we're the only
                   * process in addition to postmaster!  Also, fsync requests are
                   * subsequently to be handled by the bgwriter, not locally.
                   */
                  if (InArchiveRecovery && IsUnderPostmaster)
                  {
                        SetForwardFsyncRequests();
                        SendPostmasterSignal(PMSIGNAL_RECOVERY_STARTED);
                        bgwriterLaunched = true;
                  }

                  /*
                   * main redo apply loop
                   */
                  do
                  {
#ifdef WAL_DEBUG
                        if (XLOG_DEBUG)
                        {
                              StringInfoData buf;

                              initStringInfo(&buf);
                              appendStringInfo(&buf, "REDO @ %X/%X; LSN %X/%X: ",
                                                       ReadRecPtr.xlogid, ReadRecPtr.xrecoff,
                                                       EndRecPtr.xlogid, EndRecPtr.xrecoff);
                              xlog_outrec(&buf, record);
                              appendStringInfo(&buf, " - ");
                              RmgrTable[record->xl_rmid].rm_desc(&buf,
                                                                                 record->xl_info,
                                                                               XLogRecGetData(record));
                              elog(LOG, "%s", buf.data);
                              pfree(buf.data);
                        }
#endif

                        /*
                         * Check if we were requested to re-read config file.
                         */
                        if (got_SIGHUP)
                        {
                              got_SIGHUP = false;
                              ProcessConfigFile(PGC_SIGHUP);
                        }

                        /*
                         * Check if we were requested to exit without finishing
                         * recovery.
                         */
                        if (shutdown_requested)
                              proc_exit(1);

                        /*
                         * Have we passed our safe starting point? If so, we can tell
                         * postmaster that the database is consistent now.
                         */
                        if (!reachedMinRecoveryPoint &&
                              XLByteLT(minRecoveryPoint, EndRecPtr))
                        {
                              reachedMinRecoveryPoint = true;
                              if (InArchiveRecovery)
                              {
                                    ereport(LOG,
                                            (errmsg("consistent recovery state reached")));
                                    if (IsUnderPostmaster)
                                          SendPostmasterSignal(PMSIGNAL_RECOVERY_CONSISTENT);
                              }
                        }

                        /*
                         * Have we reached our recovery target?
                         */
                        if (recoveryStopsHere(record, &recoveryApply))
                        {
                              reachedStopPoint = true;      /* see below */
                              recoveryContinue = false;
                              if (!recoveryApply)
                                    break;
                        }

                        /* Setup error traceback support for ereport() */
                        errcontext.callback = rm_redo_error_callback;
                        errcontext.arg = (void *) record;
                        errcontext.previous = error_context_stack;
                        error_context_stack = &errcontext;

                        /* nextXid must be beyond record's xid */
                        if (TransactionIdFollowsOrEquals(record->xl_xid,
                                                                         ShmemVariableCache->nextXid))
                        {
                              ShmemVariableCache->nextXid = record->xl_xid;
                              TransactionIdAdvance(ShmemVariableCache->nextXid);
                        }

                        /*
                         * Update shared replayEndRecPtr before replaying this record,
                         * so that XLogFlush will update minRecoveryPoint correctly.
                         */
                        SpinLockAcquire(&xlogctl->info_lck);
                        xlogctl->replayEndRecPtr = EndRecPtr;
                        SpinLockRelease(&xlogctl->info_lck);

                        RmgrTable[record->xl_rmid].rm_redo(EndRecPtr, record);

                        /* Pop the error context stack */
                        error_context_stack = errcontext.previous;

                        LastRec = ReadRecPtr;

                        record = ReadRecord(NULL, LOG);
                  } while (record != NULL && recoveryContinue);

                  /*
                   * end of main redo apply loop
                   */

                  ereport(LOG,
                              (errmsg("redo done at %X/%X",
                                          ReadRecPtr.xlogid, ReadRecPtr.xrecoff)));
                  if (recoveryLastXTime)
                        ereport(LOG,
                               (errmsg("last completed transaction was at log time %s",
                                           timestamptz_to_str(recoveryLastXTime))));
                  InRedo = false;
            }
            else
            {
                  /* there are no WAL records following the checkpoint */
                  ereport(LOG,
                              (errmsg("redo is not required")));
            }
      }

      /*
       * Re-fetch the last valid or last applied record, so we can identify the
       * exact endpoint of what we consider the valid portion of WAL.
       */
      record = ReadRecord(&LastRec, PANIC);
      EndOfLog = EndRecPtr;
      XLByteToPrevSeg(EndOfLog, endLogId, endLogSeg);

      /*
       * Complain if we did not roll forward far enough to render the backup
       * dump consistent.  Note: it is indeed okay to look at the local variable
       * minRecoveryPoint here, even though ControlFile->minRecoveryPoint might
       * be further ahead --- ControlFile->minRecoveryPoint cannot have been
       * advanced beyond the WAL we processed.
       */
      if (InRecovery && XLByteLT(EndOfLog, minRecoveryPoint))
      {
            if (reachedStopPoint)   /* stopped because of stop request */
                  ereport(FATAL,
                              (errmsg("requested recovery stop point is before consistent recovery point")));
            else  /* ran off end of WAL */
                  ereport(FATAL,
                              (errmsg("WAL ends before consistent recovery point")));
      }

      /*
       * Consider whether we need to assign a new timeline ID.
       *
       * If we are doing an archive recovery, we always assign a new ID.      This
       * handles a couple of issues.      If we stopped short of the end of WAL
       * during recovery, then we are clearly generating a new timeline and must
       * assign it a unique new ID.  Even if we ran to the end, modifying the
       * current last segment is problematic because it may result in trying to
       * overwrite an already-archived copy of that segment, and we encourage
       * DBAs to make their archive_commands reject that.  We can dodge the
       * problem by making the new active segment have a new timeline ID.
       *
       * In a normal crash recovery, we can just extend the timeline we were in.
       */
      if (InArchiveRecovery)
      {
            ThisTimeLineID = findNewestTimeLine(recoveryTargetTLI) + 1;
            ereport(LOG,
                        (errmsg("selected new timeline ID: %u", ThisTimeLineID)));
            writeTimeLineHistory(ThisTimeLineID, recoveryTargetTLI,
                                           curFileTLI, endLogId, endLogSeg);
      }

      /* Save the selected TimeLineID in shared memory, too */
      XLogCtl->ThisTimeLineID = ThisTimeLineID;

      /*
       * We are now done reading the old WAL.  Turn off archive fetching if it
       * was active, and make a writable copy of the last WAL segment. (Note
       * that we also have a copy of the last block of the old WAL in readBuf;
       * we will use that below.)
       */
      if (InArchiveRecovery)
            exitArchiveRecovery(curFileTLI, endLogId, endLogSeg);

      /*
       * Prepare to write WAL starting at EndOfLog position, and init xlog
       * buffer cache using the block containing the last record from the
       * previous incarnation.
       */
      openLogId = endLogId;
      openLogSeg = endLogSeg;
      openLogFile = XLogFileOpen(openLogId, openLogSeg);
      openLogOff = 0;
      Insert = &XLogCtl->Insert;
      Insert->PrevRecord = LastRec;
      XLogCtl->xlblocks[0].xlogid = openLogId;
      XLogCtl->xlblocks[0].xrecoff =
            ((EndOfLog.xrecoff - 1) / XLOG_BLCKSZ + 1) * XLOG_BLCKSZ;

      /*
       * Tricky point here: readBuf contains the *last* block that the LastRec
       * record spans, not the one it starts in.      The last block is indeed the
       * one we want to use.
       */
      Assert(readOff == (XLogCtl->xlblocks[0].xrecoff - XLOG_BLCKSZ) % XLogSegSize);
      memcpy((char *) Insert->currpage, readBuf, XLOG_BLCKSZ);
      Insert->currpos = (char *) Insert->currpage +
            (EndOfLog.xrecoff + XLOG_BLCKSZ - XLogCtl->xlblocks[0].xrecoff);

      LogwrtResult.Write = LogwrtResult.Flush = EndOfLog;

      XLogCtl->Write.LogwrtResult = LogwrtResult;
      Insert->LogwrtResult = LogwrtResult;
      XLogCtl->LogwrtResult = LogwrtResult;

      XLogCtl->LogwrtRqst.Write = EndOfLog;
      XLogCtl->LogwrtRqst.Flush = EndOfLog;

      freespace = INSERT_FREESPACE(Insert);
      if (freespace > 0)
      {
            /* Make sure rest of page is zero */
            MemSet(Insert->currpos, 0, freespace);
            XLogCtl->Write.curridx = 0;
      }
      else
      {
            /*
             * Whenever Write.LogwrtResult points to exactly the end of a page,
             * Write.curridx must point to the *next* page (see XLogWrite()).
             *
             * Note: it might seem we should do AdvanceXLInsertBuffer() here, but
             * this is sufficient.  The first actual attempt to insert a log
             * record will advance the insert state.
             */
            XLogCtl->Write.curridx = NextBufIdx(0);
      }

      /* Pre-scan prepared transactions to find out the range of XIDs present */
      oldestActiveXID = PrescanPreparedTransactions();

      if (InRecovery)
      {
            int               rmid;

            /*
             * Allow resource managers to do any required cleanup.
             */
            for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
            {
                  if (RmgrTable[rmid].rm_cleanup != NULL)
                        RmgrTable[rmid].rm_cleanup();
            }

            /*
             * Check to see if the XLOG sequence contained any unresolved
             * references to uninitialized pages.
             */
            XLogCheckInvalidPages();

            /*
             * Perform a checkpoint to update all our recovery activity to disk.
             *
             * Note that we write a shutdown checkpoint rather than an on-line
             * one. This is not particularly critical, but since we may be
             * assigning a new TLI, using a shutdown checkpoint allows us to have
             * the rule that TLI only changes in shutdown checkpoints, which
             * allows some extra error checking in xlog_redo.
             */
            if (bgwriterLaunched)
                  RequestCheckpoint(CHECKPOINT_END_OF_RECOVERY |
                                            CHECKPOINT_IMMEDIATE |
                                            CHECKPOINT_WAIT);
            else
                  CreateCheckPoint(CHECKPOINT_END_OF_RECOVERY | CHECKPOINT_IMMEDIATE);

            /*
             * And finally, execute the recovery_end_command, if any.
             */
            if (recoveryEndCommand)
                  ExecuteRecoveryEndCommand();
      }

      /*
       * Preallocate additional log files, if wanted.
       */
      PreallocXlogFiles(EndOfLog);

      /*
       * Okay, we're officially UP.
       */
      InRecovery = false;

      LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
      ControlFile->state = DB_IN_PRODUCTION;
      ControlFile->time = (pg_time_t) time(NULL);
      UpdateControlFile();
      LWLockRelease(ControlFileLock);

      /* start the archive_timeout timer running */
      XLogCtl->Write.lastSegSwitchTime = (pg_time_t) time(NULL);

      /* initialize shared-memory copy of latest checkpoint XID/epoch */
      XLogCtl->ckptXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
      XLogCtl->ckptXid = ControlFile->checkPointCopy.nextXid;

      /* also initialize latestCompletedXid, to nextXid - 1 */
      ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
      TransactionIdRetreat(ShmemVariableCache->latestCompletedXid);

      /* Start up the commit log and related stuff, too */
      StartupCLOG();
      StartupSUBTRANS(oldestActiveXID);
      StartupMultiXact();

      /* Reload shared-memory state for prepared transactions */
      RecoverPreparedTransactions();

      /* Shut down readFile facility, free space */
      if (readFile >= 0)
      {
            close(readFile);
            readFile = -1;
      }
      if (readBuf)
      {
            free(readBuf);
            readBuf = NULL;
      }
      if (readRecordBuf)
      {
            free(readRecordBuf);
            readRecordBuf = NULL;
            readRecordBufSize = 0;
      }

      /*
       * All done.  Allow backends to write WAL.  (Although the bool flag is
       * probably atomic in itself, we use the info_lck here to ensure that
       * there are no race conditions concerning visibility of other recent
       * updates to shared memory.)
       */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            xlogctl->SharedRecoveryInProgress = false;
            SpinLockRelease(&xlogctl->info_lck);
      }
}

/*
 * Is the system still in recovery?
 *
 * Unlike testing InRecovery, this works in any process that's connected to
 * shared memory.
 *
 * As a side-effect, we initialize the local TimeLineID and RedoRecPtr
 * variables the first time we see that recovery is finished.
 */
bool
RecoveryInProgress(void)
{
      /*
       * We check shared state each time only until we leave recovery mode.
       * We can't re-enter recovery, so there's no need to keep checking after
       * the shared variable has once been seen false.
       */
      if (!LocalRecoveryInProgress)
            return false;
      else
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            /* spinlock is essential on machines with weak memory ordering! */
            SpinLockAcquire(&xlogctl->info_lck);
            LocalRecoveryInProgress = xlogctl->SharedRecoveryInProgress;
            SpinLockRelease(&xlogctl->info_lck);

            /*
             * Initialize TimeLineID and RedoRecPtr when we discover that recovery
             * is finished.  (If you change this, see also
             * LocalSetXLogInsertAllowed.)
             */
            if (!LocalRecoveryInProgress)
                  InitXLOGAccess();

            return LocalRecoveryInProgress;
      }
}

/*
 * Is this process allowed to insert new WAL records?
 *
 * Ordinarily this is essentially equivalent to !RecoveryInProgress().
 * But we also have provisions for forcing the result "true" or "false"
 * within specific processes regardless of the global state.
 */
bool
XLogInsertAllowed(void)
{
      /*
       * If value is "unconditionally true" or "unconditionally false",
       * just return it.  This provides the normal fast path once recovery
       * is known done.
       */
      if (LocalXLogInsertAllowed >= 0)
            return (bool) LocalXLogInsertAllowed;

      /*
       * Else, must check to see if we're still in recovery.
       */
      if (RecoveryInProgress())
            return false;

      /*
       * On exit from recovery, reset to "unconditionally true", since there
       * is no need to keep checking.
       */
      LocalXLogInsertAllowed = 1;
      return true;
}

/*
 * Make XLogInsertAllowed() return true in the current process only.
 */
static void
LocalSetXLogInsertAllowed(void)
{
      Assert(LocalXLogInsertAllowed == -1);
      LocalXLogInsertAllowed = 1;

      /* Initialize as RecoveryInProgress() would do when switching state */
      InitXLOGAccess();
}

/*
 * Subroutine to try to fetch and validate a prior checkpoint record.
 *
 * whichChkpt identifies the checkpoint (merely for reporting purposes).
 * 1 for "primary", 2 for "secondary", 0 for "other" (backup_label)
 */
static XLogRecord *
ReadCheckpointRecord(XLogRecPtr RecPtr, int whichChkpt)
{
      XLogRecord *record;

      if (!XRecOffIsValid(RecPtr.xrecoff))
      {
            switch (whichChkpt)
            {
                  case 1:
                        ereport(LOG,
                        (errmsg("invalid primary checkpoint link in control file")));
                        break;
                  case 2:
                        ereport(LOG,
                                    (errmsg("invalid secondary checkpoint link in control file")));
                        break;
                  default:
                        ereport(LOG,
                           (errmsg("invalid checkpoint link in backup_label file")));
                        break;
            }
            return NULL;
      }

      record = ReadRecord(&RecPtr, LOG);

      if (record == NULL)
      {
            switch (whichChkpt)
            {
                  case 1:
                        ereport(LOG,
                                    (errmsg("invalid primary checkpoint record")));
                        break;
                  case 2:
                        ereport(LOG,
                                    (errmsg("invalid secondary checkpoint record")));
                        break;
                  default:
                        ereport(LOG,
                                    (errmsg("invalid checkpoint record")));
                        break;
            }
            return NULL;
      }
      if (record->xl_rmid != RM_XLOG_ID)
      {
            switch (whichChkpt)
            {
                  case 1:
                        ereport(LOG,
                                    (errmsg("invalid resource manager ID in primary checkpoint record")));
                        break;
                  case 2:
                        ereport(LOG,
                                    (errmsg("invalid resource manager ID in secondary checkpoint record")));
                        break;
                  default:
                        ereport(LOG,
                        (errmsg("invalid resource manager ID in checkpoint record")));
                        break;
            }
            return NULL;
      }
      if (record->xl_info != XLOG_CHECKPOINT_SHUTDOWN &&
            record->xl_info != XLOG_CHECKPOINT_ONLINE)
      {
            switch (whichChkpt)
            {
                  case 1:
                        ereport(LOG,
                           (errmsg("invalid xl_info in primary checkpoint record")));
                        break;
                  case 2:
                        ereport(LOG,
                         (errmsg("invalid xl_info in secondary checkpoint record")));
                        break;
                  default:
                        ereport(LOG,
                                    (errmsg("invalid xl_info in checkpoint record")));
                        break;
            }
            return NULL;
      }
      if (record->xl_len != sizeof(CheckPoint) ||
            record->xl_tot_len != SizeOfXLogRecord + sizeof(CheckPoint))
      {
            switch (whichChkpt)
            {
                  case 1:
                        ereport(LOG,
                              (errmsg("invalid length of primary checkpoint record")));
                        break;
                  case 2:
                        ereport(LOG,
                          (errmsg("invalid length of secondary checkpoint record")));
                        break;
                  default:
                        ereport(LOG,
                                    (errmsg("invalid length of checkpoint record")));
                        break;
            }
            return NULL;
      }
      return record;
}

/*
 * This must be called during startup of a backend process, except that
 * it need not be called in a standalone backend (which does StartupXLOG
 * instead).  We need to initialize the local copies of ThisTimeLineID and
 * RedoRecPtr.
 *
 * Note: before Postgres 8.0, we went to some effort to keep the postmaster
 * process's copies of ThisTimeLineID and RedoRecPtr valid too.  This was
 * unnecessary however, since the postmaster itself never touches XLOG anyway.
 */
void
InitXLOGAccess(void)
{
      /* ThisTimeLineID doesn't change so we need no lock to copy it */
      ThisTimeLineID = XLogCtl->ThisTimeLineID;
      Assert(ThisTimeLineID != 0);

      /* Use GetRedoRecPtr to copy the RedoRecPtr safely */
      (void) GetRedoRecPtr();
}

/*
 * Once spawned, a backend may update its local RedoRecPtr from
 * XLogCtl->Insert.RedoRecPtr; it must hold the insert lock or info_lck
 * to do so.  This is done in XLogInsert() or GetRedoRecPtr().
 */
XLogRecPtr
GetRedoRecPtr(void)
{
      /* use volatile pointer to prevent code rearrangement */
      volatile XLogCtlData *xlogctl = XLogCtl;

      SpinLockAcquire(&xlogctl->info_lck);
      Assert(XLByteLE(RedoRecPtr, xlogctl->Insert.RedoRecPtr));
      RedoRecPtr = xlogctl->Insert.RedoRecPtr;
      SpinLockRelease(&xlogctl->info_lck);

      return RedoRecPtr;
}

/*
 * GetInsertRecPtr -- Returns the current insert position.
 *
 * NOTE: The value *actually* returned is the position of the last full
 * xlog page. It lags behind the real insert position by at most 1 page.
 * For that, we don't need to acquire WALInsertLock which can be quite
 * heavily contended, and an approximation is enough for the current
 * usage of this function.
 */
XLogRecPtr
GetInsertRecPtr(void)
{
      /* use volatile pointer to prevent code rearrangement */
      volatile XLogCtlData *xlogctl = XLogCtl;
      XLogRecPtr  recptr;

      SpinLockAcquire(&xlogctl->info_lck);
      recptr = xlogctl->LogwrtRqst.Write;
      SpinLockRelease(&xlogctl->info_lck);

      return recptr;
}

/*
 * Get the time of the last xlog segment switch
 */
pg_time_t
GetLastSegSwitchTime(void)
{
      pg_time_t   result;

      /* Need WALWriteLock, but shared lock is sufficient */
      LWLockAcquire(WALWriteLock, LW_SHARED);
      result = XLogCtl->Write.lastSegSwitchTime;
      LWLockRelease(WALWriteLock);

      return result;
}

/*
 * GetNextXidAndEpoch - get the current nextXid value and associated epoch
 *
 * This is exported for use by code that would like to have 64-bit XIDs.
 * We don't really support such things, but all XIDs within the system
 * can be presumed "close to" the result, and thus the epoch associated
 * with them can be determined.
 */
void
GetNextXidAndEpoch(TransactionId *xid, uint32 *epoch)
{
      uint32            ckptXidEpoch;
      TransactionId ckptXid;
      TransactionId nextXid;

      /* Must read checkpoint info first, else have race condition */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            ckptXidEpoch = xlogctl->ckptXidEpoch;
            ckptXid = xlogctl->ckptXid;
            SpinLockRelease(&xlogctl->info_lck);
      }

      /* Now fetch current nextXid */
      nextXid = ReadNewTransactionId();

      /*
       * nextXid is certainly logically later than ckptXid.  So if it's
       * numerically less, it must have wrapped into the next epoch.
       */
      if (nextXid < ckptXid)
            ckptXidEpoch++;

      *xid = nextXid;
      *epoch = ckptXidEpoch;
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownXLOG(int code, Datum arg)
{
      ereport(LOG,
                  (errmsg("shutting down")));

      if (RecoveryInProgress())
            CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
      else
      {
            /*
             * If archiving is enabled, rotate the last XLOG file so that all the
             * remaining records are archived (postmaster wakes up the archiver
             * process one more time at the end of shutdown). The checkpoint
             * record will go to the next XLOG file and won't be archived (yet).
             */
            if (XLogArchivingActive() && XLogArchiveCommandSet())
                  RequestXLogSwitch();

            CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
      }
      ShutdownCLOG();
      ShutdownSUBTRANS();
      ShutdownMultiXact();

      ereport(LOG,
                  (errmsg("database system is shut down")));
}

/*
 * Log start of a checkpoint.
 */
static void
LogCheckpointStart(int flags, bool restartpoint)
{
      const char *msg;

      /*
       * XXX: This is hopelessly untranslatable. We could call gettext_noop for
       * the main message, but what about all the flags?
       */
      if (restartpoint)
            msg = "restartpoint starting:%s%s%s%s%s%s%s";
      else
            msg = "checkpoint starting:%s%s%s%s%s%s%s";

      elog(LOG, msg,
             (flags & CHECKPOINT_IS_SHUTDOWN) ? " shutdown" : "",
             (flags & CHECKPOINT_END_OF_RECOVERY) ? " end-of-recovery" : "",
             (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
             (flags & CHECKPOINT_FORCE) ? " force" : "",
             (flags & CHECKPOINT_WAIT) ? " wait" : "",
             (flags & CHECKPOINT_CAUSE_XLOG) ? " xlog" : "",
             (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "");
}

/*
 * Log end of a checkpoint.
 */
static void
LogCheckpointEnd(bool restartpoint)
{
      long        write_secs,
                        sync_secs,
                        total_secs;
      int               write_usecs,
                        sync_usecs,
                        total_usecs;

      CheckpointStats.ckpt_end_t = GetCurrentTimestamp();

      TimestampDifference(CheckpointStats.ckpt_start_t,
                                    CheckpointStats.ckpt_end_t,
                                    &total_secs, &total_usecs);

      TimestampDifference(CheckpointStats.ckpt_write_t,
                                    CheckpointStats.ckpt_sync_t,
                                    &write_secs, &write_usecs);

      TimestampDifference(CheckpointStats.ckpt_sync_t,
                                    CheckpointStats.ckpt_sync_end_t,
                                    &sync_secs, &sync_usecs);

      if (restartpoint)
            elog(LOG, "restartpoint complete: wrote %d buffers (%.1f%%); "
                   "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s",
                   CheckpointStats.ckpt_bufs_written,
                   (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
                   write_secs, write_usecs / 1000,
                   sync_secs, sync_usecs / 1000,
                   total_secs, total_usecs / 1000);
      else
            elog(LOG, "checkpoint complete: wrote %d buffers (%.1f%%); "
                   "%d transaction log file(s) added, %d removed, %d recycled; "
                   "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s",
                   CheckpointStats.ckpt_bufs_written,
                   (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
                   CheckpointStats.ckpt_segs_added,
                   CheckpointStats.ckpt_segs_removed,
                   CheckpointStats.ckpt_segs_recycled,
                   write_secs, write_usecs / 1000,
                   sync_secs, sync_usecs / 1000,
                   total_secs, total_usecs / 1000);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 *
 * flags is a bitwise OR of the following:
 *    CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
 *    CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
 *    CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
 *          ignoring checkpoint_completion_target parameter.
 *    CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occured
 *          since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
 *          CHECKPOINT_END_OF_RECOVERY).
 *
 * Note: flags contains other bits, of interest here only for logging purposes.
 * In particular note that this routine is synchronous and does not pay
 * attention to CHECKPOINT_WAIT.
 */
void
CreateCheckPoint(int flags)
{
      bool        shutdown;
      CheckPoint  checkPoint;
      XLogRecPtr  recptr;
      XLogCtlInsert *Insert = &XLogCtl->Insert;
      XLogRecData rdata;
      uint32            freespace;
      uint32            _logId;
      uint32            _logSeg;
      TransactionId *inCommitXids;
      int               nInCommit;

      /*
       * An end-of-recovery checkpoint is really a shutdown checkpoint, just
       * issued at a different time.
       */
      if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
            shutdown = true;
      else
            shutdown = false;

      /* sanity check */
      if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
            elog(ERROR, "can't create a checkpoint during recovery");

      /*
       * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
       * (This is just pro forma, since in the present system structure there is
       * only one process that is allowed to issue checkpoints at any given
       * time.)
       */
      LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

      /*
       * Prepare to accumulate statistics.
       *
       * Note: because it is possible for log_checkpoints to change while a
       * checkpoint proceeds, we always accumulate stats, even if
       * log_checkpoints is currently off.
       */
      MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
      CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

      /*
       * Use a critical section to force system panic if we have trouble.
       */
      START_CRIT_SECTION();

      if (shutdown)
      {
            LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
            ControlFile->state = DB_SHUTDOWNING;
            ControlFile->time = (pg_time_t) time(NULL);
            UpdateControlFile();
            LWLockRelease(ControlFileLock);
      }

      /*
       * Let smgr prepare for checkpoint; this has to happen before we determine
       * the REDO pointer.  Note that smgr must not do anything that'd have to
       * be undone if we decide no checkpoint is needed.
       */
      smgrpreckpt();

      /* Begin filling in the checkpoint WAL record */
      MemSet(&checkPoint, 0, sizeof(checkPoint));
      checkPoint.time = (pg_time_t) time(NULL);

      /*
       * We must hold WALInsertLock while examining insert state to determine
       * the checkpoint REDO pointer.
       */
      LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);

      /*
       * If this isn't a shutdown or forced checkpoint, and we have not inserted
       * any XLOG records since the start of the last checkpoint, skip the
       * checkpoint.    The idea here is to avoid inserting duplicate checkpoints
       * when the system is idle. That wastes log space, and more importantly it
       * exposes us to possible loss of both current and previous checkpoint
       * records if the machine crashes just as we're writing the update.
       * (Perhaps it'd make even more sense to checkpoint only when the previous
       * checkpoint record is in a different xlog page?)
       *
       * We have to make two tests to determine that nothing has happened since
       * the start of the last checkpoint: current insertion point must match
       * the end of the last checkpoint record, and its redo pointer must point
       * to itself.
       */
      if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
                          CHECKPOINT_FORCE)) == 0)
      {
            XLogRecPtr  curInsert;

            INSERT_RECPTR(curInsert, Insert, Insert->curridx);
            if (curInsert.xlogid == ControlFile->checkPoint.xlogid &&
                  curInsert.xrecoff == ControlFile->checkPoint.xrecoff +
                  MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
                  ControlFile->checkPoint.xlogid ==
                  ControlFile->checkPointCopy.redo.xlogid &&
                  ControlFile->checkPoint.xrecoff ==
                  ControlFile->checkPointCopy.redo.xrecoff)
            {
                  LWLockRelease(WALInsertLock);
                  LWLockRelease(CheckpointLock);
                  END_CRIT_SECTION();
                  return;
            }
      }

      /*
       * Compute new REDO record ptr = location of next XLOG record.
       *
       * NB: this is NOT necessarily where the checkpoint record itself will be,
       * since other backends may insert more XLOG records while we're off doing
       * the buffer flush work.  Those XLOG records are logically after the
       * checkpoint, even though physically before it.  Got that?
       */
      freespace = INSERT_FREESPACE(Insert);
      if (freespace < SizeOfXLogRecord)
      {
            (void) AdvanceXLInsertBuffer(false);
            /* OK to ignore update return flag, since we will do flush anyway */
            freespace = INSERT_FREESPACE(Insert);
      }
      INSERT_RECPTR(checkPoint.redo, Insert, Insert->curridx);

      /*
       * Here we update the shared RedoRecPtr for future XLogInsert calls; this
       * must be done while holding the insert lock AND the info_lck.
       *
       * Note: if we fail to complete the checkpoint, RedoRecPtr will be left
       * pointing past where it really needs to point.  This is okay; the only
       * consequence is that XLogInsert might back up whole buffers that it
       * didn't really need to.  We can't postpone advancing RedoRecPtr because
       * XLogInserts that happen while we are dumping buffers must assume that
       * their buffer changes are not included in the checkpoint.
       */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            RedoRecPtr = xlogctl->Insert.RedoRecPtr = checkPoint.redo;
            SpinLockRelease(&xlogctl->info_lck);
      }

      /*
       * Now we can release WAL insert lock, allowing other xacts to proceed
       * while we are flushing disk buffers.
       */
      LWLockRelease(WALInsertLock);

      /*
       * If enabled, log checkpoint start.  We postpone this until now so as not
       * to log anything if we decided to skip the checkpoint.
       */
      if (log_checkpoints)
            LogCheckpointStart(flags, false);

      TRACE_POSTGRESQL_CHECKPOINT_START(flags);

      /*
       * Before flushing data, we must wait for any transactions that are
       * currently in their commit critical sections.  If an xact inserted its
       * commit record into XLOG just before the REDO point, then a crash
       * restart from the REDO point would not replay that record, which means
       * that our flushing had better include the xact's update of pg_clog.  So
       * we wait till he's out of his commit critical section before proceeding.
       * See notes in RecordTransactionCommit().
       *
       * Because we've already released WALInsertLock, this test is a bit fuzzy:
       * it is possible that we will wait for xacts we didn't really need to
       * wait for.  But the delay should be short and it seems better to make
       * checkpoint take a bit longer than to hold locks longer than necessary.
       * (In fact, the whole reason we have this issue is that xact.c does
       * commit record XLOG insertion and clog update as two separate steps
       * protected by different locks, but again that seems best on grounds of
       * minimizing lock contention.)
       *
       * A transaction that has not yet set inCommit when we look cannot be at
       * risk, since he's not inserted his commit record yet; and one that's
       * already cleared it is not at risk either, since he's done fixing clog
       * and we will correctly flush the update below.  So we cannot miss any
       * xacts we need to wait for.
       */
      nInCommit = GetTransactionsInCommit(&inCommitXids);
      if (nInCommit > 0)
      {
            do
            {
                  pg_usleep(10000L);      /* wait for 10 msec */
            } while (HaveTransactionsInCommit(inCommitXids, nInCommit));
      }
      pfree(inCommitXids);

      /*
       * Get the other info we need for the checkpoint record.
       */
      LWLockAcquire(XidGenLock, LW_SHARED);
      checkPoint.nextXid = ShmemVariableCache->nextXid;
      LWLockRelease(XidGenLock);

      /* Increase XID epoch if we've wrapped around since last checkpoint */
      checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
      if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
            checkPoint.nextXidEpoch++;

      LWLockAcquire(OidGenLock, LW_SHARED);
      checkPoint.nextOid = ShmemVariableCache->nextOid;
      if (!shutdown)
            checkPoint.nextOid += ShmemVariableCache->oidCount;
      LWLockRelease(OidGenLock);

      MultiXactGetCheckptMulti(shutdown,
                                           &checkPoint.nextMulti,
                                           &checkPoint.nextMultiOffset);

      /*
       * Having constructed the checkpoint record, ensure all shmem disk buffers
       * and commit-log buffers are flushed to disk.
       *
       * This I/O could fail for various reasons.  If so, we will fail to
       * complete the checkpoint, but there is no reason to force a system
       * panic. Accordingly, exit critical section while doing it.
       */
      END_CRIT_SECTION();

      CheckPointGuts(checkPoint.redo, flags);

      START_CRIT_SECTION();

      /*
       * An end-of-recovery checkpoint is created before anyone is allowed to
       * write WAL. To allow us to write the checkpoint record, temporarily
       * enable XLogInsertAllowed.
       */
      if (flags & CHECKPOINT_END_OF_RECOVERY)
            LocalSetXLogInsertAllowed();

      /*
       * This needs to be done after LocalSetXLogInsertAllowed(), else
       * ThisTimeLineID might still be uninitialized.
       */
      checkPoint.ThisTimeLineID = ThisTimeLineID;

      /*
       * Now insert the checkpoint record into XLOG.
       */
      rdata.data = (char *) (&checkPoint);
      rdata.len = sizeof(checkPoint);
      rdata.buffer = InvalidBuffer;
      rdata.next = NULL;

      recptr = XLogInsert(RM_XLOG_ID,
                                    shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
                                    XLOG_CHECKPOINT_ONLINE,
                                    &rdata);

      XLogFlush(recptr);

      /*
       * We mustn't write any new WAL after a shutdown checkpoint, or it will
       * be overwritten at next startup.  No-one should even try, this just
       * allows sanity-checking.  In the case of an end-of-recovery checkpoint,
       * we want to just temporarily disable writing until the system has exited
       * recovery.
       */
      if (shutdown)
      {
            if (flags & CHECKPOINT_END_OF_RECOVERY)
                  LocalXLogInsertAllowed = -1;  /* return to "check" state */
            else
                  LocalXLogInsertAllowed = 0;         /* never again write WAL */
      }

      /*
       * We now have ProcLastRecPtr = start of actual checkpoint record, recptr
       * = end of actual checkpoint record.
       */
      if (shutdown && !XLByteEQ(checkPoint.redo, ProcLastRecPtr))
            ereport(PANIC,
                        (errmsg("concurrent transaction log activity while database system is shutting down")));

      /*
       * Select point at which we can truncate the log, which we base on the
       * prior checkpoint's earliest info.
       */
      XLByteToSeg(ControlFile->checkPointCopy.redo, _logId, _logSeg);

      /*
       * Update the control file.
       */
      LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
      if (shutdown)
            ControlFile->state = DB_SHUTDOWNED;
      ControlFile->prevCheckPoint = ControlFile->checkPoint;
      ControlFile->checkPoint = ProcLastRecPtr;
      ControlFile->checkPointCopy = checkPoint;
      ControlFile->time = (pg_time_t) time(NULL);
      UpdateControlFile();
      LWLockRelease(ControlFileLock);

      /* Update shared-memory copy of checkpoint XID/epoch */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
            xlogctl->ckptXid = checkPoint.nextXid;
            SpinLockRelease(&xlogctl->info_lck);
      }

      /*
       * We are now done with critical updates; no need for system panic if we
       * have trouble while fooling with old log segments.
       */
      END_CRIT_SECTION();

      /*
       * Let smgr do post-checkpoint cleanup (eg, deleting old files).
       */
      smgrpostckpt();

      /*
       * Delete old log files (those no longer needed even for previous
       * checkpoint).
       */
      if (_logId || _logSeg)
      {
            PrevLogSeg(_logId, _logSeg);
            RemoveOldXlogFiles(_logId, _logSeg, recptr);
      }

      /*
       * Make more log segments if needed.  (Do this after recycling old log
       * segments, since that may supply some of the needed files.)
       */
      if (!shutdown)
            PreallocXlogFiles(recptr);

      /*
       * Truncate pg_subtrans if possible.  We can throw away all data before
       * the oldest XMIN of any running transaction.  No future transaction will
       * attempt to reference any pg_subtrans entry older than that (see Asserts
       * in subtrans.c).      During recovery, though, we mustn't do this because
       * StartupSUBTRANS hasn't been called yet.
       */
      if (!RecoveryInProgress())
            TruncateSUBTRANS(GetOldestXmin(true, false));

      /* All real work is done, but log before releasing lock. */
      if (log_checkpoints)
            LogCheckpointEnd(false);

      TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
                                                       NBuffers,
                                                       CheckpointStats.ckpt_segs_added,
                                                       CheckpointStats.ckpt_segs_removed,
                                                       CheckpointStats.ckpt_segs_recycled);

      LWLockRelease(CheckpointLock);
}

/*
 * Flush all data in shared memory to disk, and fsync
 *
 * This is the common code shared between regular checkpoints and
 * recovery restartpoints.
 */
static void
CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
{
      CheckPointCLOG();
      CheckPointSUBTRANS();
      CheckPointMultiXact();
      CheckPointBuffers(flags);     /* performs all required fsyncs */
      /* We deliberately delay 2PC checkpointing as long as possible */
      CheckPointTwoPhase(checkPointRedo);
}

/*
 * Save a checkpoint for recovery restart if appropriate
 *
 * This function is called each time a checkpoint record is read from XLOG.
 * It must determine whether the checkpoint represents a safe restartpoint or
 * not.  If so, the checkpoint record is stashed in shared memory so that
 * CreateRestartPoint can consult it.  (Note that the latter function is
 * executed by the bgwriter, while this one will be executed by the startup
 * process.)
 */
static void
RecoveryRestartPoint(const CheckPoint *checkPoint)
{
      int               rmid;

      /* use volatile pointer to prevent code rearrangement */
      volatile XLogCtlData *xlogctl = XLogCtl;

      /*
       * Is it safe to checkpoint?  We must ask each of the resource managers
       * whether they have any partial state information that might prevent a
       * correct restart from this point.  If so, we skip this opportunity, but
       * return at the next checkpoint record for another try.
       */
      for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
      {
            if (RmgrTable[rmid].rm_safe_restartpoint != NULL)
                  if (!(RmgrTable[rmid].rm_safe_restartpoint()))
                  {
                        elog(DEBUG2, "RM %d not safe to record restart point at %X/%X",
                               rmid,
                               checkPoint->redo.xlogid,
                               checkPoint->redo.xrecoff);
                        return;
                  }
      }

      /*
       * Copy the checkpoint record to shared memory, so that bgwriter can use
       * it the next time it wants to perform a restartpoint.
       */
      SpinLockAcquire(&xlogctl->info_lck);
      XLogCtl->lastCheckPointRecPtr = ReadRecPtr;
      memcpy(&XLogCtl->lastCheckPoint, checkPoint, sizeof(CheckPoint));
      SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Establish a restartpoint if possible.
 *
 * This is similar to CreateCheckPoint, but is used during WAL recovery
 * to establish a point from which recovery can roll forward without
 * replaying the entire recovery log.
 *
 * Returns true if a new restartpoint was established. We can only establish
 * a restartpoint if we have replayed a safe checkpoint record since last
 * restartpoint.
 */
bool
CreateRestartPoint(int flags)
{
      XLogRecPtr  lastCheckPointRecPtr;
      CheckPoint  lastCheckPoint;

      /* use volatile pointer to prevent code rearrangement */
      volatile XLogCtlData *xlogctl = XLogCtl;

      /*
       * Acquire CheckpointLock to ensure only one restartpoint or checkpoint
       * happens at a time.
       */
      LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

      /* Get a local copy of the last safe checkpoint record. */
      SpinLockAcquire(&xlogctl->info_lck);
      lastCheckPointRecPtr = xlogctl->lastCheckPointRecPtr;
      memcpy(&lastCheckPoint, &XLogCtl->lastCheckPoint, sizeof(CheckPoint));
      SpinLockRelease(&xlogctl->info_lck);

      /*
       * Check that we're still in recovery mode. It's ok if we exit recovery
       * mode after this check, the restart point is valid anyway.
       */
      if (!RecoveryInProgress())
      {
            ereport(DEBUG2,
                    (errmsg("skipping restartpoint, recovery has already ended")));
            LWLockRelease(CheckpointLock);
            return false;
      }

      /*
       * If the last checkpoint record we've replayed is already our last
       * restartpoint, we can't perform a new restart point. We still update
       * minRecoveryPoint in that case, so that if this is a shutdown restart
       * point, we won't start up earlier than before. That's not strictly
       * necessary, but when we get hot standby capability, it would be rather
       * weird if the database opened up for read-only connections at a
       * point-in-time before the last shutdown. Such time travel is still
       * possible in case of immediate shutdown, though.
       *
       * We don't explicitly advance minRecoveryPoint when we do create a
       * restartpoint. It's assumed that flushing the buffers will do that as a
       * side-effect.
       */
      if (XLogRecPtrIsInvalid(lastCheckPointRecPtr) ||
            XLByteLE(lastCheckPoint.redo, ControlFile->checkPointCopy.redo))
      {
            XLogRecPtr  InvalidXLogRecPtr = {0, 0};

            ereport(DEBUG2,
                        (errmsg("skipping restartpoint, already performed at %X/%X",
                          lastCheckPoint.redo.xlogid, lastCheckPoint.redo.xrecoff)));

            UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);
            LWLockRelease(CheckpointLock);
            return false;
      }

      if (log_checkpoints)
      {
            /*
             * Prepare to accumulate statistics.
             */
            MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
            CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

            LogCheckpointStart(flags, true);
      }

      CheckPointGuts(lastCheckPoint.redo, flags);

      /*
       * Update pg_control, using current time.  Check that it still shows
       * IN_ARCHIVE_RECOVERY state and an older checkpoint, else do nothing;
       * this is a quick hack to make sure nothing really bad happens if
       * somehow we get here after the end-of-recovery checkpoint.
       */
      LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
      if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&
            XLByteLT(ControlFile->checkPointCopy.redo, lastCheckPoint.redo))
      {
            ControlFile->prevCheckPoint = ControlFile->checkPoint;
            ControlFile->checkPoint = lastCheckPointRecPtr;
            ControlFile->checkPointCopy = lastCheckPoint;
            ControlFile->time = (pg_time_t) time(NULL);
            UpdateControlFile();
      }
      LWLockRelease(ControlFileLock);

      /*
       * Currently, there is no need to truncate pg_subtrans during recovery. If
       * we did do that, we will need to have called StartupSUBTRANS() already
       * and then TruncateSUBTRANS() would go here.
       */

      /* All real work is done, but log before releasing lock. */
      if (log_checkpoints)
            LogCheckpointEnd(true);

      ereport((log_checkpoints ? LOG : DEBUG2),
                  (errmsg("recovery restart point at %X/%X",
                          lastCheckPoint.redo.xlogid, lastCheckPoint.redo.xrecoff)));

      /* XXX this is currently BROKEN because we are in the wrong process */
      if (recoveryLastXTime)
            ereport((log_checkpoints ? LOG : DEBUG2),
                        (errmsg("last completed transaction was at log time %s",
                                    timestamptz_to_str(recoveryLastXTime))));

      LWLockRelease(CheckpointLock);
      return true;
}

/*
 * Write a NEXTOID log record
 */
void
XLogPutNextOid(Oid nextOid)
{
      XLogRecData rdata;

      rdata.data = (char *) (&nextOid);
      rdata.len = sizeof(Oid);
      rdata.buffer = InvalidBuffer;
      rdata.next = NULL;
      (void) XLogInsert(RM_XLOG_ID, XLOG_NEXTOID, &rdata);

      /*
       * We need not flush the NEXTOID record immediately, because any of the
       * just-allocated OIDs could only reach disk as part of a tuple insert or
       * update that would have its own XLOG record that must follow the NEXTOID
       * record.  Therefore, the standard buffer LSN interlock applied to those
       * records will ensure no such OID reaches disk before the NEXTOID record
       * does.
       *
       * Note, however, that the above statement only covers state "within" the
       * database.  When we use a generated OID as a file or directory name, we
       * are in a sense violating the basic WAL rule, because that filesystem
       * change may reach disk before the NEXTOID WAL record does.  The impact
       * of this is that if a database crash occurs immediately afterward, we
       * might after restart re-generate the same OID and find that it conflicts
       * with the leftover file or directory.  But since for safety's sake we
       * always loop until finding a nonconflicting filename, this poses no real
       * problem in practice. See pgsql-hackers discussion 27-Sep-2006.
       */
}

/*
 * Write an XLOG SWITCH record.
 *
 * Here we just blindly issue an XLogInsert request for the record.
 * All the magic happens inside XLogInsert.
 *
 * The return value is either the end+1 address of the switch record,
 * or the end+1 address of the prior segment if we did not need to
 * write a switch record because we are already at segment start.
 */
XLogRecPtr
RequestXLogSwitch(void)
{
      XLogRecPtr  RecPtr;
      XLogRecData rdata;

      /* XLOG SWITCH, alone among xlog record types, has no data */
      rdata.buffer = InvalidBuffer;
      rdata.data = NULL;
      rdata.len = 0;
      rdata.next = NULL;

      RecPtr = XLogInsert(RM_XLOG_ID, XLOG_SWITCH, &rdata);

      return RecPtr;
}

/*
 * XLOG resource manager's routines
 *
 * Definitions of info values are in include/catalog/pg_control.h, though
 * not all record types are related to control file updates.
 */
void
xlog_redo(XLogRecPtr lsn, XLogRecord *record)
{
      uint8       info = record->xl_info & ~XLR_INFO_MASK;

      /* Backup blocks are not used in xlog records */
      Assert(!(record->xl_info & XLR_BKP_BLOCK_MASK));

      if (info == XLOG_NEXTOID)
      {
            Oid               nextOid;

            memcpy(&nextOid, XLogRecGetData(record), sizeof(Oid));
            if (ShmemVariableCache->nextOid < nextOid)
            {
                  ShmemVariableCache->nextOid = nextOid;
                  ShmemVariableCache->oidCount = 0;
            }
      }
      else if (info == XLOG_CHECKPOINT_SHUTDOWN)
      {
            CheckPoint  checkPoint;

            memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
            /* In a SHUTDOWN checkpoint, believe the counters exactly */
            ShmemVariableCache->nextXid = checkPoint.nextXid;
            ShmemVariableCache->nextOid = checkPoint.nextOid;
            ShmemVariableCache->oidCount = 0;
            MultiXactSetNextMXact(checkPoint.nextMulti,
                                            checkPoint.nextMultiOffset);

            /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
            ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
            ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

            /*
             * TLI may change in a shutdown checkpoint, but it shouldn't decrease
             */
            if (checkPoint.ThisTimeLineID != ThisTimeLineID)
            {
                  if (checkPoint.ThisTimeLineID < ThisTimeLineID ||
                        !list_member_int(expectedTLIs,
                                                 (int) checkPoint.ThisTimeLineID))
                        ereport(PANIC,
                                    (errmsg("unexpected timeline ID %u (after %u) in checkpoint record",
                                                checkPoint.ThisTimeLineID, ThisTimeLineID)));
                  /* Following WAL records should be run with new TLI */
                  ThisTimeLineID = checkPoint.ThisTimeLineID;
            }

            RecoveryRestartPoint(&checkPoint);
      }
      else if (info == XLOG_CHECKPOINT_ONLINE)
      {
            CheckPoint  checkPoint;

            memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
            /* In an ONLINE checkpoint, treat the counters like NEXTOID */
            if (TransactionIdPrecedes(ShmemVariableCache->nextXid,
                                                  checkPoint.nextXid))
                  ShmemVariableCache->nextXid = checkPoint.nextXid;
            if (ShmemVariableCache->nextOid < checkPoint.nextOid)
            {
                  ShmemVariableCache->nextOid = checkPoint.nextOid;
                  ShmemVariableCache->oidCount = 0;
            }
            MultiXactAdvanceNextMXact(checkPoint.nextMulti,
                                                  checkPoint.nextMultiOffset);

            /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
            ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
            ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

            /* TLI should not change in an on-line checkpoint */
            if (checkPoint.ThisTimeLineID != ThisTimeLineID)
                  ereport(PANIC,
                              (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
                                          checkPoint.ThisTimeLineID, ThisTimeLineID)));

            RecoveryRestartPoint(&checkPoint);
      }
      else if (info == XLOG_NOOP)
      {
            /* nothing to do here */
      }
      else if (info == XLOG_SWITCH)
      {
            /* nothing to do here */
      }
}

void
xlog_desc(StringInfo buf, uint8 xl_info, char *rec)
{
      uint8       info = xl_info & ~XLR_INFO_MASK;

      if (info == XLOG_CHECKPOINT_SHUTDOWN ||
            info == XLOG_CHECKPOINT_ONLINE)
      {
            CheckPoint *checkpoint = (CheckPoint *) rec;

            appendStringInfo(buf, "checkpoint: redo %X/%X; "
                                     "tli %u; xid %u/%u; oid %u; multi %u; offset %u; %s",
                                     checkpoint->redo.xlogid, checkpoint->redo.xrecoff,
                                     checkpoint->ThisTimeLineID,
                                     checkpoint->nextXidEpoch, checkpoint->nextXid,
                                     checkpoint->nextOid,
                                     checkpoint->nextMulti,
                                     checkpoint->nextMultiOffset,
                         (info == XLOG_CHECKPOINT_SHUTDOWN) ? "shutdown" : "online");
      }
      else if (info == XLOG_NOOP)
      {
            appendStringInfo(buf, "xlog no-op");
      }
      else if (info == XLOG_NEXTOID)
      {
            Oid               nextOid;

            memcpy(&nextOid, rec, sizeof(Oid));
            appendStringInfo(buf, "nextOid: %u", nextOid);
      }
      else if (info == XLOG_SWITCH)
      {
            appendStringInfo(buf, "xlog switch");
      }
      else
            appendStringInfo(buf, "UNKNOWN");
}

#ifdef WAL_DEBUG

static void
xlog_outrec(StringInfo buf, XLogRecord *record)
{
      int               i;

      appendStringInfo(buf, "prev %X/%X; xid %u",
                               record->xl_prev.xlogid, record->xl_prev.xrecoff,
                               record->xl_xid);

      for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
      {
            if (record->xl_info & XLR_SET_BKP_BLOCK(i))
                  appendStringInfo(buf, "; bkpb%d", i + 1);
      }

      appendStringInfo(buf, ": %s", RmgrTable[record->xl_rmid].rm_name);
}
#endif   /* WAL_DEBUG */


/*
 * Return the (possible) sync flag used for opening a file, depending on the
 * value of the GUC wal_sync_method.
 */
static int
get_sync_bit(int method)
{
      /* If fsync is disabled, never open in sync mode */
      if (!enableFsync)
            return 0;

      switch (method)
      {
                  /*
                   * enum values for all sync options are defined even if they are
                   * not supported on the current platform.  But if not, they are
                   * not included in the enum option array, and therefore will never
                   * be seen here.
                   */
            case SYNC_METHOD_FSYNC:
            case SYNC_METHOD_FSYNC_WRITETHROUGH:
            case SYNC_METHOD_FDATASYNC:
                  return 0;
#ifdef OPEN_SYNC_FLAG
            case SYNC_METHOD_OPEN:
                  return OPEN_SYNC_FLAG;
#endif
#ifdef OPEN_DATASYNC_FLAG
            case SYNC_METHOD_OPEN_DSYNC:
                  return OPEN_DATASYNC_FLAG;
#endif
            default:
                  /* can't happen (unless we are out of sync with option array) */
                  elog(ERROR, "unrecognized wal_sync_method: %d", method);
                  return 0;               /* silence warning */
      }
}

/*
 * GUC support
 */
bool
assign_xlog_sync_method(int new_sync_method, bool doit, GucSource source)
{
      if (!doit)
            return true;

      if (sync_method != new_sync_method)
      {
            /*
             * To ensure that no blocks escape unsynced, force an fsync on the
             * currently open log segment (if any).  Also, if the open flag is
             * changing, close the log file so it will be reopened (with new flag
             * bit) at next use.
             */
            if (openLogFile >= 0)
            {
                  if (pg_fsync(openLogFile) != 0)
                        ereport(PANIC,
                                    (errcode_for_file_access(),
                                     errmsg("could not fsync log file %u, segment %u: %m",
                                                openLogId, openLogSeg)));
                  if (get_sync_bit(sync_method) != get_sync_bit(new_sync_method))
                        XLogFileClose();
            }
      }

      return true;
}


/*
 * Issue appropriate kind of fsync (if any) on the current XLOG output file
 */
static void
issue_xlog_fsync(void)
{
      switch (sync_method)
      {
            case SYNC_METHOD_FSYNC:
                  if (pg_fsync_no_writethrough(openLogFile) != 0)
                        ereport(PANIC,
                                    (errcode_for_file_access(),
                                     errmsg("could not fsync log file %u, segment %u: %m",
                                                openLogId, openLogSeg)));
                  break;
#ifdef HAVE_FSYNC_WRITETHROUGH
            case SYNC_METHOD_FSYNC_WRITETHROUGH:
                  if (pg_fsync_writethrough(openLogFile) != 0)
                        ereport(PANIC,
                                    (errcode_for_file_access(),
                                     errmsg("could not fsync write-through log file %u, segment %u: %m",
                                                openLogId, openLogSeg)));
                  break;
#endif
#ifdef HAVE_FDATASYNC
            case SYNC_METHOD_FDATASYNC:
                  if (pg_fdatasync(openLogFile) != 0)
                        ereport(PANIC,
                                    (errcode_for_file_access(),
                              errmsg("could not fdatasync log file %u, segment %u: %m",
                                       openLogId, openLogSeg)));
                  break;
#endif
            case SYNC_METHOD_OPEN:
            case SYNC_METHOD_OPEN_DSYNC:
                  /* write synced it already */
                  break;
            default:
                  elog(PANIC, "unrecognized wal_sync_method: %d", sync_method);
                  break;
      }
}


/*
 * pg_start_backup: set up for taking an on-line backup dump
 *
 * Essentially what this does is to create a backup label file in $PGDATA,
 * where it will be archived as part of the backup dump.  The label file
 * contains the user-supplied label string (typically this would be used
 * to tell where the backup dump will be stored) and the starting time and
 * starting WAL location for the dump.
 */
Datum
pg_start_backup(PG_FUNCTION_ARGS)
{
      text     *backupid = PG_GETARG_TEXT_P(0);
      bool        fast = PG_GETARG_BOOL(1);
      char     *backupidstr;
      XLogRecPtr  checkpointloc;
      XLogRecPtr  startpoint;
      pg_time_t   stamp_time;
      char        strfbuf[128];
      char        xlogfilename[MAXFNAMELEN];
      uint32            _logId;
      uint32            _logSeg;
      struct stat stat_buf;
      FILE     *fp;

      if (!superuser())
            ereport(ERROR,
                        (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                         errmsg("must be superuser to run a backup")));

      if (!XLogArchivingActive())
            ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("WAL archiving is not active"),
                         errhint("archive_mode must be enabled at server start.")));

      if (!XLogArchiveCommandSet())
            ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("WAL archiving is not active"),
                         errhint("archive_command must be defined before "
                                     "online backups can be made safely.")));

      backupidstr = text_to_cstring(backupid);

      /*
       * Mark backup active in shared memory.  We must do full-page WAL writes
       * during an on-line backup even if not doing so at other times, because
       * it's quite possible for the backup dump to obtain a "torn" (partially
       * written) copy of a database page if it reads the page concurrently with
       * our write to the same page.      This can be fixed as long as the first
       * write to the page in the WAL sequence is a full-page write. Hence, we
       * turn on forcePageWrites and then force a CHECKPOINT, to ensure there
       * are no dirty pages in shared memory that might get dumped while the
       * backup is in progress without having a corresponding WAL record.  (Once
       * the backup is complete, we need not force full-page writes anymore,
       * since we expect that any pages not modified during the backup interval
       * must have been correctly captured by the backup.)
       *
       * We must hold WALInsertLock to change the value of forcePageWrites, to
       * ensure adequate interlocking against XLogInsert().
       */
      LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
      if (XLogCtl->Insert.forcePageWrites)
      {
            LWLockRelease(WALInsertLock);
            ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("a backup is already in progress"),
                         errhint("Run pg_stop_backup() and try again.")));
      }
      XLogCtl->Insert.forcePageWrites = true;
      LWLockRelease(WALInsertLock);

      /*
       * Force an XLOG file switch before the checkpoint, to ensure that the WAL
       * segment the checkpoint is written to doesn't contain pages with old
       * timeline IDs. That would otherwise happen if you called
       * pg_start_backup() right after restoring from a PITR archive: the first
       * WAL segment containing the startup checkpoint has pages in the
       * beginning with the old timeline ID. That can cause trouble at recovery:
       * we won't have a history file covering the old timeline if pg_xlog
       * directory was not included in the base backup and the WAL archive was
       * cleared too before starting the backup.
       */
      RequestXLogSwitch();

      /* Ensure we release forcePageWrites if fail below */
      PG_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) 0);
      {
            /*
             * Force a CHECKPOINT.  Aside from being necessary to prevent torn
             * page problems, this guarantees that two successive backup runs will
             * have different checkpoint positions and hence different history
             * file names, even if nothing happened in between.
             *
             * We use CHECKPOINT_IMMEDIATE only if requested by user (via passing
             * fast = true).  Otherwise this can take awhile.
             */
            RequestCheckpoint(CHECKPOINT_FORCE | CHECKPOINT_WAIT |
                                      (fast ? CHECKPOINT_IMMEDIATE : 0));

            /*
             * Now we need to fetch the checkpoint record location, and also its
             * REDO pointer.  The oldest point in WAL that would be needed to
             * restore starting from the checkpoint is precisely the REDO pointer.
             */
            LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
            checkpointloc = ControlFile->checkPoint;
            startpoint = ControlFile->checkPointCopy.redo;
            LWLockRelease(ControlFileLock);

            XLByteToSeg(startpoint, _logId, _logSeg);
            XLogFileName(xlogfilename, ThisTimeLineID, _logId, _logSeg);

            /* Use the log timezone here, not the session timezone */
            stamp_time = (pg_time_t) time(NULL);
            pg_strftime(strfbuf, sizeof(strfbuf),
                              "%Y-%m-%d %H:%M:%S %Z",
                              pg_localtime(&stamp_time, log_timezone));

            /*
             * Check for existing backup label --- implies a backup is already
             * running.  (XXX given that we checked forcePageWrites above, maybe
             * it would be OK to just unlink any such label file?)
             */
            if (stat(BACKUP_LABEL_FILE, &stat_buf) != 0)
            {
                  if (errno != ENOENT)
                        ereport(ERROR,
                                    (errcode_for_file_access(),
                                     errmsg("could not stat file \"%s\": %m",
                                                BACKUP_LABEL_FILE)));
            }
            else
                  ereport(ERROR,
                              (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                               errmsg("a backup is already in progress"),
                               errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
                                           BACKUP_LABEL_FILE)));

            /*
             * Okay, write the file
             */
            fp = AllocateFile(BACKUP_LABEL_FILE, "w");
            if (!fp)
                  ereport(ERROR,
                              (errcode_for_file_access(),
                               errmsg("could not create file \"%s\": %m",
                                          BACKUP_LABEL_FILE)));
            fprintf(fp, "START WAL LOCATION: %X/%X (file %s)\n",
                        startpoint.xlogid, startpoint.xrecoff, xlogfilename);
            fprintf(fp, "CHECKPOINT LOCATION: %X/%X\n",
                        checkpointloc.xlogid, checkpointloc.xrecoff);
            fprintf(fp, "START TIME: %s\n", strfbuf);
            fprintf(fp, "LABEL: %s\n", backupidstr);
            if (fflush(fp) || ferror(fp) || FreeFile(fp))
                  ereport(ERROR,
                              (errcode_for_file_access(),
                               errmsg("could not write file \"%s\": %m",
                                          BACKUP_LABEL_FILE)));
      }
      PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) 0);

      /*
       * We're done.  As a convenience, return the starting WAL location.
       */
      snprintf(xlogfilename, sizeof(xlogfilename), "%X/%X",
                   startpoint.xlogid, startpoint.xrecoff);
      PG_RETURN_TEXT_P(cstring_to_text(xlogfilename));
}

/* Error cleanup callback for pg_start_backup */
static void
pg_start_backup_callback(int code, Datum arg)
{
      /* Turn off forcePageWrites on failure */
      LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
      XLogCtl->Insert.forcePageWrites = false;
      LWLockRelease(WALInsertLock);
}

/*
 * pg_stop_backup: finish taking an on-line backup dump
 *
 * We remove the backup label file created by pg_start_backup, and instead
 * create a backup history file in pg_xlog (whence it will immediately be
 * archived).  The backup history file contains the same info found in
 * the label file, plus the backup-end time and WAL location.
 * Note: different from CancelBackup which just cancels online backup mode.
 */
Datum
pg_stop_backup(PG_FUNCTION_ARGS)
{
      XLogRecPtr  startpoint;
      XLogRecPtr  stoppoint;
      pg_time_t   stamp_time;
      char        strfbuf[128];
      char        histfilepath[MAXPGPATH];
      char        startxlogfilename[MAXFNAMELEN];
      char        stopxlogfilename[MAXFNAMELEN];
      char        lastxlogfilename[MAXFNAMELEN];
      char        histfilename[MAXFNAMELEN];
      uint32            _logId;
      uint32            _logSeg;
      FILE     *lfp;
      FILE     *fp;
      char        ch;
      int               ich;
      int               seconds_before_warning;
      int               waits = 0;

      if (!superuser())
            ereport(ERROR,
                        (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                         (errmsg("must be superuser to run a backup"))));

      if (!XLogArchivingActive())
            ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("WAL archiving is not active"),
                         errhint("archive_mode must be enabled at server start.")));

      /*
       * OK to clear forcePageWrites
       */
      LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
      XLogCtl->Insert.forcePageWrites = false;
      LWLockRelease(WALInsertLock);

      /*
       * Force a switch to a new xlog segment file, so that the backup is valid
       * as soon as archiver moves out the current segment file. We'll report
       * the end address of the XLOG SWITCH record as the backup stopping point.
       */
      stoppoint = RequestXLogSwitch();

      XLByteToSeg(stoppoint, _logId, _logSeg);
      XLogFileName(stopxlogfilename, ThisTimeLineID, _logId, _logSeg);

      /* Use the log timezone here, not the session timezone */
      stamp_time = (pg_time_t) time(NULL);
      pg_strftime(strfbuf, sizeof(strfbuf),
                        "%Y-%m-%d %H:%M:%S %Z",
                        pg_localtime(&stamp_time, log_timezone));

      /*
       * Open the existing label file
       */
      lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
      if (!lfp)
      {
            if (errno != ENOENT)
                  ereport(ERROR,
                              (errcode_for_file_access(),
                               errmsg("could not read file \"%s\": %m",
                                          BACKUP_LABEL_FILE)));
            ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("a backup is not in progress")));
      }

      /*
       * Read and parse the START WAL LOCATION line (this code is pretty crude,
       * but we are not expecting any variability in the file format).
       */
      if (fscanf(lfp, "START WAL LOCATION: %X/%X (file %24s)%c",
                     &startpoint.xlogid, &startpoint.xrecoff, startxlogfilename,
                     &ch) != 4 || ch != '\n')
            ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));

      /*
       * Write the backup history file
       */
      XLByteToSeg(startpoint, _logId, _logSeg);
      BackupHistoryFilePath(histfilepath, ThisTimeLineID, _logId, _logSeg,
                                      startpoint.xrecoff % XLogSegSize);
      fp = AllocateFile(histfilepath, "w");
      if (!fp)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not create file \"%s\": %m",
                                    histfilepath)));
      fprintf(fp, "START WAL LOCATION: %X/%X (file %s)\n",
                  startpoint.xlogid, startpoint.xrecoff, startxlogfilename);
      fprintf(fp, "STOP WAL LOCATION: %X/%X (file %s)\n",
                  stoppoint.xlogid, stoppoint.xrecoff, stopxlogfilename);
      /* transfer remaining lines from label to history file */
      while ((ich = fgetc(lfp)) != EOF)
            fputc(ich, fp);
      fprintf(fp, "STOP TIME: %s\n", strfbuf);
      if (fflush(fp) || ferror(fp) || FreeFile(fp))
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not write file \"%s\": %m",
                                    histfilepath)));

      /*
       * Close and remove the backup label file
       */
      if (ferror(lfp) || FreeFile(lfp))
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not read file \"%s\": %m",
                                    BACKUP_LABEL_FILE)));
      if (unlink(BACKUP_LABEL_FILE) != 0)
            ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not remove file \"%s\": %m",
                                    BACKUP_LABEL_FILE)));

      /*
       * Clean out any no-longer-needed history files.  As a side effect, this
       * will post a .ready file for the newly created history file, notifying
       * the archiver that history file may be archived immediately.
       */
      CleanupBackupHistory();

      /*
       * Wait until both the last WAL file filled during backup and the history
       * file have been archived.  We assume that the alphabetic sorting
       * property of the WAL files ensures any earlier WAL files are safely
       * archived as well.
       *
       * We wait forever, since archive_command is supposed to work and we
       * assume the admin wanted his backup to work completely. If you don't
       * wish to wait, you can set statement_timeout.
       */
      XLByteToPrevSeg(stoppoint, _logId, _logSeg);
      XLogFileName(lastxlogfilename, ThisTimeLineID, _logId, _logSeg);

      XLByteToSeg(startpoint, _logId, _logSeg);
      BackupHistoryFileName(histfilename, ThisTimeLineID, _logId, _logSeg,
                                      startpoint.xrecoff % XLogSegSize);

      seconds_before_warning = 60;
      waits = 0;

      while (XLogArchiveIsBusy(lastxlogfilename) ||
               XLogArchiveIsBusy(histfilename))
      {
            CHECK_FOR_INTERRUPTS();

            pg_usleep(1000000L);

            if (++waits >= seconds_before_warning)
            {
                  seconds_before_warning *= 2;        /* This wraps in >10 years... */
                  ereport(WARNING,
                              (errmsg("pg_stop_backup still waiting for archive to complete (%d seconds elapsed)",
                                          waits)));
            }
      }

      /*
       * We're done.  As a convenience, return the ending WAL location.
       */
      snprintf(stopxlogfilename, sizeof(stopxlogfilename), "%X/%X",
                   stoppoint.xlogid, stoppoint.xrecoff);
      PG_RETURN_TEXT_P(cstring_to_text(stopxlogfilename));
}

/*
 * pg_switch_xlog: switch to next xlog file
 */
Datum
pg_switch_xlog(PG_FUNCTION_ARGS)
{
      XLogRecPtr  switchpoint;
      char        location[MAXFNAMELEN];

      if (!superuser())
            ereport(ERROR,
                        (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                   (errmsg("must be superuser to switch transaction log files"))));

      switchpoint = RequestXLogSwitch();

      /*
       * As a convenience, return the WAL location of the switch record
       */
      snprintf(location, sizeof(location), "%X/%X",
                   switchpoint.xlogid, switchpoint.xrecoff);
      PG_RETURN_TEXT_P(cstring_to_text(location));
}

/*
 * Report the current WAL write location (same format as pg_start_backup etc)
 *
 * This is useful for determining how much of WAL is visible to an external
 * archiving process.  Note that the data before this point is written out
 * to the kernel, but is not necessarily synced to disk.
 */
Datum
pg_current_xlog_location(PG_FUNCTION_ARGS)
{
      char        location[MAXFNAMELEN];

      /* Make sure we have an up-to-date local LogwrtResult */
      {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            LogwrtResult = xlogctl->LogwrtResult;
            SpinLockRelease(&xlogctl->info_lck);
      }

      snprintf(location, sizeof(location), "%X/%X",
                   LogwrtResult.Write.xlogid, LogwrtResult.Write.xrecoff);
      PG_RETURN_TEXT_P(cstring_to_text(location));
}

/*
 * Report the current WAL insert location (same format as pg_start_backup etc)
 *
 * This function is mostly for debugging purposes.
 */
Datum
pg_current_xlog_insert_location(PG_FUNCTION_ARGS)
{
      XLogCtlInsert *Insert = &XLogCtl->Insert;
      XLogRecPtr  current_recptr;
      char        location[MAXFNAMELEN];

      /*
       * Get the current end-of-WAL position ... shared lock is sufficient
       */
      LWLockAcquire(WALInsertLock, LW_SHARED);
      INSERT_RECPTR(current_recptr, Insert, Insert->curridx);
      LWLockRelease(WALInsertLock);

      snprintf(location, sizeof(location), "%X/%X",
                   current_recptr.xlogid, current_recptr.xrecoff);
      PG_RETURN_TEXT_P(cstring_to_text(location));
}

/*
 * Compute an xlog file name and decimal byte offset given a WAL location,
 * such as is returned by pg_stop_backup() or pg_xlog_switch().
 *
 * Note that a location exactly at a segment boundary is taken to be in
 * the previous segment.  This is usually the right thing, since the
 * expected usage is to determine which xlog file(s) are ready to archive.
 */
Datum
pg_xlogfile_name_offset(PG_FUNCTION_ARGS)
{
      text     *location = PG_GETARG_TEXT_P(0);
      char     *locationstr;
      unsigned int uxlogid;
      unsigned int uxrecoff;
      uint32            xlogid;
      uint32            xlogseg;
      uint32            xrecoff;
      XLogRecPtr  locationpoint;
      char        xlogfilename[MAXFNAMELEN];
      Datum       values[2];
      bool        isnull[2];
      TupleDesc   resultTupleDesc;
      HeapTuple   resultHeapTuple;
      Datum       result;

      /*
       * Read input and parse
       */
      locationstr = text_to_cstring(location);

      if (sscanf(locationstr, "%X/%X", &uxlogid, &uxrecoff) != 2)
            ereport(ERROR,
                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                         errmsg("could not parse transaction log location \"%s\"",
                                    locationstr)));

      locationpoint.xlogid = uxlogid;
      locationpoint.xrecoff = uxrecoff;

      /*
       * Construct a tuple descriptor for the result row.  This must match this
       * function's pg_proc entry!
       */
      resultTupleDesc = CreateTemplateTupleDesc(2, false);
      TupleDescInitEntry(resultTupleDesc, (AttrNumber) 1, "file_name",
                                 TEXTOID, -1, 0);
      TupleDescInitEntry(resultTupleDesc, (AttrNumber) 2, "file_offset",
                                 INT4OID, -1, 0);

      resultTupleDesc = BlessTupleDesc(resultTupleDesc);

      /*
       * xlogfilename
       */
      XLByteToPrevSeg(locationpoint, xlogid, xlogseg);
      XLogFileName(xlogfilename, ThisTimeLineID, xlogid, xlogseg);

      values[0] = CStringGetTextDatum(xlogfilename);
      isnull[0] = false;

      /*
       * offset
       */
      xrecoff = locationpoint.xrecoff - xlogseg * XLogSegSize;

      values[1] = UInt32GetDatum(xrecoff);
      isnull[1] = false;

      /*
       * Tuple jam: Having first prepared your Datums, then squash together
       */
      resultHeapTuple = heap_form_tuple(resultTupleDesc, values, isnull);

      result = HeapTupleGetDatum(resultHeapTuple);

      PG_RETURN_DATUM(result);
}

/*
 * Compute an xlog file name given a WAL location,
 * such as is returned by pg_stop_backup() or pg_xlog_switch().
 */
Datum
pg_xlogfile_name(PG_FUNCTION_ARGS)
{
      text     *location = PG_GETARG_TEXT_P(0);
      char     *locationstr;
      unsigned int uxlogid;
      unsigned int uxrecoff;
      uint32            xlogid;
      uint32            xlogseg;
      XLogRecPtr  locationpoint;
      char        xlogfilename[MAXFNAMELEN];

      locationstr = text_to_cstring(location);

      if (sscanf(locationstr, "%X/%X", &uxlogid, &uxrecoff) != 2)
            ereport(ERROR,
                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                         errmsg("could not parse transaction log location \"%s\"",
                                    locationstr)));

      locationpoint.xlogid = uxlogid;
      locationpoint.xrecoff = uxrecoff;

      XLByteToPrevSeg(locationpoint, xlogid, xlogseg);
      XLogFileName(xlogfilename, ThisTimeLineID, xlogid, xlogseg);

      PG_RETURN_TEXT_P(cstring_to_text(xlogfilename));
}

/*
 * read_backup_label: check to see if a backup_label file is present
 *
 * If we see a backup_label during recovery, we assume that we are recovering
 * from a backup dump file, and we therefore roll forward from the checkpoint
 * identified by the label file, NOT what pg_control says.  This avoids the
 * problem that pg_control might have been archived one or more checkpoints
 * later than the start of the dump, and so if we rely on it as the start
 * point, we will fail to restore a consistent database state.
 *
 * We also attempt to retrieve the corresponding backup history file.
 * If successful, set *minRecoveryLoc to constrain valid PITR stopping
 * points.
 *
 * Returns TRUE if a backup_label was found (and fills the checkpoint
 * location into *checkPointLoc); returns FALSE if not.
 */
static bool
read_backup_label(XLogRecPtr *checkPointLoc, XLogRecPtr *minRecoveryLoc)
{
      XLogRecPtr  startpoint;
      XLogRecPtr  stoppoint;
      char        histfilename[MAXFNAMELEN];
      char        histfilepath[MAXPGPATH];
      char        startxlogfilename[MAXFNAMELEN];
      char        stopxlogfilename[MAXFNAMELEN];
      TimeLineID  tli;
      uint32            _logId;
      uint32            _logSeg;
      FILE     *lfp;
      FILE     *fp;
      char        ch;

      /* Default is to not constrain recovery stop point */
      minRecoveryLoc->xlogid = 0;
      minRecoveryLoc->xrecoff = 0;

      /*
       * See if label file is present
       */
      lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
      if (!lfp)
      {
            if (errno != ENOENT)
                  ereport(FATAL,
                              (errcode_for_file_access(),
                               errmsg("could not read file \"%s\": %m",
                                          BACKUP_LABEL_FILE)));
            return false;                 /* it's not there, all is fine */
      }

      /*
       * Read and parse the START WAL LOCATION and CHECKPOINT lines (this code
       * is pretty crude, but we are not expecting any variability in the file
       * format).
       */
      if (fscanf(lfp, "START WAL LOCATION: %X/%X (file %08X%16s)%c",
                     &startpoint.xlogid, &startpoint.xrecoff, &tli,
                     startxlogfilename, &ch) != 5 || ch != '\n')
            ereport(FATAL,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
      if (fscanf(lfp, "CHECKPOINT LOCATION: %X/%X%c",
                     &checkPointLoc->xlogid, &checkPointLoc->xrecoff,
                     &ch) != 3 || ch != '\n')
            ereport(FATAL,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
      if (ferror(lfp) || FreeFile(lfp))
            ereport(FATAL,
                        (errcode_for_file_access(),
                         errmsg("could not read file \"%s\": %m",
                                    BACKUP_LABEL_FILE)));

      /*
       * Try to retrieve the backup history file (no error if we can't)
       */
      XLByteToSeg(startpoint, _logId, _logSeg);
      BackupHistoryFileName(histfilename, tli, _logId, _logSeg,
                                      startpoint.xrecoff % XLogSegSize);

      if (InArchiveRecovery)
            RestoreArchivedFile(histfilepath, histfilename, "RECOVERYHISTORY", 0);
      else
            BackupHistoryFilePath(histfilepath, tli, _logId, _logSeg,
                                            startpoint.xrecoff % XLogSegSize);

      fp = AllocateFile(histfilepath, "r");
      if (fp)
      {
            /*
             * Parse history file to identify stop point.
             */
            if (fscanf(fp, "START WAL LOCATION: %X/%X (file %24s)%c",
                           &startpoint.xlogid, &startpoint.xrecoff, startxlogfilename,
                           &ch) != 4 || ch != '\n')
                  ereport(FATAL,
                              (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                               errmsg("invalid data in file \"%s\"", histfilename)));
            if (fscanf(fp, "STOP WAL LOCATION: %X/%X (file %24s)%c",
                           &stoppoint.xlogid, &stoppoint.xrecoff, stopxlogfilename,
                           &ch) != 4 || ch != '\n')
                  ereport(FATAL,
                              (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                               errmsg("invalid data in file \"%s\"", histfilename)));
            *minRecoveryLoc = stoppoint;
            if (ferror(fp) || FreeFile(fp))
                  ereport(FATAL,
                              (errcode_for_file_access(),
                               errmsg("could not read file \"%s\": %m",
                                          histfilepath)));
      }

      return true;
}

/*
 * Error context callback for errors occurring during rm_redo().
 */
static void
rm_redo_error_callback(void *arg)
{
      XLogRecord *record = (XLogRecord *) arg;
      StringInfoData buf;

      initStringInfo(&buf);
      RmgrTable[record->xl_rmid].rm_desc(&buf,
                                                         record->xl_info,
                                                         XLogRecGetData(record));

      /* don't bother emitting empty description */
      if (buf.len > 0)
            errcontext("xlog redo %s", buf.data);

      pfree(buf.data);
}

/*
 * BackupInProgress: check if online backup mode is active
 *
 * This is done by checking for existence of the "backup_label" file.
 */
bool
BackupInProgress(void)
{
      struct stat stat_buf;

      return (stat(BACKUP_LABEL_FILE, &stat_buf) == 0);
}

/*
 * CancelBackup: rename the "backup_label" file to cancel backup mode
 *
 * If the "backup_label" file exists, it will be renamed to "backup_label.old".
 * Note that this will render an online backup in progress useless.
 * To correctly finish an online backup, pg_stop_backup must be called.
 */
void
CancelBackup(void)
{
      struct stat stat_buf;

      /* if the file is not there, return */
      if (stat(BACKUP_LABEL_FILE, &stat_buf) < 0)
            return;

      /* remove leftover file from previously cancelled backup if it exists */
      unlink(BACKUP_LABEL_OLD);

      if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) == 0)
      {
            ereport(LOG,
                        (errmsg("online backup mode cancelled"),
                         errdetail("\"%s\" was renamed to \"%s\".",
                                       BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
      }
      else
      {
            ereport(WARNING,
                        (errcode_for_file_access(),
                         errmsg("online backup mode was not cancelled"),
                         errdetail("Could not rename \"%s\" to \"%s\": %m.",
                                       BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
      }
}

/* ------------------------------------------------------
 *    Startup Process main entry point and signal handlers
 * ------------------------------------------------------
 */

/*
 * startupproc_quickdie() occurs when signalled SIGQUIT by the postmaster.
 *
 * Some backend has bought the farm,
 * so we need to stop what we're doing and exit.
 */
static void
startupproc_quickdie(SIGNAL_ARGS)
{
      PG_SETMASK(&BlockSig);

      /*
       * We DO NOT want to run proc_exit() callbacks -- we're here because
       * shared memory may be corrupted, so we don't want to try to clean up our
       * transaction.  Just nail the windows shut and get out of town.  Now that
       * there's an atexit callback to prevent third-party code from breaking
       * things by calling exit() directly, we have to reset the callbacks
       * explicitly to make this work as intended.
       */
      on_exit_reset();

      /*
       * Note we do exit(2) not exit(0).  This is to force the postmaster into a
       * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
       * backend.  This is necessary precisely because we don't clean up our
       * shared memory state.  (The "dead man switch" mechanism in pmsignal.c
       * should ensure the postmaster sees this as a crash, too, but no harm in
       * being doubly sure.)
       */
      exit(2);
}


/* SIGHUP: set flag to re-read config file at next convenient time */
static void
StartupProcSigHupHandler(SIGNAL_ARGS)
{
      got_SIGHUP = true;
}

/* SIGTERM: set flag to abort redo and exit */
static void
StartupProcShutdownHandler(SIGNAL_ARGS)
{
      if (in_restore_command)
            proc_exit(1);
      else
            shutdown_requested = true;
}

/* Main entry point for startup process */
void
StartupProcessMain(void)
{
      /*
       * If possible, make this process a group leader, so that the postmaster
       * can signal any child processes too.
       */
#ifdef HAVE_SETSID
      if (setsid() < 0)
            elog(FATAL, "setsid() failed: %m");
#endif

      /*
       * Properly accept or ignore signals the postmaster might send us
       */
      pqsignal(SIGHUP, StartupProcSigHupHandler); /* reload config file */
      pqsignal(SIGINT, SIG_IGN);    /* ignore query cancel */
      pqsignal(SIGTERM, StartupProcShutdownHandler);        /* request shutdown */
      pqsignal(SIGQUIT, startupproc_quickdie);  /* hard crash time */
      pqsignal(SIGALRM, SIG_IGN);
      pqsignal(SIGPIPE, SIG_IGN);
      pqsignal(SIGUSR1, SIG_IGN);
      pqsignal(SIGUSR2, SIG_IGN);

      /*
       * Reset some signals that are accepted by postmaster but not here
       */
      pqsignal(SIGCHLD, SIG_DFL);
      pqsignal(SIGTTIN, SIG_DFL);
      pqsignal(SIGTTOU, SIG_DFL);
      pqsignal(SIGCONT, SIG_DFL);
      pqsignal(SIGWINCH, SIG_DFL);

      /*
       * Unblock signals (they were blocked when the postmaster forked us)
       */
      PG_SETMASK(&UnBlockSig);

      StartupXLOG();

      BuildFlatFiles(false);

      /*
       * Exit normally. Exit code 0 tells postmaster that we completed recovery
       * successfully.
       */
      proc_exit(0);
}

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