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bgwriter.c

/*-------------------------------------------------------------------------
 *
 * bgwriter.c
 *
 * The background writer (bgwriter) is new as of Postgres 8.0.    It attempts
 * to keep regular backends from having to write out dirty shared buffers
 * (which they would only do when needing to free a shared buffer to read in
 * another page).  In the best scenario all writes from shared buffers will
 * be issued by the background writer process.  However, regular backends are
 * still empowered to issue writes if the bgwriter fails to maintain enough
 * clean shared buffers.
 *
 * The bgwriter is also charged with handling all checkpoints.    It will
 * automatically dispatch a checkpoint after a certain amount of time has
 * elapsed since the last one, and it can be signaled to perform requested
 * checkpoints as well.  (The GUC parameter that mandates a checkpoint every
 * so many WAL segments is implemented by having backends signal the bgwriter
 * when they fill WAL segments; the bgwriter itself doesn't watch for the
 * condition.)
 *
 * The bgwriter is started by the postmaster as soon as the startup subprocess
 * finishes, or as soon as recovery begins if we are doing archive recovery.
 * It remains alive until the postmaster commands it to terminate.
 * Normal termination is by SIGUSR2, which instructs the bgwriter to execute
 * a shutdown checkpoint and then exit(0).      (All backends must be stopped
 * before SIGUSR2 is issued!)  Emergency termination is by SIGQUIT; like any
 * backend, the bgwriter will simply abort and exit on SIGQUIT.
 *
 * If the bgwriter exits unexpectedly, the postmaster treats that the same
 * as a backend crash: shared memory may be corrupted, so remaining backends
 * should be killed by SIGQUIT and then a recovery cycle started.  (Even if
 * shared memory isn't corrupted, we have lost information about which
 * files need to be fsync'd for the next checkpoint, and so a system
 * restart needs to be forced.)
 *
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *      $PostgreSQL: pgsql/src/backend/postmaster/bgwriter.c,v 1.62 2009/06/26 20:29:04 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>

#include "access/xlog_internal.h"
#include "catalog/pg_control.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/lwlock.h"
#include "storage/pmsignal.h"
#include "storage/shmem.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "tcop/tcopprot.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/resowner.h"


/*----------
 * Shared memory area for communication between bgwriter and backends
 *
 * The ckpt counters allow backends to watch for completion of a checkpoint
 * request they send.  Here's how it works:
 *    * At start of a checkpoint, bgwriter reads (and clears) the request flags
 *      and increments ckpt_started, while holding ckpt_lck.
 *    * On completion of a checkpoint, bgwriter sets ckpt_done to
 *      equal ckpt_started.
 *    * On failure of a checkpoint, bgwriter increments ckpt_failed
 *      and sets ckpt_done to equal ckpt_started.
 *
 * The algorithm for backends is:
 *    1. Record current values of ckpt_failed and ckpt_started, and
 *       set request flags, while holding ckpt_lck.
 *    2. Send signal to request checkpoint.
 *    3. Sleep until ckpt_started changes.  Now you know a checkpoint has
 *       begun since you started this algorithm (although *not* that it was
 *       specifically initiated by your signal), and that it is using your flags.
 *    4. Record new value of ckpt_started.
 *    5. Sleep until ckpt_done >= saved value of ckpt_started.  (Use modulo
 *       arithmetic here in case counters wrap around.)  Now you know a
 *       checkpoint has started and completed, but not whether it was
 *       successful.
 *    6. If ckpt_failed is different from the originally saved value,
 *       assume request failed; otherwise it was definitely successful.
 *
 * ckpt_flags holds the OR of the checkpoint request flags sent by all
 * requesting backends since the last checkpoint start.  The flags are
 * chosen so that OR'ing is the correct way to combine multiple requests.
 *
 * num_backend_writes is used to count the number of buffer writes performed
 * by non-bgwriter processes.  This counter should be wide enough that it
 * can't overflow during a single bgwriter cycle.
 *
 * The requests array holds fsync requests sent by backends and not yet
 * absorbed by the bgwriter.
 *
 * Unlike the checkpoint fields, num_backend_writes and the requests
 * fields are protected by BgWriterCommLock.
 *----------
 */
00114 typedef struct
{
      RelFileNode rnode;
      ForkNumber  forknum;
      BlockNumber segno;                  /* see md.c for special values */
      /* might add a real request-type field later; not needed yet */
} BgWriterRequest;

00122 typedef struct
{
      pid_t       bgwriter_pid;     /* PID of bgwriter (0 if not started) */

      slock_t           ckpt_lck;         /* protects all the ckpt_* fields */

      int               ckpt_started;     /* advances when checkpoint starts */
      int               ckpt_done;        /* advances when checkpoint done */
      int               ckpt_failed;      /* advances when checkpoint fails */

      int               ckpt_flags;       /* checkpoint flags, as defined in xlog.h */

      uint32            num_backend_writes;           /* counts non-bgwriter buffer writes */

      int               num_requests;     /* current # of requests */
      int               max_requests;     /* allocated array size */
      BgWriterRequest requests[1];  /* VARIABLE LENGTH ARRAY */
} BgWriterShmemStruct;

static BgWriterShmemStruct *BgWriterShmem;

/* interval for calling AbsorbFsyncRequests in CheckpointWriteDelay */
#define WRITES_PER_ABSORB           1000

/*
 * GUC parameters
 */
int               BgWriterDelay = 200;
int               CheckPointTimeout = 300;
int               CheckPointWarning = 30;
double            CheckPointCompletionTarget = 0.5;

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

/*
 * Private state
 */
static bool am_bg_writer = false;

static bool ckpt_active = false;

/* these values are valid when ckpt_active is true: */
static pg_time_t ckpt_start_time;
static XLogRecPtr ckpt_start_recptr;
static double ckpt_cached_elapsed;

static pg_time_t last_checkpoint_time;
static pg_time_t last_xlog_switch_time;

/* Prototypes for private functions */

static void CheckArchiveTimeout(void);
static void BgWriterNap(void);
static bool IsCheckpointOnSchedule(double progress);
static bool ImmediateCheckpointRequested(void);

/* Signal handlers */

static void bg_quickdie(SIGNAL_ARGS);
static void BgSigHupHandler(SIGNAL_ARGS);
static void ReqCheckpointHandler(SIGNAL_ARGS);
static void ReqShutdownHandler(SIGNAL_ARGS);


/*
 * Main entry point for bgwriter process
 *
 * This is invoked from BootstrapMain, which has already created the basic
 * execution environment, but not enabled signals yet.
 */
void
BackgroundWriterMain(void)
{
      sigjmp_buf  local_sigjmp_buf;
      MemoryContext bgwriter_context;

      BgWriterShmem->bgwriter_pid = MyProcPid;
      am_bg_writer = true;

      /*
       * If possible, make this process a group leader, so that the postmaster
       * can signal any child processes too.    (bgwriter probably never has any
       * child processes, but for consistency we make all postmaster child
       * processes do this.)
       */
#ifdef HAVE_SETSID
      if (setsid() < 0)
            elog(FATAL, "setsid() failed: %m");
#endif

      /*
       * Properly accept or ignore signals the postmaster might send us
       *
       * Note: we deliberately ignore SIGTERM, because during a standard Unix
       * system shutdown cycle, init will SIGTERM all processes at once.      We
       * want to wait for the backends to exit, whereupon the postmaster will
       * tell us it's okay to shut down (via SIGUSR2).
       *
       * SIGUSR1 is presently unused; keep it spare in case someday we want this
       * process to participate in sinval messaging.
       */
      pqsignal(SIGHUP, BgSigHupHandler);  /* set flag to read config file */
      pqsignal(SIGINT, ReqCheckpointHandler);         /* request checkpoint */
      pqsignal(SIGTERM, SIG_IGN); /* ignore SIGTERM */
      pqsignal(SIGQUIT, bg_quickdie);           /* hard crash time */
      pqsignal(SIGALRM, SIG_IGN);
      pqsignal(SIGPIPE, SIG_IGN);
      pqsignal(SIGUSR1, SIG_IGN); /* reserve for sinval */
      pqsignal(SIGUSR2, ReqShutdownHandler);          /* request shutdown */

      /*
       * 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);

      /* We allow SIGQUIT (quickdie) at all times */
#ifdef HAVE_SIGPROCMASK
      sigdelset(&BlockSig, SIGQUIT);
#else
      BlockSig &= ~(sigmask(SIGQUIT));
#endif

      /*
       * Initialize so that first time-driven event happens at the correct time.
       */
      last_checkpoint_time = last_xlog_switch_time = (pg_time_t) time(NULL);

      /*
       * Create a resource owner to keep track of our resources (currently only
       * buffer pins).
       */
      CurrentResourceOwner = ResourceOwnerCreate(NULL, "Background Writer");

      /*
       * Create a memory context that we will do all our work in.  We do this so
       * that we can reset the context during error recovery and thereby avoid
       * possible memory leaks.  Formerly this code just ran in
       * TopMemoryContext, but resetting that would be a really bad idea.
       */
      bgwriter_context = AllocSetContextCreate(TopMemoryContext,
                                                                   "Background Writer",
                                                                   ALLOCSET_DEFAULT_MINSIZE,
                                                                   ALLOCSET_DEFAULT_INITSIZE,
                                                                   ALLOCSET_DEFAULT_MAXSIZE);
      MemoryContextSwitchTo(bgwriter_context);

      /*
       * If an exception is encountered, processing resumes here.
       *
       * See notes in postgres.c about the design of this coding.
       */
      if (sigsetjmp(local_sigjmp_buf, 1) != 0)
      {
            /* Since not using PG_TRY, must reset error stack by hand */
            error_context_stack = NULL;

            /* Prevent interrupts while cleaning up */
            HOLD_INTERRUPTS();

            /* Report the error to the server log */
            EmitErrorReport();

            /*
             * These operations are really just a minimal subset of
             * AbortTransaction().  We don't have very many resources to worry
             * about in bgwriter, but we do have LWLocks, buffers, and temp files.
             */
            LWLockReleaseAll();
            AbortBufferIO();
            UnlockBuffers();
            /* buffer pins are released here: */
            ResourceOwnerRelease(CurrentResourceOwner,
                                           RESOURCE_RELEASE_BEFORE_LOCKS,
                                           false, true);
            /* we needn't bother with the other ResourceOwnerRelease phases */
            AtEOXact_Buffers(false);
            AtEOXact_Files();
            AtEOXact_HashTables(false);

            /* Warn any waiting backends that the checkpoint failed. */
            if (ckpt_active)
            {
                  /* use volatile pointer to prevent code rearrangement */
                  volatile BgWriterShmemStruct *bgs = BgWriterShmem;

                  SpinLockAcquire(&bgs->ckpt_lck);
                  bgs->ckpt_failed++;
                  bgs->ckpt_done = bgs->ckpt_started;
                  SpinLockRelease(&bgs->ckpt_lck);

                  ckpt_active = false;
            }

            /*
             * Now return to normal top-level context and clear ErrorContext for
             * next time.
             */
            MemoryContextSwitchTo(bgwriter_context);
            FlushErrorState();

            /* Flush any leaked data in the top-level context */
            MemoryContextResetAndDeleteChildren(bgwriter_context);

            /* Now we can allow interrupts again */
            RESUME_INTERRUPTS();

            /*
             * Sleep at least 1 second after any error.  A write error is likely
             * to be repeated, and we don't want to be filling the error logs as
             * fast as we can.
             */
            pg_usleep(1000000L);

            /*
             * Close all open files after any error.  This is helpful on Windows,
             * where holding deleted files open causes various strange errors.
             * It's not clear we need it elsewhere, but shouldn't hurt.
             */
            smgrcloseall();
      }

      /* We can now handle ereport(ERROR) */
      PG_exception_stack = &local_sigjmp_buf;

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

      /*
       * Loop forever
       */
      for (;;)
      {
            bool        do_checkpoint = false;
            int               flags = 0;
            pg_time_t   now;
            int               elapsed_secs;

            /*
             * Emergency bailout if postmaster has died.  This is to avoid the
             * necessity for manual cleanup of all postmaster children.
             */
            if (!PostmasterIsAlive(true))
                  exit(1);

            /*
             * Process any requests or signals received recently.
             */
            AbsorbFsyncRequests();

            if (got_SIGHUP)
            {
                  got_SIGHUP = false;
                  ProcessConfigFile(PGC_SIGHUP);
            }
            if (checkpoint_requested)
            {
                  checkpoint_requested = false;
                  do_checkpoint = true;
                  BgWriterStats.m_requested_checkpoints++;
            }
            if (shutdown_requested)
            {
                  /*
                   * From here on, elog(ERROR) should end with exit(1), not send
                   * control back to the sigsetjmp block above
                   */
                  ExitOnAnyError = true;
                  /* Close down the database */
                  ShutdownXLOG(0, 0);
                  /* Normal exit from the bgwriter is here */
                  proc_exit(0);           /* done */
            }

            /*
             * Force a checkpoint if too much time has elapsed since the last one.
             * Note that we count a timed checkpoint in stats only when this
             * occurs without an external request, but we set the CAUSE_TIME flag
             * bit even if there is also an external request.
             */
            now = (pg_time_t) time(NULL);
            elapsed_secs = now - last_checkpoint_time;
            if (elapsed_secs >= CheckPointTimeout)
            {
                  if (!do_checkpoint)
                        BgWriterStats.m_timed_checkpoints++;
                  do_checkpoint = true;
                  flags |= CHECKPOINT_CAUSE_TIME;
            }

            /*
             * Do a checkpoint if requested, otherwise do one cycle of
             * dirty-buffer writing.
             */
            if (do_checkpoint)
            {
                  bool        ckpt_performed = false;
                  bool        do_restartpoint;

                  /* use volatile pointer to prevent code rearrangement */
                  volatile BgWriterShmemStruct *bgs = BgWriterShmem;

                  /*
                   * Check if we should perform a checkpoint or a restartpoint. As a
                   * side-effect, RecoveryInProgress() initializes TimeLineID if
                   * it's not set yet.
                   */
                  do_restartpoint = RecoveryInProgress();

                  /*
                   * Atomically fetch the request flags to figure out what kind of a
                   * checkpoint we should perform, and increase the started-counter
                   * to acknowledge that we've started a new checkpoint.
                   */
                  SpinLockAcquire(&bgs->ckpt_lck);
                  flags |= bgs->ckpt_flags;
                  bgs->ckpt_flags = 0;
                  bgs->ckpt_started++;
                  SpinLockRelease(&bgs->ckpt_lck);

                  /*
                   * The end-of-recovery checkpoint is a real checkpoint that's
                   * performed while we're still in recovery.
                   */
                  if (flags & CHECKPOINT_END_OF_RECOVERY)
                        do_restartpoint = false;

                  /*
                   * We will warn if (a) too soon since last checkpoint (whatever
                   * caused it) and (b) somebody set the CHECKPOINT_CAUSE_XLOG flag
                   * since the last checkpoint start.  Note in particular that this
                   * implementation will not generate warnings caused by
                   * CheckPointTimeout < CheckPointWarning.
                   */
                  if (!do_restartpoint &&
                        (flags & CHECKPOINT_CAUSE_XLOG) &&
                        elapsed_secs < CheckPointWarning)
                        ereport(LOG,
                                    (errmsg_plural("checkpoints are occurring too frequently (%d second apart)",
                        "checkpoints are occurring too frequently (%d seconds apart)",
                                                         elapsed_secs,
                                                         elapsed_secs),
                                     errhint("Consider increasing the configuration parameter \"checkpoint_segments\".")));

                  /*
                   * Initialize bgwriter-private variables used during checkpoint.
                   */
                  ckpt_active = true;
                  if (!do_restartpoint)
                        ckpt_start_recptr = GetInsertRecPtr();
                  ckpt_start_time = now;
                  ckpt_cached_elapsed = 0;

                  /*
                   * Do the checkpoint.
                   */
                  if (!do_restartpoint)
                  {
                        CreateCheckPoint(flags);
                        ckpt_performed = true;
                  }
                  else
                        ckpt_performed = CreateRestartPoint(flags);

                  /*
                   * After any checkpoint, close all smgr files.  This is so we
                   * won't hang onto smgr references to deleted files indefinitely.
                   */
                  smgrcloseall();

                  /*
                   * Indicate checkpoint completion to any waiting backends.
                   */
                  SpinLockAcquire(&bgs->ckpt_lck);
                  bgs->ckpt_done = bgs->ckpt_started;
                  SpinLockRelease(&bgs->ckpt_lck);

                  if (ckpt_performed)
                  {
                        /*
                         * Note we record the checkpoint start time not end time as
                         * last_checkpoint_time.  This is so that time-driven
                         * checkpoints happen at a predictable spacing.
                         */
                        last_checkpoint_time = now;
                  }
                  else
                  {
                        /*
                         * We were not able to perform the restartpoint (checkpoints
                         * throw an ERROR in case of error).  Most likely because we
                         * have not received any new checkpoint WAL records since the
                         * last restartpoint. Try again in 15 s.
                         */
                        last_checkpoint_time = now - CheckPointTimeout + 15;
                  }

                  ckpt_active = false;
            }
            else
                  BgBufferSync();

            /* Check for archive_timeout and switch xlog files if necessary. */
            CheckArchiveTimeout();

            /* Nap for the configured time. */
            BgWriterNap();
      }
}

/*
 * CheckArchiveTimeout -- check for archive_timeout and switch xlog files
 *          if needed
 */
static void
CheckArchiveTimeout(void)
{
      pg_time_t   now;
      pg_time_t   last_time;

      if (XLogArchiveTimeout <= 0 || RecoveryInProgress())
            return;

      now = (pg_time_t) time(NULL);

      /* First we do a quick check using possibly-stale local state. */
      if ((int) (now - last_xlog_switch_time) < XLogArchiveTimeout)
            return;

      /*
       * Update local state ... note that last_xlog_switch_time is the last time
       * a switch was performed *or requested*.
       */
      last_time = GetLastSegSwitchTime();

      last_xlog_switch_time = Max(last_xlog_switch_time, last_time);

      /* Now we can do the real check */
      if ((int) (now - last_xlog_switch_time) >= XLogArchiveTimeout)
      {
            XLogRecPtr  switchpoint;

            /* OK, it's time to switch */
            switchpoint = RequestXLogSwitch();

            /*
             * If the returned pointer points exactly to a segment boundary,
             * assume nothing happened.
             */
            if ((switchpoint.xrecoff % XLogSegSize) != 0)
                  ereport(DEBUG1,
                        (errmsg("transaction log switch forced (archive_timeout=%d)",
                                    XLogArchiveTimeout)));

            /*
             * Update state in any case, so we don't retry constantly when the
             * system is idle.
             */
            last_xlog_switch_time = now;
      }
}

/*
 * BgWriterNap -- Nap for the configured time or until a signal is received.
 */
static void
BgWriterNap(void)
{
      long        udelay;

      /*
       * Send off activity statistics to the stats collector
       */
      pgstat_send_bgwriter();

      /*
       * Nap for the configured time, or sleep for 10 seconds if there is no
       * bgwriter activity configured.
       *
       * On some platforms, signals won't interrupt the sleep.  To ensure we
       * respond reasonably promptly when someone signals us, break down the
       * sleep into 1-second increments, and check for interrupts after each
       * nap.
       *
       * We absorb pending requests after each short sleep.
       */
      if (bgwriter_lru_maxpages > 0 || ckpt_active)
            udelay = BgWriterDelay * 1000L;
      else if (XLogArchiveTimeout > 0)
            udelay = 1000000L;            /* One second */
      else
            udelay = 10000000L;           /* Ten seconds */

      while (udelay > 999999L)
      {
            if (got_SIGHUP || shutdown_requested ||
            (ckpt_active ? ImmediateCheckpointRequested() : checkpoint_requested))
                  break;
            pg_usleep(1000000L);
            AbsorbFsyncRequests();
            udelay -= 1000000L;
      }

      if (!(got_SIGHUP || shutdown_requested ||
        (ckpt_active ? ImmediateCheckpointRequested() : checkpoint_requested)))
            pg_usleep(udelay);
}

/*
 * Returns true if an immediate checkpoint request is pending.    (Note that
 * this does not check the *current* checkpoint's IMMEDIATE flag, but whether
 * there is one pending behind it.)
 */
static bool
ImmediateCheckpointRequested(void)
{
      if (checkpoint_requested)
      {
            volatile BgWriterShmemStruct *bgs = BgWriterShmem;

            /*
             * We don't need to acquire the ckpt_lck in this case because we're
             * only looking at a single flag bit.
             */
            if (bgs->ckpt_flags & CHECKPOINT_IMMEDIATE)
                  return true;
      }
      return false;
}

/*
 * CheckpointWriteDelay -- yield control to bgwriter during a checkpoint
 *
 * This function is called after each page write performed by BufferSync().
 * It is responsible for keeping the bgwriter's normal activities in
 * progress during a long checkpoint, and for throttling BufferSync()'s
 * write rate to hit checkpoint_completion_target.
 *
 * The checkpoint request flags should be passed in; currently the only one
 * examined is CHECKPOINT_IMMEDIATE, which disables delays between writes.
 *
 * 'progress' is an estimate of how much of the work has been done, as a
 * fraction between 0.0 meaning none, and 1.0 meaning all done.
 */
void
CheckpointWriteDelay(int flags, double progress)
{
      static int  absorb_counter = WRITES_PER_ABSORB;

      /* Do nothing if checkpoint is being executed by non-bgwriter process */
      if (!am_bg_writer)
            return;

      /*
       * Perform the usual bgwriter duties and take a nap, unless we're behind
       * schedule, in which case we just try to catch up as quickly as possible.
       */
      if (!(flags & CHECKPOINT_IMMEDIATE) &&
            !shutdown_requested &&
            !ImmediateCheckpointRequested() &&
            IsCheckpointOnSchedule(progress))
      {
            if (got_SIGHUP)
            {
                  got_SIGHUP = false;
                  ProcessConfigFile(PGC_SIGHUP);
            }

            AbsorbFsyncRequests();
            absorb_counter = WRITES_PER_ABSORB;

            BgBufferSync();
            CheckArchiveTimeout();
            BgWriterNap();
      }
      else if (--absorb_counter <= 0)
      {
            /*
             * Absorb pending fsync requests after each WRITES_PER_ABSORB write
             * operations even when we don't sleep, to prevent overflow of the
             * fsync request queue.
             */
            AbsorbFsyncRequests();
            absorb_counter = WRITES_PER_ABSORB;
      }
}

/*
 * IsCheckpointOnSchedule -- are we on schedule to finish this checkpoint
 *           in time?
 *
 * Compares the current progress against the time/segments elapsed since last
 * checkpoint, and returns true if the progress we've made this far is greater
 * than the elapsed time/segments.
 */
static bool
IsCheckpointOnSchedule(double progress)
{
      XLogRecPtr  recptr;
      struct timeval now;
      double            elapsed_xlogs,
                        elapsed_time;

      Assert(ckpt_active);

      /* Scale progress according to checkpoint_completion_target. */
      progress *= CheckPointCompletionTarget;

      /*
       * Check against the cached value first. Only do the more expensive
       * calculations once we reach the target previously calculated. Since
       * neither time or WAL insert pointer moves backwards, a freshly
       * calculated value can only be greater than or equal to the cached value.
       */
      if (progress < ckpt_cached_elapsed)
            return false;

      /*
       * Check progress against WAL segments written and checkpoint_segments.
       *
       * We compare the current WAL insert location against the location
       * computed before calling CreateCheckPoint. The code in XLogInsert that
       * actually triggers a checkpoint when checkpoint_segments is exceeded
       * compares against RedoRecptr, so this is not completely accurate.
       * However, it's good enough for our purposes, we're only calculating an
       * estimate anyway.
       */
      if (!RecoveryInProgress())
      {
            recptr = GetInsertRecPtr();
            elapsed_xlogs =
                  (((double) (int32) (recptr.xlogid - ckpt_start_recptr.xlogid)) * XLogSegsPerFile +
                   ((double) recptr.xrecoff - (double) ckpt_start_recptr.xrecoff) / XLogSegSize) /
                  CheckPointSegments;

            if (progress < elapsed_xlogs)
            {
                  ckpt_cached_elapsed = elapsed_xlogs;
                  return false;
            }
      }

      /*
       * Check progress against time elapsed and checkpoint_timeout.
       */
      gettimeofday(&now, NULL);
      elapsed_time = ((double) ((pg_time_t) now.tv_sec - ckpt_start_time) +
                              now.tv_usec / 1000000.0) / CheckPointTimeout;

      if (progress < elapsed_time)
      {
            ckpt_cached_elapsed = elapsed_time;
            return false;
      }

      /* It looks like we're on schedule. */
      return true;
}


/* --------------------------------
 *          signal handler routines
 * --------------------------------
 */

/*
 * bg_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
bg_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
BgSigHupHandler(SIGNAL_ARGS)
{
      got_SIGHUP = true;
}

/* SIGINT: set flag to run a normal checkpoint right away */
static void
ReqCheckpointHandler(SIGNAL_ARGS)
{
      checkpoint_requested = true;
}

/* SIGUSR2: set flag to run a shutdown checkpoint and exit */
static void
ReqShutdownHandler(SIGNAL_ARGS)
{
      shutdown_requested = true;
}


/* --------------------------------
 *          communication with backends
 * --------------------------------
 */

/*
 * BgWriterShmemSize
 *          Compute space needed for bgwriter-related shared memory
 */
Size
BgWriterShmemSize(void)
{
      Size        size;

      /*
       * Currently, the size of the requests[] array is arbitrarily set equal to
       * NBuffers.  This may prove too large or small ...
       */
      size = offsetof(BgWriterShmemStruct, requests);
      size = add_size(size, mul_size(NBuffers, sizeof(BgWriterRequest)));

      return size;
}

/*
 * BgWriterShmemInit
 *          Allocate and initialize bgwriter-related shared memory
 */
void
BgWriterShmemInit(void)
{
      bool        found;

      BgWriterShmem = (BgWriterShmemStruct *)
            ShmemInitStruct("Background Writer Data",
                                    BgWriterShmemSize(),
                                    &found);
      if (BgWriterShmem == NULL)
            ereport(FATAL,
                        (errcode(ERRCODE_OUT_OF_MEMORY),
                         errmsg("not enough shared memory for background writer")));
      if (found)
            return;                             /* already initialized */

      MemSet(BgWriterShmem, 0, sizeof(BgWriterShmemStruct));
      SpinLockInit(&BgWriterShmem->ckpt_lck);
      BgWriterShmem->max_requests = NBuffers;
}

/*
 * RequestCheckpoint
 *          Called in backend processes to request a checkpoint
 *
 * 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).
 *    CHECKPOINT_WAIT: wait for completion before returning (otherwise,
 *          just signal bgwriter to do it, and return).
 *    CHECKPOINT_CAUSE_XLOG: checkpoint is requested due to xlog filling.
 *          (This affects logging, and in particular enables CheckPointWarning.)
 */
void
RequestCheckpoint(int flags)
{
      /* use volatile pointer to prevent code rearrangement */
      volatile BgWriterShmemStruct *bgs = BgWriterShmem;
      int               ntries;
      int               old_failed,
                        old_started;

      /*
       * If in a standalone backend, just do it ourselves.
       */
      if (!IsPostmasterEnvironment)
      {
            /*
             * There's no point in doing slow checkpoints in a standalone backend,
             * because there's no other backends the checkpoint could disrupt.
             */
            CreateCheckPoint(flags | CHECKPOINT_IMMEDIATE);

            /*
             * After any checkpoint, close all smgr files.  This is so we won't
             * hang onto smgr references to deleted files indefinitely.
             */
            smgrcloseall();

            return;
      }

      /*
       * Atomically set the request flags, and take a snapshot of the counters.
       * When we see ckpt_started > old_started, we know the flags we set here
       * have been seen by bgwriter.
       *
       * Note that we OR the flags with any existing flags, to avoid overriding
       * a "stronger" request by another backend.  The flag senses must be
       * chosen to make this work!
       */
      SpinLockAcquire(&bgs->ckpt_lck);

      old_failed = bgs->ckpt_failed;
      old_started = bgs->ckpt_started;
      bgs->ckpt_flags |= flags;

      SpinLockRelease(&bgs->ckpt_lck);

      /*
       * Send signal to request checkpoint.  It's possible that the bgwriter
       * hasn't started yet, or is in process of restarting, so we will retry a
       * few times if needed.  Also, if not told to wait for the checkpoint to
       * occur, we consider failure to send the signal to be nonfatal and merely
       * LOG it.
       */
      for (ntries = 0;; ntries++)
      {
            if (BgWriterShmem->bgwriter_pid == 0)
            {
                  if (ntries >= 20) /* max wait 2.0 sec */
                  {
                        elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
                        "could not request checkpoint because bgwriter not running");
                        break;
                  }
            }
            else if (kill(BgWriterShmem->bgwriter_pid, SIGINT) != 0)
            {
                  if (ntries >= 20) /* max wait 2.0 sec */
                  {
                        elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
                               "could not signal for checkpoint: %m");
                        break;
                  }
            }
            else
                  break;                        /* signal sent successfully */

            CHECK_FOR_INTERRUPTS();
            pg_usleep(100000L);           /* wait 0.1 sec, then retry */
      }

      /*
       * If requested, wait for completion.  We detect completion according to
       * the algorithm given above.
       */
      if (flags & CHECKPOINT_WAIT)
      {
            int               new_started,
                              new_failed;

            /* Wait for a new checkpoint to start. */
            for (;;)
            {
                  SpinLockAcquire(&bgs->ckpt_lck);
                  new_started = bgs->ckpt_started;
                  SpinLockRelease(&bgs->ckpt_lck);

                  if (new_started != old_started)
                        break;

                  CHECK_FOR_INTERRUPTS();
                  pg_usleep(100000L);
            }

            /*
             * We are waiting for ckpt_done >= new_started, in a modulo sense.
             */
            for (;;)
            {
                  int               new_done;

                  SpinLockAcquire(&bgs->ckpt_lck);
                  new_done = bgs->ckpt_done;
                  new_failed = bgs->ckpt_failed;
                  SpinLockRelease(&bgs->ckpt_lck);

                  if (new_done - new_started >= 0)
                        break;

                  CHECK_FOR_INTERRUPTS();
                  pg_usleep(100000L);
            }

            if (new_failed != old_failed)
                  ereport(ERROR,
                              (errmsg("checkpoint request failed"),
                               errhint("Consult recent messages in the server log for details.")));
      }
}

/*
 * ForwardFsyncRequest
 *          Forward a file-fsync request from a backend to the bgwriter
 *
 * Whenever a backend is compelled to write directly to a relation
 * (which should be seldom, if the bgwriter is getting its job done),
 * the backend calls this routine to pass over knowledge that the relation
 * is dirty and must be fsync'd before next checkpoint.  We also use this
 * opportunity to count such writes for statistical purposes.
 *
 * segno specifies which segment (not block!) of the relation needs to be
 * fsync'd.  (Since the valid range is much less than BlockNumber, we can
 * use high values for special flags; that's all internal to md.c, which
 * see for details.)
 *
 * If we are unable to pass over the request (at present, this can happen
 * if the shared memory queue is full), we return false.  That forces
 * the backend to do its own fsync.  We hope that will be even more seldom.
 *
 * Note: we presently make no attempt to eliminate duplicate requests
 * in the requests[] queue.  The bgwriter will have to eliminate dups
 * internally anyway, so we may as well avoid holding the lock longer
 * than we have to here.
 */
bool
ForwardFsyncRequest(RelFileNode rnode, ForkNumber forknum, BlockNumber segno)
{
      BgWriterRequest *request;

      if (!IsUnderPostmaster)
            return false;                 /* probably shouldn't even get here */

      if (am_bg_writer)
            elog(ERROR, "ForwardFsyncRequest must not be called in bgwriter");

      LWLockAcquire(BgWriterCommLock, LW_EXCLUSIVE);

      /* we count non-bgwriter writes even when the request queue overflows */
      BgWriterShmem->num_backend_writes++;

      if (BgWriterShmem->bgwriter_pid == 0 ||
            BgWriterShmem->num_requests >= BgWriterShmem->max_requests)
      {
            LWLockRelease(BgWriterCommLock);
            return false;
      }
      request = &BgWriterShmem->requests[BgWriterShmem->num_requests++];
      request->rnode = rnode;
      request->forknum = forknum;
      request->segno = segno;
      LWLockRelease(BgWriterCommLock);
      return true;
}

/*
 * AbsorbFsyncRequests
 *          Retrieve queued fsync requests and pass them to local smgr.
 *
 * This is exported because it must be called during CreateCheckPoint;
 * we have to be sure we have accepted all pending requests just before
 * we start fsync'ing.  Since CreateCheckPoint sometimes runs in
 * non-bgwriter processes, do nothing if not bgwriter.
 */
void
AbsorbFsyncRequests(void)
{
      BgWriterRequest *requests = NULL;
      BgWriterRequest *request;
      int               n;

      if (!am_bg_writer)
            return;

      /*
       * We have to PANIC if we fail to absorb all the pending requests (eg,
       * because our hashtable runs out of memory).  This is because the system
       * cannot run safely if we are unable to fsync what we have been told to
       * fsync.  Fortunately, the hashtable is so small that the problem is
       * quite unlikely to arise in practice.
       */
      START_CRIT_SECTION();

      /*
       * We try to avoid holding the lock for a long time by copying the request
       * array.
       */
      LWLockAcquire(BgWriterCommLock, LW_EXCLUSIVE);

      /* Transfer write count into pending pgstats message */
      BgWriterStats.m_buf_written_backend += BgWriterShmem->num_backend_writes;
      BgWriterShmem->num_backend_writes = 0;

      n = BgWriterShmem->num_requests;
      if (n > 0)
      {
            requests = (BgWriterRequest *) palloc(n * sizeof(BgWriterRequest));
            memcpy(requests, BgWriterShmem->requests, n * sizeof(BgWriterRequest));
      }
      BgWriterShmem->num_requests = 0;

      LWLockRelease(BgWriterCommLock);

      for (request = requests; n > 0; request++, n--)
            RememberFsyncRequest(request->rnode, request->forknum, request->segno);

      if (requests)
            pfree(requests);

      END_CRIT_SECTION();
}

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