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Sourcecode: postgresql-8.4 version File versions

varsup.c

/*-------------------------------------------------------------------------
 *
 * varsup.c
 *      postgres OID & XID variables support routines
 *
 * Copyright (c) 2000-2009, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *      $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.84 2009/04/23 00:23:45 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/clog.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "miscadmin.h"
#include "postmaster/autovacuum.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "utils/builtins.h"


/* Number of OIDs to prefetch (preallocate) per XLOG write */
#define VAR_OID_PREFETCH            8192

/* pointer to "variable cache" in shared memory (set up by shmem.c) */
VariableCache ShmemVariableCache = NULL;


/*
 * Allocate the next XID for my new transaction or subtransaction.
 *
 * The new XID is also stored into MyProc before returning.
 */
TransactionId
GetNewTransactionId(bool isSubXact)
{
      TransactionId xid;

      /*
       * During bootstrap initialization, we return the special bootstrap
       * transaction id.
       */
      if (IsBootstrapProcessingMode())
      {
            Assert(!isSubXact);
            MyProc->xid = BootstrapTransactionId;
            return BootstrapTransactionId;
      }

      LWLockAcquire(XidGenLock, LW_EXCLUSIVE);

      xid = ShmemVariableCache->nextXid;

      /*----------
       * Check to see if it's safe to assign another XID.  This protects against
       * catastrophic data loss due to XID wraparound.  The basic rules are:
       *
       * If we're past xidVacLimit, start trying to force autovacuum cycles.
       * If we're past xidWarnLimit, start issuing warnings.
       * If we're past xidStopLimit, refuse to execute transactions, unless
       * we are running in a standalone backend (which gives an escape hatch
       * to the DBA who somehow got past the earlier defenses).
       *
       * Test is coded to fall out as fast as possible during normal operation,
       * ie, when the vac limit is set and we haven't violated it.
       *----------
       */
      if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit) &&
            TransactionIdIsValid(ShmemVariableCache->xidVacLimit))
      {
            /*
             * To avoid swamping the postmaster with signals, we issue the autovac
             * request only once per 64K transaction starts.  This still gives
             * plenty of chances before we get into real trouble.
             */
            if (IsUnderPostmaster && (xid % 65536) == 0)
                  SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

            if (IsUnderPostmaster &&
             TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidStopLimit))
                  ereport(ERROR,
                              (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                               errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
                                          NameStr(ShmemVariableCache->limit_datname)),
                               errhint("Stop the postmaster and use a standalone backend to vacuum database \"%s\".\n"
                                           "You might also need to commit or roll back old prepared transactions.",
                                           NameStr(ShmemVariableCache->limit_datname))));
            else if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidWarnLimit))
                  ereport(WARNING,
                  (errmsg("database \"%s\" must be vacuumed within %u transactions",
                              NameStr(ShmemVariableCache->limit_datname),
                              ShmemVariableCache->xidWrapLimit - xid),
                   errhint("To avoid a database shutdown, execute a database-wide VACUUM in \"%s\".\n"
                               "You might also need to commit or roll back old prepared transactions.",
                               NameStr(ShmemVariableCache->limit_datname))));
      }

      /*
       * If we are allocating the first XID of a new page of the commit log,
       * zero out that commit-log page before returning. We must do this while
       * holding XidGenLock, else another xact could acquire and commit a later
       * XID before we zero the page.  Fortunately, a page of the commit log
       * holds 32K or more transactions, so we don't have to do this very often.
       *
       * Extend pg_subtrans too.
       */
      ExtendCLOG(xid);
      ExtendSUBTRANS(xid);

      /*
       * Now advance the nextXid counter.  This must not happen until after we
       * have successfully completed ExtendCLOG() --- if that routine fails, we
       * want the next incoming transaction to try it again.      We cannot assign
       * more XIDs until there is CLOG space for them.
       */
      TransactionIdAdvance(ShmemVariableCache->nextXid);

      /*
       * We must store the new XID into the shared ProcArray before releasing
       * XidGenLock.    This ensures that every active XID older than
       * latestCompletedXid is present in the ProcArray, which is essential for
       * correct OldestXmin tracking; see src/backend/access/transam/README.
       *
       * XXX by storing xid into MyProc without acquiring ProcArrayLock, we are
       * relying on fetch/store of an xid to be atomic, else other backends
       * might see a partially-set xid here.    But holding both locks at once
       * would be a nasty concurrency hit.  So for now, assume atomicity.
       *
       * Note that readers of PGPROC xid fields should be careful to fetch the
       * value only once, rather than assume they can read a value multiple
       * times and get the same answer each time.
       *
       * The same comments apply to the subxact xid count and overflow fields.
       *
       * A solution to the atomic-store problem would be to give each PGPROC its
       * own spinlock used only for fetching/storing that PGPROC's xid and
       * related fields.
       *
       * If there's no room to fit a subtransaction XID into PGPROC, set the
       * cache-overflowed flag instead.  This forces readers to look in
       * pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a
       * race-condition window, in that the new XID will not appear as running
       * until its parent link has been placed into pg_subtrans. However, that
       * will happen before anyone could possibly have a reason to inquire about
       * the status of the XID, so it seems OK.  (Snapshots taken during this
       * window *will* include the parent XID, so they will deliver the correct
       * answer later on when someone does have a reason to inquire.)
       */
      {
            /*
             * Use volatile pointer to prevent code rearrangement; other backends
             * could be examining my subxids info concurrently, and we don't want
             * them to see an invalid intermediate state, such as incrementing
             * nxids before filling the array entry.  Note we are assuming that
             * TransactionId and int fetch/store are atomic.
             */
            volatile PGPROC *myproc = MyProc;

            if (!isSubXact)
                  myproc->xid = xid;
            else
            {
                  int               nxids = myproc->subxids.nxids;

                  if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
                  {
                        myproc->subxids.xids[nxids] = xid;
                        myproc->subxids.nxids = nxids + 1;
                  }
                  else
                        myproc->subxids.overflowed = true;
            }
      }

      LWLockRelease(XidGenLock);

      return xid;
}

/*
 * Read nextXid but don't allocate it.
 */
TransactionId
ReadNewTransactionId(void)
{
      TransactionId xid;

      LWLockAcquire(XidGenLock, LW_SHARED);
      xid = ShmemVariableCache->nextXid;
      LWLockRelease(XidGenLock);

      return xid;
}

/*
 * Determine the last safe XID to allocate given the currently oldest
 * datfrozenxid (ie, the oldest XID that might exist in any database
 * of our cluster).
 */
void
SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
                                Name oldest_datname)
{
      TransactionId xidVacLimit;
      TransactionId xidWarnLimit;
      TransactionId xidStopLimit;
      TransactionId xidWrapLimit;
      TransactionId curXid;

      Assert(TransactionIdIsNormal(oldest_datfrozenxid));

      /*
       * The place where we actually get into deep trouble is halfway around
       * from the oldest potentially-existing XID.  (This calculation is
       * probably off by one or two counts, because the special XIDs reduce the
       * size of the loop a little bit.  But we throw in plenty of slop below,
       * so it doesn't matter.)
       */
      xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
      if (xidWrapLimit < FirstNormalTransactionId)
            xidWrapLimit += FirstNormalTransactionId;

      /*
       * We'll refuse to continue assigning XIDs in interactive mode once we get
       * within 1M transactions of data loss.  This leaves lots of room for the
       * DBA to fool around fixing things in a standalone backend, while not
       * being significant compared to total XID space. (Note that since
       * vacuuming requires one transaction per table cleaned, we had better be
       * sure there's lots of XIDs left...)
       */
      xidStopLimit = xidWrapLimit - 1000000;
      if (xidStopLimit < FirstNormalTransactionId)
            xidStopLimit -= FirstNormalTransactionId;

      /*
       * We'll start complaining loudly when we get within 10M transactions of
       * the stop point.      This is kind of arbitrary, but if you let your gas
       * gauge get down to 1% of full, would you be looking for the next gas
       * station?  We need to be fairly liberal about this number because there
       * are lots of scenarios where most transactions are done by automatic
       * clients that won't pay attention to warnings. (No, we're not gonna make
       * this configurable.  If you know enough to configure it, you know enough
       * to not get in this kind of trouble in the first place.)
       */
      xidWarnLimit = xidStopLimit - 10000000;
      if (xidWarnLimit < FirstNormalTransactionId)
            xidWarnLimit -= FirstNormalTransactionId;

      /*
       * We'll start trying to force autovacuums when oldest_datfrozenxid gets
       * to be more than autovacuum_freeze_max_age transactions old.
       *
       * Note: guc.c ensures that autovacuum_freeze_max_age is in a sane range,
       * so that xidVacLimit will be well before xidWarnLimit.
       *
       * Note: autovacuum_freeze_max_age is a PGC_POSTMASTER parameter so that
       * we don't have to worry about dealing with on-the-fly changes in its
       * value.  It doesn't look practical to update shared state from a GUC
       * assign hook (too many processes would try to execute the hook,
       * resulting in race conditions as well as crashes of those not connected
       * to shared memory).  Perhaps this can be improved someday.
       */
      xidVacLimit = oldest_datfrozenxid + autovacuum_freeze_max_age;
      if (xidVacLimit < FirstNormalTransactionId)
            xidVacLimit += FirstNormalTransactionId;

      /* Grab lock for just long enough to set the new limit values */
      LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
      ShmemVariableCache->oldestXid = oldest_datfrozenxid;
      ShmemVariableCache->xidVacLimit = xidVacLimit;
      ShmemVariableCache->xidWarnLimit = xidWarnLimit;
      ShmemVariableCache->xidStopLimit = xidStopLimit;
      ShmemVariableCache->xidWrapLimit = xidWrapLimit;
      namecpy(&ShmemVariableCache->limit_datname, oldest_datname);
      curXid = ShmemVariableCache->nextXid;
      LWLockRelease(XidGenLock);

      /* Log the info */
      ereport(DEBUG1,
         (errmsg("transaction ID wrap limit is %u, limited by database \"%s\"",
                     xidWrapLimit, NameStr(*oldest_datname))));

      /*
       * If past the autovacuum force point, immediately signal an autovac
       * request.  The reason for this is that autovac only processes one
       * database per invocation.  Once it's finished cleaning up the oldest
       * database, it'll call here, and we'll signal the postmaster to start
       * another iteration immediately if there are still any old databases.
       */
      if (TransactionIdFollowsOrEquals(curXid, xidVacLimit) &&
            IsUnderPostmaster)
            SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

      /* Give an immediate warning if past the wrap warn point */
      if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit))
            ereport(WARNING,
               (errmsg("database \"%s\" must be vacuumed within %u transactions",
                           NameStr(*oldest_datname),
                           xidWrapLimit - curXid),
                  errhint("To avoid a database shutdown, execute a database-wide VACUUM in \"%s\".\n"
                              "You might also need to commit or roll back old prepared transactions.",
                              NameStr(*oldest_datname))));
}


/*
 * GetNewObjectId -- allocate a new OID
 *
 * OIDs are generated by a cluster-wide counter.  Since they are only 32 bits
 * wide, counter wraparound will occur eventually, and therefore it is unwise
 * to assume they are unique unless precautions are taken to make them so.
 * Hence, this routine should generally not be used directly.  The only
 * direct callers should be GetNewOid() and GetNewRelFileNode() in
 * catalog/catalog.c.
 */
Oid
GetNewObjectId(void)
{
      Oid               result;

      LWLockAcquire(OidGenLock, LW_EXCLUSIVE);

      /*
       * Check for wraparound of the OID counter.  We *must* not return 0
       * (InvalidOid); and as long as we have to check that, it seems a good
       * idea to skip over everything below FirstNormalObjectId too. (This
       * basically just avoids lots of collisions with bootstrap-assigned OIDs
       * right after a wrap occurs, so as to avoid a possibly large number of
       * iterations in GetNewOid.)  Note we are relying on unsigned comparison.
       *
       * During initdb, we start the OID generator at FirstBootstrapObjectId, so
       * we only enforce wrapping to that point when in bootstrap or standalone
       * mode.  The first time through this routine after normal postmaster
       * start, the counter will be forced up to FirstNormalObjectId. This
       * mechanism leaves the OIDs between FirstBootstrapObjectId and
       * FirstNormalObjectId available for automatic assignment during initdb,
       * while ensuring they will never conflict with user-assigned OIDs.
       */
      if (ShmemVariableCache->nextOid < ((Oid) FirstNormalObjectId))
      {
            if (IsPostmasterEnvironment)
            {
                  /* wraparound in normal environment */
                  ShmemVariableCache->nextOid = FirstNormalObjectId;
                  ShmemVariableCache->oidCount = 0;
            }
            else
            {
                  /* we may be bootstrapping, so don't enforce the full range */
                  if (ShmemVariableCache->nextOid < ((Oid) FirstBootstrapObjectId))
                  {
                        /* wraparound in standalone environment? */
                        ShmemVariableCache->nextOid = FirstBootstrapObjectId;
                        ShmemVariableCache->oidCount = 0;
                  }
            }
      }

      /* If we run out of logged for use oids then we must log more */
      if (ShmemVariableCache->oidCount == 0)
      {
            XLogPutNextOid(ShmemVariableCache->nextOid + VAR_OID_PREFETCH);
            ShmemVariableCache->oidCount = VAR_OID_PREFETCH;
      }

      result = ShmemVariableCache->nextOid;

      (ShmemVariableCache->nextOid)++;
      (ShmemVariableCache->oidCount)--;

      LWLockRelease(OidGenLock);

      return result;
}

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