postgres/src/backend/access/transam/varsup.c

/*-------------------------------------------------------------------------
 *
 * varsup.c
 *	  postgres OID & XID variables support routines
 *
 * Copyright (c) 2000-2007, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.78 2007/02/15 23:23:22 alvherre 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.
 */
TransactionId
GetNewTransactionId(bool isSubXact)
{
	TransactionId xid;

	/*
	 * During bootstrap initialization, we return the special bootstrap
	 * transaction id.
	 */
	if (IsBootstrapProcessingMode())
		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\".",
							 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 full-database VACUUM in \"%s\".",
					 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 PGPROC array before releasing
	 * XidGenLock.	This ensures that when GetSnapshotData calls
	 * ReadNewTransactionId, all active XIDs before the returned value of
	 * nextXid are already present in PGPROC.  Else we have a race condition.
	 *
	 * 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 (and in fact could cause a deadlock
	 * against GetSnapshotData).  So for now, assume atomicity. Note that
	 * readers of PGPROC xid field should be careful to fetch the value only
	 * once, rather than assume they can read it 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.)
	 */
	if (MyProc != NULL)
	{
		/*
		 * 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 full-database VACUUM in \"%s\".",
					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;
}