path: root/src/backend/access/transam/transam.c

/*-------------------------------------------------------------------------
 *
 * transam.c
 *	  postgres transaction log interface routines
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.76 2008/03/26 18:48:59 alvherre Exp $
 *
 * NOTES
 *	  This file contains the high level access-method interface to the
 *	  transaction system.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/clog.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "utils/snapmgr.h"


/*
 * Single-item cache for results of TransactionLogFetch.  It's worth having
 * such a cache because we frequently find ourselves repeatedly checking the
 * same XID, for example when scanning a table just after a bulk insert,
 * update, or delete.
 */
static TransactionId cachedFetchXid = InvalidTransactionId;
static XidStatus cachedFetchXidStatus;
static XLogRecPtr cachedCommitLSN;

/* Handy constant for an invalid xlog recptr */
static const XLogRecPtr InvalidXLogRecPtr = {0, 0};

/* Local functions */
static XidStatus TransactionLogFetch(TransactionId transactionId);
static void TransactionLogUpdate(TransactionId transactionId,
					 XidStatus status, XLogRecPtr lsn);


/* ----------------------------------------------------------------
 *		Postgres log access method interface
 *
 *		TransactionLogFetch
 *		TransactionLogUpdate
 * ----------------------------------------------------------------
 */

/*
 * TransactionLogFetch --- fetch commit status of specified transaction id
 */
static XidStatus
TransactionLogFetch(TransactionId transactionId)
{
	XidStatus	xidstatus;
	XLogRecPtr	xidlsn;

	/*
	 * Before going to the commit log manager, check our single item cache to
	 * see if we didn't just check the transaction status a moment ago.
	 */
	if (TransactionIdEquals(transactionId, cachedFetchXid))
		return cachedFetchXidStatus;

	/*
	 * Also, check to see if the transaction ID is a permanent one.
	 */
	if (!TransactionIdIsNormal(transactionId))
	{
		if (TransactionIdEquals(transactionId, BootstrapTransactionId))
			return TRANSACTION_STATUS_COMMITTED;
		if (TransactionIdEquals(transactionId, FrozenTransactionId))
			return TRANSACTION_STATUS_COMMITTED;
		return TRANSACTION_STATUS_ABORTED;
	}

	/*
	 * Get the transaction status.
	 */
	xidstatus = TransactionIdGetStatus(transactionId, &xidlsn);

	/*
	 * Cache it, but DO NOT cache status for unfinished or sub-committed
	 * transactions!  We only cache status that is guaranteed not to change.
	 */
	if (xidstatus != TRANSACTION_STATUS_IN_PROGRESS &&
		xidstatus != TRANSACTION_STATUS_SUB_COMMITTED)
	{
		cachedFetchXid = transactionId;
		cachedFetchXidStatus = xidstatus;
		cachedCommitLSN = xidlsn;
	}

	return xidstatus;
}

/*
 *		TransactionLogUpdate
 *
 * Store the new status of a transaction.  The commit record LSN must be
 * passed when recording an async commit; else it should be InvalidXLogRecPtr.
 */
static inline void
TransactionLogUpdate(TransactionId transactionId,
					 XidStatus status, XLogRecPtr lsn)
{
	/*
	 * update the commit log
	 */
	TransactionIdSetStatus(transactionId, status, lsn);
}

/*
 * TransactionLogMultiUpdate
 *
 * Update multiple transaction identifiers to a given status.
 * Don't depend on this being atomic; it's not.
 */
static inline void
TransactionLogMultiUpdate(int nxids, TransactionId *xids,
						  XidStatus status, XLogRecPtr lsn)
{
	int			i;

	Assert(nxids != 0);

	for (i = 0; i < nxids; i++)
		TransactionIdSetStatus(xids[i], status, lsn);
}


/* ----------------------------------------------------------------
 *						Interface functions
 *
 *		TransactionIdDidCommit
 *		TransactionIdDidAbort
 *		========
 *		   these functions test the transaction status of
 *		   a specified transaction id.
 *
 *		TransactionIdCommit
 *		TransactionIdAbort
 *		========
 *		   these functions set the transaction status
 *		   of the specified xid.
 *
 * See also TransactionIdIsInProgress, which once was in this module
 * but now lives in procarray.c.
 * ----------------------------------------------------------------
 */

/*
 * TransactionIdDidCommit
 *		True iff transaction associated with the identifier did commit.
 *
 * Note:
 *		Assumes transaction identifier is valid.
 */
bool							/* true if given transaction committed */
TransactionIdDidCommit(TransactionId transactionId)
{
	XidStatus	xidstatus;

	xidstatus = TransactionLogFetch(transactionId);

	/*
	 * If it's marked committed, it's committed.
	 */
	if (xidstatus == TRANSACTION_STATUS_COMMITTED)
		return true;

	/*
	 * If it's marked subcommitted, we have to check the parent recursively.
	 * However, if it's older than TransactionXmin, we can't look at
	 * pg_subtrans; instead assume that the parent crashed without cleaning up
	 * its children.
	 *
	 * Originally we Assert'ed that the result of SubTransGetParent was not
	 * zero. However with the introduction of prepared transactions, there can
	 * be a window just after database startup where we do not have complete
	 * knowledge in pg_subtrans of the transactions after TransactionXmin.
	 * StartupSUBTRANS() has ensured that any missing information will be
	 * zeroed.	Since this case should not happen under normal conditions, it
	 * seems reasonable to emit a WARNING for it.
	 */
	if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
	{
		TransactionId parentXid;

		if (TransactionIdPrecedes(transactionId, TransactionXmin))
			return false;
		parentXid = SubTransGetParent(transactionId);
		if (!TransactionIdIsValid(parentXid))
		{
			elog(WARNING, "no pg_subtrans entry for subcommitted XID %u",
				 transactionId);
			return false;
		}
		return TransactionIdDidCommit(parentXid);
	}

	/*
	 * It's not committed.
	 */
	return false;
}

/*
 * TransactionIdDidAbort
 *		True iff transaction associated with the identifier did abort.
 *
 * Note:
 *		Assumes transaction identifier is valid.
 */
bool							/* true if given transaction aborted */
TransactionIdDidAbort(TransactionId transactionId)
{
	XidStatus	xidstatus;

	xidstatus = TransactionLogFetch(transactionId);

	/*
	 * If it's marked aborted, it's aborted.
	 */
	if (xidstatus == TRANSACTION_STATUS_ABORTED)
		return true;

	/*
	 * If it's marked subcommitted, we have to check the parent recursively.
	 * However, if it's older than TransactionXmin, we can't look at
	 * pg_subtrans; instead assume that the parent crashed without cleaning up
	 * its children.
	 */
	if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
	{
		TransactionId parentXid;

		if (TransactionIdPrecedes(transactionId, TransactionXmin))
			return true;
		parentXid = SubTransGetParent(transactionId);
		if (!TransactionIdIsValid(parentXid))
		{
			/* see notes in TransactionIdDidCommit */
			elog(WARNING, "no pg_subtrans entry for subcommitted XID %u",
				 transactionId);
			return true;
		}
		return TransactionIdDidAbort(parentXid);
	}

	/*
	 * It's not aborted.
	 */
	return false;
}

/*
 * TransactionIdIsKnownCompleted
 *		True iff transaction associated with the identifier is currently
 *		known to have either committed or aborted.
 *
 * This does NOT look into pg_clog but merely probes our local cache
 * (and so it's not named TransactionIdDidComplete, which would be the
 * appropriate name for a function that worked that way).  The intended
 * use is just to short-circuit TransactionIdIsInProgress calls when doing
 * repeated tqual.c checks for the same XID.  If this isn't extremely fast
 * then it will be counterproductive.
 *
 * Note:
 *		Assumes transaction identifier is valid.
 */
bool
TransactionIdIsKnownCompleted(TransactionId transactionId)
{
	if (TransactionIdEquals(transactionId, cachedFetchXid))
	{
		/* If it's in the cache at all, it must be completed. */
		return true;
	}

	return false;
}


/*
 * TransactionIdCommit
 *		Commits the transaction associated with the identifier.
 *
 * Note:
 *		Assumes transaction identifier is valid.
 */
void
TransactionIdCommit(TransactionId transactionId)
{
	TransactionLogUpdate(transactionId, TRANSACTION_STATUS_COMMITTED,
						 InvalidXLogRecPtr);
}

/*
 * TransactionIdAsyncCommit
 *		Same as above, but for async commits.  The commit record LSN is needed.
 */
void
TransactionIdAsyncCommit(TransactionId transactionId, XLogRecPtr lsn)
{
	TransactionLogUpdate(transactionId, TRANSACTION_STATUS_COMMITTED, lsn);
}

/*
 * TransactionIdAbort
 *		Aborts the transaction associated with the identifier.
 *
 * Note:
 *		Assumes transaction identifier is valid.
 *		No async version of this is needed.
 */
void
TransactionIdAbort(TransactionId transactionId)
{
	TransactionLogUpdate(transactionId, TRANSACTION_STATUS_ABORTED,
						 InvalidXLogRecPtr);
}

/*
 * TransactionIdSubCommit
 *		Marks the subtransaction associated with the identifier as
 *		sub-committed.
 *
 * Note:
 *		No async version of this is needed.
 */
void
TransactionIdSubCommit(TransactionId transactionId)
{
	TransactionLogUpdate(transactionId, TRANSACTION_STATUS_SUB_COMMITTED,
						 InvalidXLogRecPtr);
}

/*
 * TransactionIdCommitTree
 *		Marks all the given transaction ids as committed.
 *
 * The caller has to be sure that this is used only to mark subcommitted
 * subtransactions as committed, and only *after* marking the toplevel
 * parent as committed.  Otherwise there is a race condition against
 * TransactionIdDidCommit.
 */
void
TransactionIdCommitTree(int nxids, TransactionId *xids)
{
	if (nxids > 0)
		TransactionLogMultiUpdate(nxids, xids, TRANSACTION_STATUS_COMMITTED,
								  InvalidXLogRecPtr);
}

/*
 * TransactionIdAsyncCommitTree
 *		Same as above, but for async commits.  The commit record LSN is needed.
 */
void
TransactionIdAsyncCommitTree(int nxids, TransactionId *xids, XLogRecPtr lsn)
{
	if (nxids > 0)
		TransactionLogMultiUpdate(nxids, xids, TRANSACTION_STATUS_COMMITTED,
								  lsn);
}

/*
 * TransactionIdAbortTree
 *		Marks all the given transaction ids as aborted.
 *
 * We don't need to worry about the non-atomic behavior, since any onlookers
 * will consider all the xacts as not-yet-committed anyway.
 */
void
TransactionIdAbortTree(int nxids, TransactionId *xids)
{
	if (nxids > 0)
		TransactionLogMultiUpdate(nxids, xids, TRANSACTION_STATUS_ABORTED,
								  InvalidXLogRecPtr);
}


/*
 * TransactionIdPrecedes --- is id1 logically < id2?
 */
bool
TransactionIdPrecedes(TransactionId id1, TransactionId id2)
{
	/*
	 * If either ID is a permanent XID then we can just do unsigned
	 * comparison.	If both are normal, do a modulo-2^31 comparison.
	 */
	int32		diff;

	if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
		return (id1 < id2);

	diff = (int32) (id1 - id2);
	return (diff < 0);
}

/*
 * TransactionIdPrecedesOrEquals --- is id1 logically <= id2?
 */
bool
TransactionIdPrecedesOrEquals(TransactionId id1, TransactionId id2)
{
	int32		diff;

	if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
		return (id1 <= id2);

	diff = (int32) (id1 - id2);
	return (diff <= 0);
}

/*
 * TransactionIdFollows --- is id1 logically > id2?
 */
bool
TransactionIdFollows(TransactionId id1, TransactionId id2)
{
	int32		diff;

	if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
		return (id1 > id2);

	diff = (int32) (id1 - id2);
	return (diff > 0);
}

/*
 * TransactionIdFollowsOrEquals --- is id1 logically >= id2?
 */
bool
TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2)
{
	int32		diff;

	if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
		return (id1 >= id2);

	diff = (int32) (id1 - id2);
	return (diff >= 0);
}


/*
 * TransactionIdLatest --- get latest XID among a main xact and its children
 */
TransactionId
TransactionIdLatest(TransactionId mainxid,
					int nxids, const TransactionId *xids)
{
	TransactionId result;

	/*
	 * In practice it is highly likely that the xids[] array is sorted, and so
	 * we could save some cycles by just taking the last child XID, but this
	 * probably isn't so performance-critical that it's worth depending on
	 * that assumption.  But just to show we're not totally stupid, scan the
	 * array back-to-front to avoid useless assignments.
	 */
	result = mainxid;
	while (--nxids >= 0)
	{
		if (TransactionIdPrecedes(result, xids[nxids]))
			result = xids[nxids];
	}
	return result;
}


/*
 * TransactionIdGetCommitLSN
 *
 * This function returns an LSN that is late enough to be able
 * to guarantee that if we flush up to the LSN returned then we
 * will have flushed the transaction's commit record to disk.
 *
 * The result is not necessarily the exact LSN of the transaction's
 * commit record!  For example, for long-past transactions (those whose
 * clog pages already migrated to disk), we'll return InvalidXLogRecPtr.
 * Also, because we group transactions on the same clog page to conserve
 * storage, we might return the LSN of a later transaction that falls into
 * the same group.
 */
XLogRecPtr
TransactionIdGetCommitLSN(TransactionId xid)
{
	XLogRecPtr	result;

	/*
	 * Currently, all uses of this function are for xids that were just
	 * reported to be committed by TransactionLogFetch, so we expect that
	 * checking TransactionLogFetch's cache will usually succeed and avoid an
	 * extra trip to shared memory.
	 */
	if (TransactionIdEquals(xid, cachedFetchXid))
		return cachedCommitLSN;

	/* Special XIDs are always known committed */
	if (!TransactionIdIsNormal(xid))
		return InvalidXLogRecPtr;

	/*
	 * Get the transaction status.
	 */
	(void) TransactionIdGetStatus(xid, &result);

	return result;
}