transam.c

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/*-------------------------------------------------------------------------
 *
 * transam.c
 *    postgres transaction (commit) log interface routines
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/transam/transam.c
 *
 * 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;

/* Local functions */
static XidStatus TransactionLogFetch(TransactionId transactionId);


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

/*
 * 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;
}

/* ----------------------------------------------------------------
 *                      Interface functions
 *
 *      TransactionIdDidCommit
 *      TransactionIdDidAbort
 *      ========
 *         these functions test the transaction status of
 *         a specified transaction id.
 *
 *      TransactionIdCommitTree
 *      TransactionIdAsyncCommitTree
 *      TransactionIdAbortTree
 *      ========
 *         these functions set the transaction status of the specified
 *         transaction tree.
 *
 * 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 and exists in clog.
 */
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 and exists in clog.
 */
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_xact 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 heapam_visibility.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;
}

/*
 * TransactionIdCommitTree
 *      Marks the given transaction and children as committed
 *
 * "xid" is a toplevel transaction commit, and the xids array contains its
 * committed subtransactions.
 *
 * This commit operation is not guaranteed to be atomic, but if not, subxids
 * are correctly marked subcommit first.
 */
void
TransactionIdCommitTree(TransactionId xid, int nxids, TransactionId *xids)
{
    TransactionIdSetTreeStatus(xid, nxids, xids,
                               TRANSACTION_STATUS_COMMITTED,
                               InvalidXLogRecPtr);
}

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

/*
 * TransactionIdAbortTree
 *      Marks the given transaction and children as aborted.
 *
 * "xid" is a toplevel transaction commit, and the xids array contains its
 * committed subtransactions.
 *
 * 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(TransactionId xid, int nxids, TransactionId *xids)
{
    TransactionIdSetTreeStatus(xid, 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-2^32 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;
}