varsup.c

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/*-------------------------------------------------------------------------
 *
 * varsup.c
 *    postgres OID & XID variables support routines
 *
 * Copyright (c) 2000-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/access/transam/varsup.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/clog.h"
#include "access/commit_ts.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/xact.h"
#include "access/xlogutils.h"
#include "commands/dbcommands.h"
#include "miscadmin.h"
#include "postmaster/autovacuum.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "utils/syscache.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 FullTransactionId for a new transaction or
 * subtransaction.
 *
 * The new XID is also stored into MyProc->xid/ProcGlobal->xids[] before
 * returning.
 *
 * Note: when this is called, we are actually already inside a valid
 * transaction, since XIDs are now not allocated until the transaction
 * does something.  So it is safe to do a database lookup if we want to
 * issue a warning about XID wrap.
 */
FullTransactionId
GetNewTransactionId(bool isSubXact)
{
    FullTransactionId full_xid;
    TransactionId xid;

    /*
     * Workers synchronize transaction state at the beginning of each parallel
     * operation, so we can't account for new XIDs after that point.
     */
    if (IsInParallelMode())
        elog(ERROR, "cannot assign TransactionIds during a parallel operation");

    /*
     * During bootstrap initialization, we return the special bootstrap
     * transaction id.
     */
    if (IsBootstrapProcessingMode())
    {
        Assert(!isSubXact);
        MyProc->xid = BootstrapTransactionId;
        ProcGlobal->xids[MyProc->pgxactoff] = BootstrapTransactionId;
        return FullTransactionIdFromEpochAndXid(0, BootstrapTransactionId);
    }

    /* safety check, we should never get this far in a HS standby */
    if (RecoveryInProgress())
        elog(ERROR, "cannot assign TransactionIds during recovery");

    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);

    full_xid = ShmemVariableCache->nextXid;
    xid = XidFromFullTransactionId(full_xid);

    /*----------
     * 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 single-user mode (which gives an escape hatch
     * to the DBA who somehow got past the earlier defenses).
     *
     * Note that this coding also appears in GetNewMultiXactId.
     *----------
     */
    if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
    {
        /*
         * For safety's sake, we release XidGenLock while sending signals,
         * warnings, etc.  This is not so much because we care about
         * preserving concurrency in this situation, as to avoid any
         * possibility of deadlock while doing get_database_name(). First,
         * copy all the shared values we'll need in this path.
         */
        TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
        TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
        TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
        Oid         oldest_datoid = ShmemVariableCache->oldestXidDB;

        LWLockRelease(XidGenLock);

        /*
         * 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, xidStopLimit))
        {
            char       *oldest_datname = get_database_name(oldest_datoid);

            /* complain even if that DB has disappeared */
            if (oldest_datname)
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
                                oldest_datname),
                         errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
            else
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",
                                oldest_datoid),
                         errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
        }
        else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
        {
            char       *oldest_datname = get_database_name(oldest_datoid);

            /* complain even if that DB has disappeared */
            if (oldest_datname)
                ereport(WARNING,
                        (errmsg("database \"%s\" must be vacuumed within %u transactions",
                                oldest_datname,
                                xidWrapLimit - xid),
                         errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
            else
                ereport(WARNING,
                        (errmsg("database with OID %u must be vacuumed within %u transactions",
                                oldest_datoid,
                                xidWrapLimit - xid),
                         errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
        }

        /* Re-acquire lock and start over */
        LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
        full_xid = ShmemVariableCache->nextXid;
        xid = XidFromFullTransactionId(full_xid);
    }

    /*
     * 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 and pg_commit_ts too.
     */
    ExtendCLOG(xid);
    ExtendCommitTs(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.
     */
    FullTransactionIdAdvance(&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.
     *
     * Note that readers of ProcGlobal->xids/PGPROC->xid should be careful to
     * fetch the value for each proc only once, rather than assume they can
     * read a value multiple times and get the same answer each time.  Note we
     * are assuming that TransactionId and int fetch/store are atomic.
     *
     * The same comments apply to the subxact xid count and overflow fields.
     *
     * Use of a write barrier prevents dangerous code rearrangement in this
     * function; other backends could otherwise e.g. be examining my subxids
     * info concurrently, and we don't want them to see an invalid
     * intermediate state, such as an incremented nxids before the array entry
     * is filled.
     *
     * Other processes that read nxids should do so before reading xids
     * elements with a pg_read_barrier() in between, so that they can be sure
     * not to read an uninitialized array element; see
     * src/backend/storage/lmgr/README.barrier.
     *
     * 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 (!isSubXact)
    {
        Assert(ProcGlobal->subxidStates[MyProc->pgxactoff].count == 0);
        Assert(!ProcGlobal->subxidStates[MyProc->pgxactoff].overflowed);
        Assert(MyProc->subxidStatus.count == 0);
        Assert(!MyProc->subxidStatus.overflowed);

        /* LWLockRelease acts as barrier */
        MyProc->xid = xid;
        ProcGlobal->xids[MyProc->pgxactoff] = xid;
    }
    else
    {
        XidCacheStatus *substat = &ProcGlobal->subxidStates[MyProc->pgxactoff];
        int         nxids = MyProc->subxidStatus.count;

        Assert(substat->count == MyProc->subxidStatus.count);
        Assert(substat->overflowed == MyProc->subxidStatus.overflowed);

        if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
        {
            MyProc->subxids.xids[nxids] = xid;
            pg_write_barrier();
            MyProc->subxidStatus.count = substat->count = nxids + 1;
        }
        else
            MyProc->subxidStatus.overflowed = substat->overflowed = true;
    }

    LWLockRelease(XidGenLock);

    return full_xid;
}

/*
 * Read nextXid but don't allocate it.
 */
FullTransactionId
ReadNextFullTransactionId(void)
{
    FullTransactionId fullXid;

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

    return fullXid;
}

/*
 * Advance nextXid to the value after a given xid.  The epoch is inferred.
 * This must only be called during recovery or from two-phase start-up code.
 */
void
AdvanceNextFullTransactionIdPastXid(TransactionId xid)
{
    FullTransactionId newNextFullXid;
    TransactionId next_xid;
    uint32      epoch;

    /*
     * It is safe to read nextXid without a lock, because this is only called
     * from the startup process or single-process mode, meaning that no other
     * process can modify it.
     */
    Assert(AmStartupProcess() || !IsUnderPostmaster);

    /* Fast return if this isn't an xid high enough to move the needle. */
    next_xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
    if (!TransactionIdFollowsOrEquals(xid, next_xid))
        return;

    /*
     * Compute the FullTransactionId that comes after the given xid.  To do
     * this, we preserve the existing epoch, but detect when we've wrapped
     * into a new epoch.  This is necessary because WAL records and 2PC state
     * currently contain 32 bit xids.  The wrap logic is safe in those cases
     * because the span of active xids cannot exceed one epoch at any given
     * point in the WAL stream.
     */
    TransactionIdAdvance(xid);
    epoch = EpochFromFullTransactionId(ShmemVariableCache->nextXid);
    if (unlikely(xid < next_xid))
        ++epoch;
    newNextFullXid = FullTransactionIdFromEpochAndXid(epoch, xid);

    /*
     * We still need to take a lock to modify the value when there are
     * concurrent readers.
     */
    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
    ShmemVariableCache->nextXid = newNextFullXid;
    LWLockRelease(XidGenLock);
}

/*
 * Advance the cluster-wide value for the oldest valid clog entry.
 *
 * We must acquire XactTruncationLock to advance the oldestClogXid. It's not
 * necessary to hold the lock during the actual clog truncation, only when we
 * advance the limit, as code looking up arbitrary xids is required to hold
 * XactTruncationLock from when it tests oldestClogXid through to when it
 * completes the clog lookup.
 */
void
AdvanceOldestClogXid(TransactionId oldest_datfrozenxid)
{
    LWLockAcquire(XactTruncationLock, LW_EXCLUSIVE);
    if (TransactionIdPrecedes(ShmemVariableCache->oldestClogXid,
                              oldest_datfrozenxid))
    {
        ShmemVariableCache->oldestClogXid = oldest_datfrozenxid;
    }
    LWLockRelease(XactTruncationLock);
}

/*
 * Determine the last safe XID to allocate using the currently oldest
 * datfrozenxid (ie, the oldest XID that might exist in any database
 * of our cluster), and the OID of the (or a) database with that value.
 */
void
SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
{
    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 3M 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. (VACUUM requires an XID
     * if it truncates at wal_level!=minimal.  "VACUUM (ANALYZE)", which a DBA
     * might do by reflex, assigns an XID.  Hence, we had better be sure
     * there's lots of XIDs left...)  Also, at default BLCKSZ, this leaves two
     * completely-idle segments.  In the event of edge-case bugs involving
     * page or segment arithmetic, idle segments render the bugs unreachable
     * outside of single-user mode.
     */
    xidStopLimit = xidWrapLimit - 3000000;
    if (xidStopLimit < FirstNormalTransactionId)
        xidStopLimit -= FirstNormalTransactionId;

    /*
     * We'll start complaining loudly when we get within 40M transactions of
     * data loss.  This is kind of arbitrary, but if you let your gas gauge
     * get down to 2% 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 = xidWrapLimit - 40000000;
    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.  See also
     * SetMultiXactIdLimit.
     */
    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;
    ShmemVariableCache->oldestXidDB = oldest_datoid;
    curXid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
    LWLockRelease(XidGenLock);

    /* Log the info */
    ereport(DEBUG1,
            (errmsg_internal("transaction ID wrap limit is %u, limited by database with OID %u",
                             xidWrapLimit, oldest_datoid)));

    /*
     * 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 && !InRecovery)
        SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

    /* Give an immediate warning if past the wrap warn point */
    if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit) && !InRecovery)
    {
        char       *oldest_datname;

        /*
         * We can be called when not inside a transaction, for example during
         * StartupXLOG().  In such a case we cannot do database access, so we
         * must just report the oldest DB's OID.
         *
         * Note: it's also possible that get_database_name fails and returns
         * NULL, for example because the database just got dropped.  We'll
         * still warn, even though the warning might now be unnecessary.
         */
        if (IsTransactionState())
            oldest_datname = get_database_name(oldest_datoid);
        else
            oldest_datname = NULL;

        if (oldest_datname)
            ereport(WARNING,
                    (errmsg("database \"%s\" must be vacuumed within %u transactions",
                            oldest_datname,
                            xidWrapLimit - curXid),
                     errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
                             "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
        else
            ereport(WARNING,
                    (errmsg("database with OID %u must be vacuumed within %u transactions",
                            oldest_datoid,
                            xidWrapLimit - curXid),
                     errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
                             "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
    }
}


/*
 * ForceTransactionIdLimitUpdate -- does the XID wrap-limit data need updating?
 *
 * We primarily check whether oldestXidDB is valid.  The cases we have in
 * mind are that that database was dropped, or the field was reset to zero
 * by pg_resetwal.  In either case we should force recalculation of the
 * wrap limit.  Also do it if oldestXid is old enough to be forcing
 * autovacuums or other actions; this ensures we update our state as soon
 * as possible once extra overhead is being incurred.
 */
bool
ForceTransactionIdLimitUpdate(void)
{
    TransactionId nextXid;
    TransactionId xidVacLimit;
    TransactionId oldestXid;
    Oid         oldestXidDB;

    /* Locking is probably not really necessary, but let's be careful */
    LWLockAcquire(XidGenLock, LW_SHARED);
    nextXid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
    xidVacLimit = ShmemVariableCache->xidVacLimit;
    oldestXid = ShmemVariableCache->oldestXid;
    oldestXidDB = ShmemVariableCache->oldestXidDB;
    LWLockRelease(XidGenLock);

    if (!TransactionIdIsNormal(oldestXid))
        return true;            /* shouldn't happen, but just in case */
    if (!TransactionIdIsValid(xidVacLimit))
        return true;            /* this shouldn't happen anymore either */
    if (TransactionIdFollowsOrEquals(nextXid, xidVacLimit))
        return true;            /* past xidVacLimit, don't delay updating */
    if (!SearchSysCacheExists1(DATABASEOID, ObjectIdGetDatum(oldestXidDB)))
        return true;            /* could happen, per comments above */
    return false;
}


/*
 * 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 GetNewOidWithIndex() and GetNewRelFileNode() in
 * catalog/catalog.c.
 */
Oid
GetNewObjectId(void)
{
    Oid         result;

    /* safety check, we should never get this far in a HS standby */
    if (RecoveryInProgress())
        elog(ERROR, "cannot assign OIDs during recovery");

    LWLockAcquire(OidGenLock, LW_EXCLUSIVE);

    /*
     * Check for wraparound of the OID counter.  We *must* not return 0
     * (InvalidOid), and in normal operation we mustn't return anything below
     * FirstNormalObjectId since that range is reserved for initdb (see
     * IsCatalogRelationOid()).  Note we are relying on unsigned comparison.
     *
     * During initdb, we start the OID generator at FirstGenbkiObjectId, so we
     * only wrap if before 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 FirstGenbkiObjectId 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, or first post-initdb assignment, in normal mode */
            ShmemVariableCache->nextOid = FirstNormalObjectId;
            ShmemVariableCache->oidCount = 0;
        }
        else
        {
            /* we may be bootstrapping, so don't enforce the full range */
            if (ShmemVariableCache->nextOid < ((Oid) FirstGenbkiObjectId))
            {
                /* wraparound in standalone mode (unlikely but possible) */
                ShmemVariableCache->nextOid = FirstNormalObjectId;
                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;
}

/*
 * SetNextObjectId
 *
 * This may only be called during initdb; it advances the OID counter
 * to the specified value.
 */
static void
SetNextObjectId(Oid nextOid)
{
    /* Safety check, this is only allowable during initdb */
    if (IsPostmasterEnvironment)
        elog(ERROR, "cannot advance OID counter anymore");

    /* Taking the lock is, therefore, just pro forma; but do it anyway */
    LWLockAcquire(OidGenLock, LW_EXCLUSIVE);

    if (ShmemVariableCache->nextOid > nextOid)
        elog(ERROR, "too late to advance OID counter to %u, it is now %u",
             nextOid, ShmemVariableCache->nextOid);

    ShmemVariableCache->nextOid = nextOid;
    ShmemVariableCache->oidCount = 0;

    LWLockRelease(OidGenLock);
}

/*
 * StopGeneratingPinnedObjectIds
 *
 * This is called once during initdb to force the OID counter up to
 * FirstUnpinnedObjectId.  This supports letting initdb's post-bootstrap
 * processing create some pinned objects early on.  Once it's done doing
 * so, it calls this (via pg_stop_making_pinned_objects()) so that the
 * remaining objects it makes will be considered un-pinned.
 */
void
StopGeneratingPinnedObjectIds(void)
{
    SetNextObjectId(FirstUnpinnedObjectId);
}


#ifdef USE_ASSERT_CHECKING

/*
 * Assert that xid is between [oldestXid, nextXid], which is the range we
 * expect XIDs coming from tables etc to be in.
 *
 * As ShmemVariableCache->oldestXid could change just after this call without
 * further precautions, and as a wrapped-around xid could again fall within
 * the valid range, this assertion can only detect if something is definitely
 * wrong, but not establish correctness.
 *
 * This intentionally does not expose a return value, to avoid code being
 * introduced that depends on the return value.
 */
void
AssertTransactionIdInAllowableRange(TransactionId xid)
{
    TransactionId oldest_xid;
    TransactionId next_xid;

    Assert(TransactionIdIsValid(xid));

    /* we may see bootstrap / frozen */
    if (!TransactionIdIsNormal(xid))
        return;

    /*
     * We can't acquire XidGenLock, as this may be called with XidGenLock
     * already held (or with other locks that don't allow XidGenLock to be
     * nested). That's ok for our purposes though, since we already rely on
     * 32bit reads to be atomic. While nextXid is 64 bit, we only look at the
     * lower 32bit, so a skewed read doesn't hurt.
     *
     * There's no increased danger of falling outside [oldest, next] by
     * accessing them without a lock. xid needs to have been created with
     * GetNewTransactionId() in the originating session, and the locks there
     * pair with the memory barrier below.  We do however accept xid to be <=
     * to next_xid, instead of just <, as xid could be from the procarray,
     * before we see the updated nextXid value.
     */
    pg_memory_barrier();
    oldest_xid = ShmemVariableCache->oldestXid;
    next_xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);

    Assert(TransactionIdFollowsOrEquals(xid, oldest_xid) ||
           TransactionIdPrecedesOrEquals(xid, next_xid));
}
#endif