transam.h

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/*-------------------------------------------------------------------------
 *
 * transam.h
 *    postgres transaction access method support code
 *
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/access/transam.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef TRANSAM_H
#define TRANSAM_H
 
#include "access/xlogdefs.h"
 
 
/* ----------------
 *      Special transaction ID values
 *
 * BootstrapTransactionId is the XID for "bootstrap" operations, and
 * FrozenTransactionId is used for very old tuples.  Both should
 * always be considered valid.
 *
 * FirstNormalTransactionId is the first "normal" transaction id.
 * Note: if you need to change it, you must change pg_class.h as well.
 * ----------------
 */
#define InvalidTransactionId        ((TransactionId) 0)
#define BootstrapTransactionId      ((TransactionId) 1)
#define FrozenTransactionId         ((TransactionId) 2)
#define FirstNormalTransactionId    ((TransactionId) 3)
#define MaxTransactionId            ((TransactionId) 0xFFFFFFFF)
 
/* ----------------
 *      transaction ID manipulation macros
 * ----------------
 */
#define TransactionIdIsValid(xid)       ((xid) != InvalidTransactionId)
#define TransactionIdIsNormal(xid)      ((xid) >= FirstNormalTransactionId)
#define TransactionIdEquals(id1, id2)   ((id1) == (id2))
#define TransactionIdStore(xid, dest)   (*(dest) = (xid))
#define StoreInvalidTransactionId(dest) (*(dest) = InvalidTransactionId)
 
#define EpochFromFullTransactionId(x)   ((uint32) ((x).value >> 32))
#define XidFromFullTransactionId(x)     ((uint32) (x).value)
#define U64FromFullTransactionId(x)     ((x).value)
#define FullTransactionIdEquals(a, b)   ((a).value == (b).value)
#define FullTransactionIdPrecedes(a, b) ((a).value < (b).value)
#define FullTransactionIdPrecedesOrEquals(a, b) ((a).value <= (b).value)
#define FullTransactionIdFollows(a, b) ((a).value > (b).value)
#define FullTransactionIdFollowsOrEquals(a, b) ((a).value >= (b).value)
#define FullTransactionIdIsValid(x)     TransactionIdIsValid(XidFromFullTransactionId(x))
#define InvalidFullTransactionId        FullTransactionIdFromEpochAndXid(0, InvalidTransactionId)
#define FirstNormalFullTransactionId    FullTransactionIdFromEpochAndXid(0, FirstNormalTransactionId)
#define FullTransactionIdIsNormal(x)    FullTransactionIdFollowsOrEquals(x, FirstNormalFullTransactionId)
 
/*
 * A 64 bit value that contains an epoch and a TransactionId.  This is
 * wrapped in a struct to prevent implicit conversion to/from TransactionId.
 * Not all values represent valid normal XIDs.
 */
typedef struct FullTransactionId
{
    uint64      value;
} FullTransactionId;
 
static inline FullTransactionId
FullTransactionIdFromEpochAndXid(uint32 epoch, TransactionId xid)
{
    FullTransactionId result;
 
    result.value = ((uint64) epoch) << 32 | xid;
 
    return result;
}
 
static inline FullTransactionId
FullTransactionIdFromU64(uint64 value)
{
    FullTransactionId result;
 
    result.value = value;
 
    return result;
}
 
/* advance a transaction ID variable, handling wraparound correctly */
#define TransactionIdAdvance(dest)  \
    do { \
        (dest)++; \
        if ((dest) < FirstNormalTransactionId) \
            (dest) = FirstNormalTransactionId; \
    } while(0)
 
/*
 * Retreat a FullTransactionId variable, stepping over xids that would appear
 * to be special only when viewed as 32bit XIDs.
 */
static inline void
FullTransactionIdRetreat(FullTransactionId *dest)
{
    dest->value--;
 
    /*
     * In contrast to 32bit XIDs don't step over the "actual" special xids.
     * For 64bit xids these can't be reached as part of a wraparound as they
     * can in the 32bit case.
     */
    if (FullTransactionIdPrecedes(*dest, FirstNormalFullTransactionId))
        return;
 
    /*
     * But we do need to step over XIDs that'd appear special only for 32bit
     * XIDs.
     */
    while (XidFromFullTransactionId(*dest) < FirstNormalTransactionId)
        dest->value--;
}
 
/*
 * Advance a FullTransactionId variable, stepping over xids that would appear
 * to be special only when viewed as 32bit XIDs.
 */
static inline void
FullTransactionIdAdvance(FullTransactionId *dest)
{
    dest->value++;
 
    /* see FullTransactionIdAdvance() */
    if (FullTransactionIdPrecedes(*dest, FirstNormalFullTransactionId))
        return;
 
    while (XidFromFullTransactionId(*dest) < FirstNormalTransactionId)
        dest->value++;
}
 
/* back up a transaction ID variable, handling wraparound correctly */
#define TransactionIdRetreat(dest)  \
    do { \
        (dest)--; \
    } while ((dest) < FirstNormalTransactionId)
 
/* compare two XIDs already known to be normal; this is a macro for speed */
#define NormalTransactionIdPrecedes(id1, id2) \
    (AssertMacro(TransactionIdIsNormal(id1) && TransactionIdIsNormal(id2)), \
    (int32) ((id1) - (id2)) < 0)
 
/* compare two XIDs already known to be normal; this is a macro for speed */
#define NormalTransactionIdFollows(id1, id2) \
    (AssertMacro(TransactionIdIsNormal(id1) && TransactionIdIsNormal(id2)), \
    (int32) ((id1) - (id2)) > 0)
 
/* ----------
 *      Object ID (OID) zero is InvalidOid.
 *
 *      OIDs 1-9999 are reserved for manual assignment (see .dat files in
 *      src/include/catalog/).  Of these, 8000-9999 are reserved for
 *      development purposes (such as in-progress patches and forks);
 *      they should not appear in released versions.
 *
 *      OIDs 10000-11999 are reserved for assignment by genbki.pl, for use
 *      when the .dat files in src/include/catalog/ do not specify an OID
 *      for a catalog entry that requires one.  Note that genbki.pl assigns
 *      these OIDs independently in each catalog, so they're not guaranteed
 *      to be globally unique.  Furthermore, the bootstrap backend and
 *      initdb's post-bootstrap processing can also assign OIDs in this range.
 *      The normal OID-generation logic takes care of any OID conflicts that
 *      might arise from that.
 *
 *      OIDs 12000-16383 are reserved for unpinned objects created by initdb's
 *      post-bootstrap processing.  initdb forces the OID generator up to
 *      12000 as soon as it's made the pinned objects it's responsible for.
 *
 *      OIDs beginning at 16384 are assigned from the OID generator
 *      during normal multiuser operation.  (We force the generator up to
 *      16384 as soon as we are in normal operation.)
 *
 * The choices of 8000, 10000 and 12000 are completely arbitrary, and can be
 * moved if we run low on OIDs in any category.  Changing the macros below,
 * and updating relevant documentation (see bki.sgml and RELEASE_CHANGES),
 * should be sufficient to do this.  Moving the 16384 boundary between
 * initdb-assigned OIDs and user-defined objects would be substantially
 * more painful, however, since some user-defined OIDs will appear in
 * on-disk data; such a change would probably break pg_upgrade.
 *
 * NOTE: if the OID generator wraps around, we skip over OIDs 0-16383
 * and resume with 16384.  This minimizes the odds of OID conflict, by not
 * reassigning OIDs that might have been assigned during initdb.  Critically,
 * it also ensures that no user-created object will be considered pinned.
 * ----------
 */
#define FirstGenbkiObjectId     10000
#define FirstUnpinnedObjectId   12000
#define FirstNormalObjectId     16384
 
/*
 * TransamVariables is a data structure in shared memory that is used to track
 * OID and XID assignment state.  For largely historical reasons, there is
 * just one struct with different fields that are protected by different
 * LWLocks.
 *
 * Note: xidWrapLimit and oldestXidDB are not "active" values, but are
 * used just to generate useful messages when xidWarnLimit or xidStopLimit
 * are exceeded.
 */
typedef struct TransamVariablesData
{
    /*
     * These fields are protected by OidGenLock.
     */
    Oid         nextOid;        /* next OID to assign */
    uint32      oidCount;       /* OIDs available before must do XLOG work */
 
    /*
     * These fields are protected by XidGenLock.
     */
    FullTransactionId nextXid;  /* next XID to assign */
 
    TransactionId oldestXid;    /* cluster-wide minimum datfrozenxid */
    TransactionId xidVacLimit;  /* start forcing autovacuums here */
    TransactionId xidWarnLimit; /* start complaining here */
    TransactionId xidStopLimit; /* refuse to advance nextXid beyond here */
    TransactionId xidWrapLimit; /* where the world ends */
    Oid         oldestXidDB;    /* database with minimum datfrozenxid */
 
    /*
     * These fields are protected by CommitTsLock
     */
    TransactionId oldestCommitTsXid;
    TransactionId newestCommitTsXid;
 
    /*
     * These fields are protected by ProcArrayLock.
     */
    FullTransactionId latestCompletedXid;   /* newest full XID that has
                                             * committed or aborted */
 
    /*
     * Number of top-level transactions with xids (i.e. which may have
     * modified the database) that completed in some form since the start of
     * the server. This currently is solely used to check whether
     * GetSnapshotData() needs to recompute the contents of the snapshot, or
     * not. There are likely other users of this.  Always above 1.
     */
    uint64      xactCompletionCount;
 
    /*
     * These fields are protected by XactTruncationLock
     */
    TransactionId oldestClogXid;    /* oldest it's safe to look up in clog */
 
} TransamVariablesData;
 
 
/* ----------------
 *      extern declarations
 * ----------------
 */
 
/* in transam/xact.c */
extern bool TransactionStartedDuringRecovery(void);
 
/* in transam/varsup.c */
extern PGDLLIMPORT TransamVariablesData *TransamVariables;
 
/*
 * prototypes for functions in transam/transam.c
 */
extern bool TransactionIdDidCommit(TransactionId transactionId);
extern bool TransactionIdDidAbort(TransactionId transactionId);
extern void TransactionIdCommitTree(TransactionId xid, int nxids, TransactionId *xids);
extern void TransactionIdAsyncCommitTree(TransactionId xid, int nxids, TransactionId *xids, XLogRecPtr lsn);
extern void TransactionIdAbortTree(TransactionId xid, int nxids, TransactionId *xids);
extern bool TransactionIdPrecedes(TransactionId id1, TransactionId id2);
extern bool TransactionIdPrecedesOrEquals(TransactionId id1, TransactionId id2);
extern bool TransactionIdFollows(TransactionId id1, TransactionId id2);
extern bool TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2);
extern TransactionId TransactionIdLatest(TransactionId mainxid,
                                         int nxids, const TransactionId *xids);
extern XLogRecPtr TransactionIdGetCommitLSN(TransactionId xid);
 
/* in transam/varsup.c */
extern Size VarsupShmemSize(void);
extern void VarsupShmemInit(void);
extern FullTransactionId GetNewTransactionId(bool isSubXact);
extern void AdvanceNextFullTransactionIdPastXid(TransactionId xid);
extern FullTransactionId ReadNextFullTransactionId(void);
extern void SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
                                  Oid oldest_datoid);
extern void AdvanceOldestClogXid(TransactionId oldest_datfrozenxid);
extern bool ForceTransactionIdLimitUpdate(void);
extern Oid  GetNewObjectId(void);
extern void StopGeneratingPinnedObjectIds(void);
 
#ifdef USE_ASSERT_CHECKING
extern void AssertTransactionIdInAllowableRange(TransactionId xid);
#else
#define AssertTransactionIdInAllowableRange(xid) ((void)true)
#endif
 
/*
 * Some frontend programs include this header.  For compilers that emit static
 * inline functions even when they're unused, that leads to unsatisfied
 * external references; hence hide them with #ifndef FRONTEND.
 */
#ifndef FRONTEND
 
/*
 * For callers that just need the XID part of the next transaction ID.
 */
static inline TransactionId
ReadNextTransactionId(void)
{
    return XidFromFullTransactionId(ReadNextFullTransactionId());
}
 
/* return transaction ID backed up by amount, handling wraparound correctly */
static inline TransactionId
TransactionIdRetreatedBy(TransactionId xid, uint32 amount)
{
    xid -= amount;
 
    while (xid < FirstNormalTransactionId)
        xid--;
 
    return xid;
}
 
/* return the older of the two IDs */
static inline TransactionId
TransactionIdOlder(TransactionId a, TransactionId b)
{
    if (!TransactionIdIsValid(a))
        return b;
 
    if (!TransactionIdIsValid(b))
        return a;
 
    if (TransactionIdPrecedes(a, b))
        return a;
    return b;
}
 
/* return the older of the two IDs, assuming they're both normal */
static inline TransactionId
NormalTransactionIdOlder(TransactionId a, TransactionId b)
{
    Assert(TransactionIdIsNormal(a));
    Assert(TransactionIdIsNormal(b));
    if (NormalTransactionIdPrecedes(a, b))
        return a;
    return b;
}
 
/* return the newer of the two IDs */
static inline FullTransactionId
FullTransactionIdNewer(FullTransactionId a, FullTransactionId b)
{
    if (!FullTransactionIdIsValid(a))
        return b;
 
    if (!FullTransactionIdIsValid(b))
        return a;
 
    if (FullTransactionIdFollows(a, b))
        return a;
    return b;
}
 
/*
 * Compute FullTransactionId for the given TransactionId, assuming xid was
 * between [oldestXid, nextXid] at the time when TransamVariables->nextXid was
 * nextFullXid.  When adding calls, evaluate what prevents xid from preceding
 * oldestXid if SetTransactionIdLimit() runs between the collection of xid and
 * the collection of nextFullXid.
 */
static inline FullTransactionId
FullTransactionIdFromAllowableAt(FullTransactionId nextFullXid,
                                 TransactionId xid)
{
    uint32      epoch;
 
    /* Special transaction ID. */
    if (!TransactionIdIsNormal(xid))
        return FullTransactionIdFromEpochAndXid(0, xid);
 
    Assert(TransactionIdPrecedesOrEquals(xid,
                                         XidFromFullTransactionId(nextFullXid)));
 
    /*
     * The 64 bit result must be <= nextFullXid, since nextFullXid hadn't been
     * issued yet when xid was in the past.  The xid must therefore be from
     * the epoch of nextFullXid or the epoch before.  We know this because we
     * must remove (by freezing) an XID before assigning the XID half an epoch
     * ahead of it.
     *
     * The unlikely() branch hint is dubious.  It's perfect for the first 2^32
     * XIDs of a cluster's life.  Right at 2^32 XIDs, misprediction shoots to
     * 100%, then improves until perfection returns 2^31 XIDs later.  Since
     * current callers pass relatively-recent XIDs, expect >90% prediction
     * accuracy overall.  This favors average latency over tail latency.
     */
    epoch = EpochFromFullTransactionId(nextFullXid);
    if (unlikely(xid > XidFromFullTransactionId(nextFullXid)))
    {
        Assert(epoch != 0);
        epoch--;
    }
 
    return FullTransactionIdFromEpochAndXid(epoch, xid);
}
 
#endif                          /* FRONTEND */
 
#endif                          /* TRANSAM_H */