xid_wraparound.c

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/*--------------------------------------------------------------------------
 *
 * xid_wraparound.c
 *      Utilities for testing XID wraparound
 *
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *      src/test/modules/xid_wraparound/xid_wraparound.c
 *
 * -------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/xact.h"
#include "miscadmin.h"
#include "storage/proc.h"
#include "utils/xid8.h"
 
PG_MODULE_MAGIC;
 
static int64 consume_xids_shortcut(void);
static FullTransactionId consume_xids_common(FullTransactionId untilxid, uint64 nxids);
 
/*
 * Consume the specified number of XIDs.
 */
PG_FUNCTION_INFO_V1(consume_xids);
Datum
consume_xids(PG_FUNCTION_ARGS)
{
    int64       nxids = PG_GETARG_INT64(0);
    FullTransactionId lastxid;
 
    if (nxids < 0)
        elog(ERROR, "invalid nxids argument: %lld", (long long) nxids);
 
    if (nxids == 0)
        lastxid = ReadNextFullTransactionId();
    else
        lastxid = consume_xids_common(InvalidFullTransactionId, (uint64) nxids);
 
    PG_RETURN_FULLTRANSACTIONID(lastxid);
}
 
/*
 * Consume XIDs, up to the given XID.
 */
PG_FUNCTION_INFO_V1(consume_xids_until);
Datum
consume_xids_until(PG_FUNCTION_ARGS)
{
    FullTransactionId targetxid = PG_GETARG_FULLTRANSACTIONID(0);
    FullTransactionId lastxid;
 
    if (!FullTransactionIdIsNormal(targetxid))
        elog(ERROR, "targetxid %llu is not normal",
             (unsigned long long) U64FromFullTransactionId(targetxid));
 
    lastxid = consume_xids_common(targetxid, 0);
 
    PG_RETURN_FULLTRANSACTIONID(lastxid);
}
 
/*
 * Common functionality between the two public functions.
 */
static FullTransactionId
consume_xids_common(FullTransactionId untilxid, uint64 nxids)
{
    FullTransactionId lastxid;
    uint64      last_reported_at = 0;
    uint64      consumed = 0;
 
    /* Print a NOTICE every REPORT_INTERVAL xids */
#define REPORT_INTERVAL (10 * 1000000)
 
    /* initialize 'lastxid' with the system's current next XID */
    lastxid = ReadNextFullTransactionId();
 
    /*
     * We consume XIDs by calling GetNewTransactionId(true), which marks the
     * consumed XIDs as subtransactions of the current top-level transaction.
     * For that to work, this transaction must have a top-level XID.
     *
     * GetNewTransactionId registers them in the subxid cache in PGPROC, until
     * the cache overflows, but beyond that, we don't keep track of the
     * consumed XIDs.
     */
    (void) GetTopTransactionId();
 
    for (;;)
    {
        uint64      xids_left;
 
        CHECK_FOR_INTERRUPTS();
 
        /* How many XIDs do we have left to consume? */
        if (nxids > 0)
        {
            if (consumed >= nxids)
                break;
            xids_left = nxids - consumed;
        }
        else
        {
            if (FullTransactionIdFollowsOrEquals(lastxid, untilxid))
                break;
            xids_left = U64FromFullTransactionId(untilxid) - U64FromFullTransactionId(lastxid);
        }
 
        /*
         * If we still have plenty of XIDs to consume, try to take a shortcut
         * and bump up the nextXid counter directly.
         */
        if (xids_left > 2000 &&
            consumed - last_reported_at < REPORT_INTERVAL &&
            MyProc->subxidStatus.overflowed)
        {
            int64       consumed_by_shortcut = consume_xids_shortcut();
 
            if (consumed_by_shortcut > 0)
            {
                consumed += consumed_by_shortcut;
                continue;
            }
        }
 
        /* Slow path: Call GetNewTransactionId to allocate a new XID. */
        lastxid = GetNewTransactionId(true);
        consumed++;
 
        /* Report progress */
        if (consumed - last_reported_at >= REPORT_INTERVAL)
        {
            if (nxids > 0)
                elog(NOTICE, "consumed %llu / %llu XIDs, latest %u:%u",
                     (unsigned long long) consumed, (unsigned long long) nxids,
                     EpochFromFullTransactionId(lastxid),
                     XidFromFullTransactionId(lastxid));
            else
                elog(NOTICE, "consumed up to %u:%u / %u:%u",
                     EpochFromFullTransactionId(lastxid),
                     XidFromFullTransactionId(lastxid),
                     EpochFromFullTransactionId(untilxid),
                     XidFromFullTransactionId(untilxid));
            last_reported_at = consumed;
        }
    }
 
    return lastxid;
}
 
/*
 * These constants copied from .c files, because they're private.
 */
#define COMMIT_TS_XACTS_PER_PAGE (BLCKSZ / 10)
#define SUBTRANS_XACTS_PER_PAGE (BLCKSZ / sizeof(TransactionId))
#define CLOG_XACTS_PER_BYTE 4
#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
 
/*
 * All the interesting action in GetNewTransactionId happens when we extend
 * the SLRUs, or at the uint32 wraparound. If the nextXid counter is not close
 * to any of those interesting values, take a shortcut and bump nextXID
 * directly, close to the next "interesting" value.
 */
static inline uint32
XidSkip(FullTransactionId fullxid)
{
    uint32      low = XidFromFullTransactionId(fullxid);
    uint32      rem;
    uint32      distance;
 
    if (low < 5 || low >= UINT32_MAX - 5)
        return 0;
    distance = UINT32_MAX - 5 - low;
 
    rem = low % COMMIT_TS_XACTS_PER_PAGE;
    if (rem == 0)
        return 0;
    distance = Min(distance, COMMIT_TS_XACTS_PER_PAGE - rem);
 
    rem = low % SUBTRANS_XACTS_PER_PAGE;
    if (rem == 0)
        return 0;
    distance = Min(distance, SUBTRANS_XACTS_PER_PAGE - rem);
 
    rem = low % CLOG_XACTS_PER_PAGE;
    if (rem == 0)
        return 0;
    distance = Min(distance, CLOG_XACTS_PER_PAGE - rem);
 
    return distance;
}
 
static int64
consume_xids_shortcut(void)
{
    FullTransactionId nextXid;
    uint32      consumed;
 
    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
    nextXid = TransamVariables->nextXid;
 
    /*
     * Go slow near the "interesting values". The interesting zones include 5
     * transactions before and after SLRU page switches.
     */
    consumed = XidSkip(nextXid);
    if (consumed > 0)
        TransamVariables->nextXid.value += (uint64) consumed;
 
    LWLockRelease(XidGenLock);
 
    return consumed;
}