subtrans.c

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/*-------------------------------------------------------------------------
 *
 * subtrans.c
 *      PostgreSQL subtransaction-log manager
 *
 * The pg_subtrans manager is a pg_xact-like manager that stores the parent
 * transaction Id for each transaction.  It is a fundamental part of the
 * nested transactions implementation.  A main transaction has a parent
 * of InvalidTransactionId, and each subtransaction has its immediate parent.
 * The tree can easily be walked from child to parent, but not in the
 * opposite direction.
 *
 * This code is based on xact.c, but the robustness requirements
 * are completely different from pg_xact, because we only need to remember
 * pg_subtrans information for currently-open transactions.  Thus, there is
 * no need to preserve data over a crash and restart.
 *
 * There are no XLOG interactions since we do not care about preserving
 * data across crashes.  During database startup, we simply force the
 * currently-active page of SUBTRANS to zeroes.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/subtrans.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/slru.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "utils/guc_hooks.h"
#include "utils/snapmgr.h"
 
 
/*
 * Defines for SubTrans page sizes.  A page is the same BLCKSZ as is used
 * everywhere else in Postgres.
 *
 * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
 * SubTrans page numbering also wraps around at
 * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE, and segment numbering at
 * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need take no
 * explicit notice of that fact in this module, except when comparing segment
 * and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes) and zeroing
 * them in StartupSUBTRANS.
 */
 
/* We need four bytes per xact */
#define SUBTRANS_XACTS_PER_PAGE (BLCKSZ / sizeof(TransactionId))
 
/*
 * Although we return an int64 the actual value can't currently exceed
 * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE.
 */
static inline int64
TransactionIdToPage(TransactionId xid)
{
    return xid / (int64) SUBTRANS_XACTS_PER_PAGE;
}
 
#define TransactionIdToEntry(xid) ((xid) % (TransactionId) SUBTRANS_XACTS_PER_PAGE)
 
 
/*
 * Link to shared-memory data structures for SUBTRANS control
 */
static SlruCtlData SubTransCtlData;
 
#define SubTransCtl  (&SubTransCtlData)
 
 
static bool SubTransPagePrecedes(int64 page1, int64 page2);
 
 
/*
 * Record the parent of a subtransaction in the subtrans log.
 */
void
SubTransSetParent(TransactionId xid, TransactionId parent)
{
    int64       pageno = TransactionIdToPage(xid);
    int         entryno = TransactionIdToEntry(xid);
    int         slotno;
    LWLock     *lock;
    TransactionId *ptr;
 
    Assert(TransactionIdIsValid(parent));
    Assert(TransactionIdFollows(xid, parent));
 
    lock = SimpleLruGetBankLock(SubTransCtl, pageno);
    LWLockAcquire(lock, LW_EXCLUSIVE);
 
    slotno = SimpleLruReadPage(SubTransCtl, pageno, true, xid);
    ptr = (TransactionId *) SubTransCtl->shared->page_buffer[slotno];
    ptr += entryno;
 
    /*
     * It's possible we'll try to set the parent xid multiple times but we
     * shouldn't ever be changing the xid from one valid xid to another valid
     * xid, which would corrupt the data structure.
     */
    if (*ptr != parent)
    {
        Assert(*ptr == InvalidTransactionId);
        *ptr = parent;
        SubTransCtl->shared->page_dirty[slotno] = true;
    }
 
    LWLockRelease(lock);
}
 
/*
 * Interrogate the parent of a transaction in the subtrans log.
 */
TransactionId
SubTransGetParent(TransactionId xid)
{
    int64       pageno = TransactionIdToPage(xid);
    int         entryno = TransactionIdToEntry(xid);
    int         slotno;
    TransactionId *ptr;
    TransactionId parent;
 
    /* Can't ask about stuff that might not be around anymore */
    Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));
 
    /* Bootstrap and frozen XIDs have no parent */
    if (!TransactionIdIsNormal(xid))
        return InvalidTransactionId;
 
    /* lock is acquired by SimpleLruReadPage_ReadOnly */
 
    slotno = SimpleLruReadPage_ReadOnly(SubTransCtl, pageno, xid);
    ptr = (TransactionId *) SubTransCtl->shared->page_buffer[slotno];
    ptr += entryno;
 
    parent = *ptr;
 
    LWLockRelease(SimpleLruGetBankLock(SubTransCtl, pageno));
 
    return parent;
}
 
/*
 * SubTransGetTopmostTransaction
 *
 * Returns the topmost transaction of the given transaction id.
 *
 * Because we cannot look back further than TransactionXmin, it is possible
 * that this function will lie and return an intermediate subtransaction ID
 * instead of the true topmost parent ID.  This is OK, because in practice
 * we only care about detecting whether the topmost parent is still running
 * or is part of a current snapshot's list of still-running transactions.
 * Therefore, any XID before TransactionXmin is as good as any other.
 */
TransactionId
SubTransGetTopmostTransaction(TransactionId xid)
{
    TransactionId parentXid = xid,
                previousXid = xid;
 
    /* Can't ask about stuff that might not be around anymore */
    Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));
 
    while (TransactionIdIsValid(parentXid))
    {
        previousXid = parentXid;
        if (TransactionIdPrecedes(parentXid, TransactionXmin))
            break;
        parentXid = SubTransGetParent(parentXid);
 
        /*
         * By convention the parent xid gets allocated first, so should always
         * precede the child xid. Anything else points to a corrupted data
         * structure that could lead to an infinite loop, so exit.
         */
        if (!TransactionIdPrecedes(parentXid, previousXid))
            elog(ERROR, "pg_subtrans contains invalid entry: xid %u points to parent xid %u",
                 previousXid, parentXid);
    }
 
    Assert(TransactionIdIsValid(previousXid));
 
    return previousXid;
}
 
/*
 * Number of shared SUBTRANS buffers.
 *
 * If asked to autotune, use 2MB for every 1GB of shared buffers, up to 8MB.
 * Otherwise just cap the configured amount to be between 16 and the maximum
 * allowed.
 */
static int
SUBTRANSShmemBuffers(void)
{
    /* auto-tune based on shared buffers */
    if (subtransaction_buffers == 0)
        return SimpleLruAutotuneBuffers(512, 1024);
 
    return Min(Max(16, subtransaction_buffers), SLRU_MAX_ALLOWED_BUFFERS);
}
 
/*
 * Initialization of shared memory for SUBTRANS
 */
Size
SUBTRANSShmemSize(void)
{
    return SimpleLruShmemSize(SUBTRANSShmemBuffers(), 0);
}
 
void
SUBTRANSShmemInit(void)
{
    /* If auto-tuning is requested, now is the time to do it */
    if (subtransaction_buffers == 0)
    {
        char        buf[32];
 
        snprintf(buf, sizeof(buf), "%d", SUBTRANSShmemBuffers());
        SetConfigOption("subtransaction_buffers", buf, PGC_POSTMASTER,
                        PGC_S_DYNAMIC_DEFAULT);
 
        /*
         * We prefer to report this value's source as PGC_S_DYNAMIC_DEFAULT.
         * However, if the DBA explicitly set subtransaction_buffers = 0 in
         * the config file, then PGC_S_DYNAMIC_DEFAULT will fail to override
         * that and we must force the matter with PGC_S_OVERRIDE.
         */
        if (subtransaction_buffers == 0)    /* failed to apply it? */
            SetConfigOption("subtransaction_buffers", buf, PGC_POSTMASTER,
                            PGC_S_OVERRIDE);
    }
    Assert(subtransaction_buffers != 0);
 
    SubTransCtl->PagePrecedes = SubTransPagePrecedes;
    SimpleLruInit(SubTransCtl, "subtransaction", SUBTRANSShmemBuffers(), 0,
                  "pg_subtrans", LWTRANCHE_SUBTRANS_BUFFER,
                  LWTRANCHE_SUBTRANS_SLRU, SYNC_HANDLER_NONE, false);
    SlruPagePrecedesUnitTests(SubTransCtl, SUBTRANS_XACTS_PER_PAGE);
}
 
/*
 * GUC check_hook for subtransaction_buffers
 */
bool
check_subtrans_buffers(int *newval, void **extra, GucSource source)
{
    return check_slru_buffers("subtransaction_buffers", newval);
}
 
/*
 * This func must be called ONCE on system install.  It creates
 * the initial SUBTRANS segment.  (The SUBTRANS directory is assumed to
 * have been created by the initdb shell script, and SUBTRANSShmemInit
 * must have been called already.)
 *
 * Note: it's not really necessary to create the initial segment now,
 * since slru.c would create it on first write anyway.  But we may as well
 * do it to be sure the directory is set up correctly.
 */
void
BootStrapSUBTRANS(void)
{
    /* Zero the initial page and flush it to disk */
    SimpleLruZeroAndWritePage(SubTransCtl, 0);
}
 
/*
 * This must be called ONCE during postmaster or standalone-backend startup,
 * after StartupXLOG has initialized TransamVariables->nextXid.
 *
 * oldestActiveXID is the oldest XID of any prepared transaction, or nextXid
 * if there are none.
 */
void
StartupSUBTRANS(TransactionId oldestActiveXID)
{
    FullTransactionId nextXid;
    int64       startPage;
    int64       endPage;
    LWLock     *prevlock = NULL;
    LWLock     *lock;
 
    /*
     * Since we don't expect pg_subtrans to be valid across crashes, we
     * initialize the currently-active page(s) to zeroes during startup.
     * Whenever we advance into a new page, ExtendSUBTRANS will likewise zero
     * the new page without regard to whatever was previously on disk.
     */
    startPage = TransactionIdToPage(oldestActiveXID);
    nextXid = TransamVariables->nextXid;
    endPage = TransactionIdToPage(XidFromFullTransactionId(nextXid));
 
    for (;;)
    {
        lock = SimpleLruGetBankLock(SubTransCtl, startPage);
        if (prevlock != lock)
        {
            if (prevlock)
                LWLockRelease(prevlock);
            LWLockAcquire(lock, LW_EXCLUSIVE);
            prevlock = lock;
        }
 
        (void) SimpleLruZeroPage(SubTransCtl, startPage);
        if (startPage == endPage)
            break;
 
        startPage++;
        /* must account for wraparound */
        if (startPage > TransactionIdToPage(MaxTransactionId))
            startPage = 0;
    }
 
    LWLockRelease(lock);
}
 
/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 */
void
CheckPointSUBTRANS(void)
{
    /*
     * Write dirty SUBTRANS pages to disk
     *
     * This is not actually necessary from a correctness point of view. We do
     * it merely to improve the odds that writing of dirty pages is done by
     * the checkpoint process and not by backends.
     */
    TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_START(true);
    SimpleLruWriteAll(SubTransCtl, true);
    TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_DONE(true);
}
 
 
/*
 * Make sure that SUBTRANS has room for a newly-allocated XID.
 *
 * NB: this is called while holding XidGenLock.  We want it to be very fast
 * most of the time; even when it's not so fast, no actual I/O need happen
 * unless we're forced to write out a dirty subtrans page to make room
 * in shared memory.
 */
void
ExtendSUBTRANS(TransactionId newestXact)
{
    int64       pageno;
    LWLock     *lock;
 
    /*
     * No work except at first XID of a page.  But beware: just after
     * wraparound, the first XID of page zero is FirstNormalTransactionId.
     */
    if (TransactionIdToEntry(newestXact) != 0 &&
        !TransactionIdEquals(newestXact, FirstNormalTransactionId))
        return;
 
    pageno = TransactionIdToPage(newestXact);
 
    lock = SimpleLruGetBankLock(SubTransCtl, pageno);
    LWLockAcquire(lock, LW_EXCLUSIVE);
 
    /* Zero the page */
    SimpleLruZeroPage(SubTransCtl, pageno);
 
    LWLockRelease(lock);
}
 
 
/*
 * Remove all SUBTRANS segments before the one holding the passed transaction ID
 *
 * oldestXact is the oldest TransactionXmin of any running transaction.  This
 * is called only during checkpoint.
 */
void
TruncateSUBTRANS(TransactionId oldestXact)
{
    int64       cutoffPage;
 
    /*
     * The cutoff point is the start of the segment containing oldestXact. We
     * pass the *page* containing oldestXact to SimpleLruTruncate.  We step
     * back one transaction to avoid passing a cutoff page that hasn't been
     * created yet in the rare case that oldestXact would be the first item on
     * a page and oldestXact == next XID.  In that case, if we didn't subtract
     * one, we'd trigger SimpleLruTruncate's wraparound detection.
     */
    TransactionIdRetreat(oldestXact);
    cutoffPage = TransactionIdToPage(oldestXact);
 
    SimpleLruTruncate(SubTransCtl, cutoffPage);
}
 
 
/*
 * Decide whether a SUBTRANS page number is "older" for truncation purposes.
 * Analogous to CLOGPagePrecedes().
 */
static bool
SubTransPagePrecedes(int64 page1, int64 page2)
{
    TransactionId xid1;
    TransactionId xid2;
 
    xid1 = ((TransactionId) page1) * SUBTRANS_XACTS_PER_PAGE;
    xid1 += FirstNormalTransactionId + 1;
    xid2 = ((TransactionId) page2) * SUBTRANS_XACTS_PER_PAGE;
    xid2 += FirstNormalTransactionId + 1;
 
    return (TransactionIdPrecedes(xid1, xid2) &&
            TransactionIdPrecedes(xid1, xid2 + SUBTRANS_XACTS_PER_PAGE - 1));
}