clog.c

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/*-------------------------------------------------------------------------
 *
 * clog.c
 *      PostgreSQL transaction-commit-log manager
 *
 * This module replaces the old "pg_log" access code, which treated pg_log
 * essentially like a relation, in that it went through the regular buffer
 * manager.  The problem with that was that there wasn't any good way to
 * recycle storage space for transactions so old that they'll never be
 * looked up again.  Now we use specialized access code so that the commit
 * log can be broken into relatively small, independent segments.
 *
 * XLOG interactions: this module generates an XLOG record whenever a new
 * CLOG page is initialized to zeroes.  Other writes of CLOG come from
 * recording of transaction commit or abort in xact.c, which generates its
 * own XLOG records for these events and will re-perform the status update
 * on redo; so we need make no additional XLOG entry here.  For synchronous
 * transaction commits, the XLOG is guaranteed flushed through the XLOG commit
 * record before we are called to log a commit, so the WAL rule "write xlog
 * before data" is satisfied automatically.  However, for async commits we
 * must track the latest LSN affecting each CLOG page, so that we can flush
 * XLOG that far and satisfy the WAL rule.  We don't have to worry about this
 * for aborts (whether sync or async), since the post-crash assumption would
 * be that such transactions failed anyway.
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/clog.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/clog.h"
#include "access/slru.h"
#include "access/transam.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "storage/proc.h"
#include "storage/sync.h"
 
/*
 * Defines for CLOG 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,
 * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
 * and CLOG segment numbering at
 * 0xFFFFFFFF/CLOG_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 TruncateCLOG (see CLOGPagePrecedes).
 */
 
/* We need two bits per xact, so four xacts fit in a byte */
#define CLOG_BITS_PER_XACT  2
#define CLOG_XACTS_PER_BYTE 4
#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
#define CLOG_XACT_BITMASK   ((1 << CLOG_BITS_PER_XACT) - 1)
 
 
/*
 * Although we return an int64 the actual value can't currently exceed
 * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE.
 */
static inline int64
TransactionIdToPage(TransactionId xid)
{
    return xid / (int64) CLOG_XACTS_PER_PAGE;
}
 
#define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
#define TransactionIdToByte(xid)    (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
#define TransactionIdToBIndex(xid)  ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)
 
/* We store the latest async LSN for each group of transactions */
#define CLOG_XACTS_PER_LSN_GROUP    32  /* keep this a power of 2 */
#define CLOG_LSNS_PER_PAGE  (CLOG_XACTS_PER_PAGE / CLOG_XACTS_PER_LSN_GROUP)
 
#define GetLSNIndex(slotno, xid)    ((slotno) * CLOG_LSNS_PER_PAGE + \
    ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP)
 
/*
 * The number of subtransactions below which we consider to apply clog group
 * update optimization.  Testing reveals that the number higher than this can
 * hurt performance.
 */
#define THRESHOLD_SUBTRANS_CLOG_OPT 5
 
/*
 * Link to shared-memory data structures for CLOG control
 */
static SlruCtlData XactCtlData;
 
#define XactCtl (&XactCtlData)
 
 
static int  ZeroCLOGPage(int64 pageno, bool writeXlog);
static bool CLOGPagePrecedes(int64 page1, int64 page2);
static void WriteZeroPageXlogRec(int64 pageno);
static void WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact,
                                 Oid oldestXactDb);
static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
                                       TransactionId *subxids, XidStatus status,
                                       XLogRecPtr lsn, int64 pageno,
                                       bool all_xact_same_page);
static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status,
                                      XLogRecPtr lsn, int slotno);
static void set_status_by_pages(int nsubxids, TransactionId *subxids,
                                XidStatus status, XLogRecPtr lsn);
static bool TransactionGroupUpdateXidStatus(TransactionId xid,
                                            XidStatus status, XLogRecPtr lsn, int64 pageno);
static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
                                               TransactionId *subxids, XidStatus status,
                                               XLogRecPtr lsn, int64 pageno);
 
 
/*
 * TransactionIdSetTreeStatus
 *
 * Record the final state of transaction entries in the commit log for
 * a transaction and its subtransaction tree. Take care to ensure this is
 * efficient, and as atomic as possible.
 *
 * xid is a single xid to set status for. This will typically be
 * the top level transactionid for a top level commit or abort. It can
 * also be a subtransaction when we record transaction aborts.
 *
 * subxids is an array of xids of length nsubxids, representing subtransactions
 * in the tree of xid. In various cases nsubxids may be zero.
 *
 * lsn must be the WAL location of the commit record when recording an async
 * commit.  For a synchronous commit it can be InvalidXLogRecPtr, since the
 * caller guarantees the commit record is already flushed in that case.  It
 * should be InvalidXLogRecPtr for abort cases, too.
 *
 * In the commit case, atomicity is limited by whether all the subxids are in
 * the same CLOG page as xid.  If they all are, then the lock will be grabbed
 * only once, and the status will be set to committed directly.  Otherwise
 * we must
 *   1. set sub-committed all subxids that are not on the same page as the
 *      main xid
 *   2. atomically set committed the main xid and the subxids on the same page
 *   3. go over the first bunch again and set them committed
 * Note that as far as concurrent checkers are concerned, main transaction
 * commit as a whole is still atomic.
 *
 * Example:
 *      TransactionId t commits and has subxids t1, t2, t3, t4
 *      t is on page p1, t1 is also on p1, t2 and t3 are on p2, t4 is on p3
 *      1. update pages2-3:
 *                  page2: set t2,t3 as sub-committed
 *                  page3: set t4 as sub-committed
 *      2. update page1:
 *                  page1: set t,t1 as committed
 *      3. update pages2-3:
 *                  page2: set t2,t3 as committed
 *                  page3: set t4 as committed
 *
 * NB: this is a low-level routine and is NOT the preferred entry point
 * for most uses; functions in transam.c are the intended callers.
 *
 * XXX Think about issuing POSIX_FADV_WILLNEED on pages that we will need,
 * but aren't yet in cache, as well as hinting pages not to fall out of
 * cache yet.
 */
void
TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
                           TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
{
    int64       pageno = TransactionIdToPage(xid);  /* get page of parent */
    int         i;
 
    Assert(status == TRANSACTION_STATUS_COMMITTED ||
           status == TRANSACTION_STATUS_ABORTED);
 
    /*
     * See how many subxids, if any, are on the same page as the parent, if
     * any.
     */
    for (i = 0; i < nsubxids; i++)
    {
        if (TransactionIdToPage(subxids[i]) != pageno)
            break;
    }
 
    /*
     * Do all items fit on a single page?
     */
    if (i == nsubxids)
    {
        /*
         * Set the parent and all subtransactions in a single call
         */
        TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
                                   pageno, true);
    }
    else
    {
        int         nsubxids_on_first_page = i;
 
        /*
         * If this is a commit then we care about doing this correctly (i.e.
         * using the subcommitted intermediate status).  By here, we know
         * we're updating more than one page of clog, so we must mark entries
         * that are *not* on the first page so that they show as subcommitted
         * before we then return to update the status to fully committed.
         *
         * To avoid touching the first page twice, skip marking subcommitted
         * for the subxids on that first page.
         */
        if (status == TRANSACTION_STATUS_COMMITTED)
            set_status_by_pages(nsubxids - nsubxids_on_first_page,
                                subxids + nsubxids_on_first_page,
                                TRANSACTION_STATUS_SUB_COMMITTED, lsn);
 
        /*
         * Now set the parent and subtransactions on same page as the parent,
         * if any
         */
        pageno = TransactionIdToPage(xid);
        TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status,
                                   lsn, pageno, false);
 
        /*
         * Now work through the rest of the subxids one clog page at a time,
         * starting from the second page onwards, like we did above.
         */
        set_status_by_pages(nsubxids - nsubxids_on_first_page,
                            subxids + nsubxids_on_first_page,
                            status, lsn);
    }
}
 
/*
 * Helper for TransactionIdSetTreeStatus: set the status for a bunch of
 * transactions, chunking in the separate CLOG pages involved. We never
 * pass the whole transaction tree to this function, only subtransactions
 * that are on different pages to the top level transaction id.
 */
static void
set_status_by_pages(int nsubxids, TransactionId *subxids,
                    XidStatus status, XLogRecPtr lsn)
{
    int64       pageno = TransactionIdToPage(subxids[0]);
    int         offset = 0;
    int         i = 0;
 
    Assert(nsubxids > 0);       /* else the pageno fetch above is unsafe */
 
    while (i < nsubxids)
    {
        int         num_on_page = 0;
        int64       nextpageno;
 
        do
        {
            nextpageno = TransactionIdToPage(subxids[i]);
            if (nextpageno != pageno)
                break;
            num_on_page++;
            i++;
        } while (i < nsubxids);
 
        TransactionIdSetPageStatus(InvalidTransactionId,
                                   num_on_page, subxids + offset,
                                   status, lsn, pageno, false);
        offset = i;
        pageno = nextpageno;
    }
}
 
/*
 * Record the final state of transaction entries in the commit log for all
 * entries on a single page.  Atomic only on this page.
 */
static void
TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
                           TransactionId *subxids, XidStatus status,
                           XLogRecPtr lsn, int64 pageno,
                           bool all_xact_same_page)
{
    /* Can't use group update when PGPROC overflows. */
    StaticAssertDecl(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS,
                     "group clog threshold less than PGPROC cached subxids");
 
    /*
     * When there is contention on XactSLRULock, we try to group multiple
     * updates; a single leader process will perform transaction status
     * updates for multiple backends so that the number of times XactSLRULock
     * needs to be acquired is reduced.
     *
     * For this optimization to be safe, the XID and subxids in MyProc must be
     * the same as the ones for which we're setting the status.  Check that
     * this is the case.
     *
     * For this optimization to be efficient, we shouldn't have too many
     * sub-XIDs and all of the XIDs for which we're adjusting clog should be
     * on the same page.  Check those conditions, too.
     */
    if (all_xact_same_page && xid == MyProc->xid &&
        nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
        nsubxids == MyProc->subxidStatus.count &&
        (nsubxids == 0 ||
         memcmp(subxids, MyProc->subxids.xids,
                nsubxids * sizeof(TransactionId)) == 0))
    {
        /*
         * If we can immediately acquire XactSLRULock, we update the status of
         * our own XID and release the lock.  If not, try use group XID
         * update.  If that doesn't work out, fall back to waiting for the
         * lock to perform an update for this transaction only.
         */
        if (LWLockConditionalAcquire(XactSLRULock, LW_EXCLUSIVE))
        {
            /* Got the lock without waiting!  Do the update. */
            TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
                                               lsn, pageno);
            LWLockRelease(XactSLRULock);
            return;
        }
        else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
        {
            /* Group update mechanism has done the work. */
            return;
        }
 
        /* Fall through only if update isn't done yet. */
    }
 
    /* Group update not applicable, or couldn't accept this page number. */
    LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
    TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
                                       lsn, pageno);
    LWLockRelease(XactSLRULock);
}
 
/*
 * Record the final state of transaction entry in the commit log
 *
 * We don't do any locking here; caller must handle that.
 */
static void
TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
                                   TransactionId *subxids, XidStatus status,
                                   XLogRecPtr lsn, int64 pageno)
{
    int         slotno;
    int         i;
 
    Assert(status == TRANSACTION_STATUS_COMMITTED ||
           status == TRANSACTION_STATUS_ABORTED ||
           (status == TRANSACTION_STATUS_SUB_COMMITTED && !TransactionIdIsValid(xid)));
    Assert(LWLockHeldByMeInMode(XactSLRULock, LW_EXCLUSIVE));
 
    /*
     * If we're doing an async commit (ie, lsn is valid), then we must wait
     * for any active write on the page slot to complete.  Otherwise our
     * update could reach disk in that write, which will not do since we
     * mustn't let it reach disk until we've done the appropriate WAL flush.
     * But when lsn is invalid, it's OK to scribble on a page while it is
     * write-busy, since we don't care if the update reaches disk sooner than
     * we think.
     */
    slotno = SimpleLruReadPage(XactCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
 
    /*
     * Set the main transaction id, if any.
     *
     * If we update more than one xid on this page while it is being written
     * out, we might find that some of the bits go to disk and others don't.
     * If we are updating commits on the page with the top-level xid that
     * could break atomicity, so we subcommit the subxids first before we mark
     * the top-level commit.
     */
    if (TransactionIdIsValid(xid))
    {
        /* Subtransactions first, if needed ... */
        if (status == TRANSACTION_STATUS_COMMITTED)
        {
            for (i = 0; i < nsubxids; i++)
            {
                Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
                TransactionIdSetStatusBit(subxids[i],
                                          TRANSACTION_STATUS_SUB_COMMITTED,
                                          lsn, slotno);
            }
        }
 
        /* ... then the main transaction */
        TransactionIdSetStatusBit(xid, status, lsn, slotno);
    }
 
    /* Set the subtransactions */
    for (i = 0; i < nsubxids; i++)
    {
        Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
        TransactionIdSetStatusBit(subxids[i], status, lsn, slotno);
    }
 
    XactCtl->shared->page_dirty[slotno] = true;
}
 
/*
 * When we cannot immediately acquire XactSLRULock in exclusive mode at
 * commit time, add ourselves to a list of processes that need their XIDs
 * status update.  The first process to add itself to the list will acquire
 * XactSLRULock in exclusive mode and set transaction status as required
 * on behalf of all group members.  This avoids a great deal of contention
 * around XactSLRULock when many processes are trying to commit at once,
 * since the lock need not be repeatedly handed off from one committing
 * process to the next.
 *
 * Returns true when transaction status has been updated in clog; returns
 * false if we decided against applying the optimization because the page
 * number we need to update differs from those processes already waiting.
 */
static bool
TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
                                XLogRecPtr lsn, int64 pageno)
{
    volatile PROC_HDR *procglobal = ProcGlobal;
    PGPROC     *proc = MyProc;
    uint32      nextidx;
    uint32      wakeidx;
 
    /* We should definitely have an XID whose status needs to be updated. */
    Assert(TransactionIdIsValid(xid));
 
    /*
     * Add ourselves to the list of processes needing a group XID status
     * update.
     */
    proc->clogGroupMember = true;
    proc->clogGroupMemberXid = xid;
    proc->clogGroupMemberXidStatus = status;
    proc->clogGroupMemberPage = pageno;
    proc->clogGroupMemberLsn = lsn;
 
    nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
 
    while (true)
    {
        /*
         * Add the proc to list, if the clog page where we need to update the
         * current transaction status is same as group leader's clog page.
         *
         * There is a race condition here, which is that after doing the below
         * check and before adding this proc's clog update to a group, the
         * group leader might have already finished the group update for this
         * page and becomes group leader of another group. This will lead to a
         * situation where a single group can have different clog page
         * updates.  This isn't likely and will still work, just maybe a bit
         * less efficiently.
         */
        if (nextidx != INVALID_PGPROCNO &&
            ProcGlobal->allProcs[nextidx].clogGroupMemberPage != proc->clogGroupMemberPage)
        {
            /*
             * Ensure that this proc is not a member of any clog group that
             * needs an XID status update.
             */
            proc->clogGroupMember = false;
            pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PGPROCNO);
            return false;
        }
 
        pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
 
        if (pg_atomic_compare_exchange_u32(&procglobal->clogGroupFirst,
                                           &nextidx,
                                           (uint32) proc->pgprocno))
            break;
    }
 
    /*
     * If the list was not empty, the leader will update the status of our
     * XID. It is impossible to have followers without a leader because the
     * first process that has added itself to the list will always have
     * nextidx as INVALID_PGPROCNO.
     */
    if (nextidx != INVALID_PGPROCNO)
    {
        int         extraWaits = 0;
 
        /* Sleep until the leader updates our XID status. */
        pgstat_report_wait_start(WAIT_EVENT_XACT_GROUP_UPDATE);
        for (;;)
        {
            /* acts as a read barrier */
            PGSemaphoreLock(proc->sem);
            if (!proc->clogGroupMember)
                break;
            extraWaits++;
        }
        pgstat_report_wait_end();
 
        Assert(pg_atomic_read_u32(&proc->clogGroupNext) == INVALID_PGPROCNO);
 
        /* Fix semaphore count for any absorbed wakeups */
        while (extraWaits-- > 0)
            PGSemaphoreUnlock(proc->sem);
        return true;
    }
 
    /* We are the leader.  Acquire the lock on behalf of everyone. */
    LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
 
    /*
     * Now that we've got the lock, clear the list of processes waiting for
     * group XID status update, saving a pointer to the head of the list.
     * Trying to pop elements one at a time could lead to an ABA problem.
     */
    nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst,
                                     INVALID_PGPROCNO);
 
    /* Remember head of list so we can perform wakeups after dropping lock. */
    wakeidx = nextidx;
 
    /* Walk the list and update the status of all XIDs. */
    while (nextidx != INVALID_PGPROCNO)
    {
        PGPROC     *nextproc = &ProcGlobal->allProcs[nextidx];
 
        /*
         * Transactions with more than THRESHOLD_SUBTRANS_CLOG_OPT sub-XIDs
         * should not use group XID status update mechanism.
         */
        Assert(nextproc->subxidStatus.count <= THRESHOLD_SUBTRANS_CLOG_OPT);
 
        TransactionIdSetPageStatusInternal(nextproc->clogGroupMemberXid,
                                           nextproc->subxidStatus.count,
                                           nextproc->subxids.xids,
                                           nextproc->clogGroupMemberXidStatus,
                                           nextproc->clogGroupMemberLsn,
                                           nextproc->clogGroupMemberPage);
 
        /* Move to next proc in list. */
        nextidx = pg_atomic_read_u32(&nextproc->clogGroupNext);
    }
 
    /* We're done with the lock now. */
    LWLockRelease(XactSLRULock);
 
    /*
     * Now that we've released the lock, go back and wake everybody up.  We
     * don't do this under the lock so as to keep lock hold times to a
     * minimum.
     */
    while (wakeidx != INVALID_PGPROCNO)
    {
        PGPROC     *wakeproc = &ProcGlobal->allProcs[wakeidx];
 
        wakeidx = pg_atomic_read_u32(&wakeproc->clogGroupNext);
        pg_atomic_write_u32(&wakeproc->clogGroupNext, INVALID_PGPROCNO);
 
        /* ensure all previous writes are visible before follower continues. */
        pg_write_barrier();
 
        wakeproc->clogGroupMember = false;
 
        if (wakeproc != MyProc)
            PGSemaphoreUnlock(wakeproc->sem);
    }
 
    return true;
}
 
/*
 * Sets the commit status of a single transaction.
 *
 * Must be called with XactSLRULock held
 */
static void
TransactionIdSetStatusBit(TransactionId xid, XidStatus status, XLogRecPtr lsn, int slotno)
{
    int         byteno = TransactionIdToByte(xid);
    int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
    char       *byteptr;
    char        byteval;
    char        curval;
 
    byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
    curval = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
 
    /*
     * When replaying transactions during recovery we still need to perform
     * the two phases of subcommit and then commit. However, some transactions
     * are already correctly marked, so we just treat those as a no-op which
     * allows us to keep the following Assert as restrictive as possible.
     */
    if (InRecovery && status == TRANSACTION_STATUS_SUB_COMMITTED &&
        curval == TRANSACTION_STATUS_COMMITTED)
        return;
 
    /*
     * Current state change should be from 0 or subcommitted to target state
     * or we should already be there when replaying changes during recovery.
     */
    Assert(curval == 0 ||
           (curval == TRANSACTION_STATUS_SUB_COMMITTED &&
            status != TRANSACTION_STATUS_IN_PROGRESS) ||
           curval == status);
 
    /* note this assumes exclusive access to the clog page */
    byteval = *byteptr;
    byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
    byteval |= (status << bshift);
    *byteptr = byteval;
 
    /*
     * Update the group LSN if the transaction completion LSN is higher.
     *
     * Note: lsn will be invalid when supplied during InRecovery processing,
     * so we don't need to do anything special to avoid LSN updates during
     * recovery. After recovery completes the next clog change will set the
     * LSN correctly.
     */
    if (!XLogRecPtrIsInvalid(lsn))
    {
        int         lsnindex = GetLSNIndex(slotno, xid);
 
        if (XactCtl->shared->group_lsn[lsnindex] < lsn)
            XactCtl->shared->group_lsn[lsnindex] = lsn;
    }
}
 
/*
 * Interrogate the state of a transaction in the commit log.
 *
 * Aside from the actual commit status, this function returns (into *lsn)
 * an LSN that is late enough to be able to guarantee that if we flush up to
 * that LSN 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.
 *
 * NB: this is a low-level routine and is NOT the preferred entry point
 * for most uses; TransactionLogFetch() in transam.c is the intended caller.
 */
XidStatus
TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
{
    int64       pageno = TransactionIdToPage(xid);
    int         byteno = TransactionIdToByte(xid);
    int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
    int         slotno;
    int         lsnindex;
    char       *byteptr;
    XidStatus   status;
 
    /* lock is acquired by SimpleLruReadPage_ReadOnly */
 
    slotno = SimpleLruReadPage_ReadOnly(XactCtl, pageno, xid);
    byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
 
    status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
 
    lsnindex = GetLSNIndex(slotno, xid);
    *lsn = XactCtl->shared->group_lsn[lsnindex];
 
    LWLockRelease(XactSLRULock);
 
    return status;
}
 
/*
 * Number of shared CLOG buffers.
 *
 * On larger multi-processor systems, it is possible to have many CLOG page
 * requests in flight at one time which could lead to disk access for CLOG
 * page if the required page is not found in memory.  Testing revealed that we
 * can get the best performance by having 128 CLOG buffers, more than that it
 * doesn't improve performance.
 *
 * Unconditionally keeping the number of CLOG buffers to 128 did not seem like
 * a good idea, because it would increase the minimum amount of shared memory
 * required to start, which could be a problem for people running very small
 * configurations.  The following formula seems to represent a reasonable
 * compromise: people with very low values for shared_buffers will get fewer
 * CLOG buffers as well, and everyone else will get 128.
 */
Size
CLOGShmemBuffers(void)
{
    return Min(128, Max(4, NBuffers / 512));
}
 
/*
 * Initialization of shared memory for CLOG
 */
Size
CLOGShmemSize(void)
{
    return SimpleLruShmemSize(CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE);
}
 
void
CLOGShmemInit(void)
{
    XactCtl->PagePrecedes = CLOGPagePrecedes;
    SimpleLruInit(XactCtl, "Xact", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE,
                  XactSLRULock, "pg_xact", LWTRANCHE_XACT_BUFFER,
                  SYNC_HANDLER_CLOG, false);
    SlruPagePrecedesUnitTests(XactCtl, CLOG_XACTS_PER_PAGE);
}
 
/*
 * This func must be called ONCE on system install.  It creates
 * the initial CLOG segment.  (The CLOG directory is assumed to
 * have been created by initdb, and CLOGShmemInit must have been
 * called already.)
 */
void
BootStrapCLOG(void)
{
    int         slotno;
 
    LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
 
    /* Create and zero the first page of the commit log */
    slotno = ZeroCLOGPage(0, false);
 
    /* Make sure it's written out */
    SimpleLruWritePage(XactCtl, slotno);
    Assert(!XactCtl->shared->page_dirty[slotno]);
 
    LWLockRelease(XactSLRULock);
}
 
/*
 * Initialize (or reinitialize) a page of CLOG to zeroes.
 * If writeXlog is true, also emit an XLOG record saying we did this.
 *
 * The page is not actually written, just set up in shared memory.
 * The slot number of the new page is returned.
 *
 * Control lock must be held at entry, and will be held at exit.
 */
static int
ZeroCLOGPage(int64 pageno, bool writeXlog)
{
    int         slotno;
 
    slotno = SimpleLruZeroPage(XactCtl, pageno);
 
    if (writeXlog)
        WriteZeroPageXlogRec(pageno);
 
    return slotno;
}
 
/*
 * This must be called ONCE during postmaster or standalone-backend startup,
 * after StartupXLOG has initialized ShmemVariableCache->nextXid.
 */
void
StartupCLOG(void)
{
    TransactionId xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
    int64       pageno = TransactionIdToPage(xid);
 
    LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
 
    /*
     * Initialize our idea of the latest page number.
     */
    XactCtl->shared->latest_page_number = pageno;
 
    LWLockRelease(XactSLRULock);
}
 
/*
 * This must be called ONCE at the end of startup/recovery.
 */
void
TrimCLOG(void)
{
    TransactionId xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
    int64       pageno = TransactionIdToPage(xid);
 
    LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
 
    /*
     * Zero out the remainder of the current clog page.  Under normal
     * circumstances it should be zeroes already, but it seems at least
     * theoretically possible that XLOG replay will have settled on a nextXID
     * value that is less than the last XID actually used and marked by the
     * previous database lifecycle (since subtransaction commit writes clog
     * but makes no WAL entry).  Let's just be safe. (We need not worry about
     * pages beyond the current one, since those will be zeroed when first
     * used.  For the same reason, there is no need to do anything when
     * nextXid is exactly at a page boundary; and it's likely that the
     * "current" page doesn't exist yet in that case.)
     */
    if (TransactionIdToPgIndex(xid) != 0)
    {
        int         byteno = TransactionIdToByte(xid);
        int         bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
        int         slotno;
        char       *byteptr;
 
        slotno = SimpleLruReadPage(XactCtl, pageno, false, xid);
        byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
 
        /* Zero so-far-unused positions in the current byte */
        *byteptr &= (1 << bshift) - 1;
        /* Zero the rest of the page */
        MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
 
        XactCtl->shared->page_dirty[slotno] = true;
    }
 
    LWLockRelease(XactSLRULock);
}
 
/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 */
void
CheckPointCLOG(void)
{
    /*
     * Write dirty CLOG pages to disk.  This may result in sync requests
     * queued for later handling by ProcessSyncRequests(), as part of the
     * checkpoint.
     */
    TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
    SimpleLruWriteAll(XactCtl, true);
    TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
}
 
 
/*
 * Make sure that CLOG 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 clog or xlog page to make room
 * in shared memory.
 */
void
ExtendCLOG(TransactionId newestXact)
{
    int64       pageno;
 
    /*
     * No work except at first XID of a page.  But beware: just after
     * wraparound, the first XID of page zero is FirstNormalTransactionId.
     */
    if (TransactionIdToPgIndex(newestXact) != 0 &&
        !TransactionIdEquals(newestXact, FirstNormalTransactionId))
        return;
 
    pageno = TransactionIdToPage(newestXact);
 
    LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
 
    /* Zero the page and make an XLOG entry about it */
    ZeroCLOGPage(pageno, true);
 
    LWLockRelease(XactSLRULock);
}
 
 
/*
 * Remove all CLOG segments before the one holding the passed transaction ID
 *
 * Before removing any CLOG data, we must flush XLOG to disk, to ensure
 * that any recently-emitted FREEZE_PAGE records have reached disk; otherwise
 * a crash and restart might leave us with some unfrozen tuples referencing
 * removed CLOG data.  We choose to emit a special TRUNCATE XLOG record too.
 * Replaying the deletion from XLOG is not critical, since the files could
 * just as well be removed later, but doing so prevents a long-running hot
 * standby server from acquiring an unreasonably bloated CLOG directory.
 *
 * Since CLOG segments hold a large number of transactions, the opportunity to
 * actually remove a segment is fairly rare, and so it seems best not to do
 * the XLOG flush unless we have confirmed that there is a removable segment.
 */
void
TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
{
    int64       cutoffPage;
 
    /*
     * The cutoff point is the start of the segment containing oldestXact. We
     * pass the *page* containing oldestXact to SimpleLruTruncate.
     */
    cutoffPage = TransactionIdToPage(oldestXact);
 
    /* Check to see if there's any files that could be removed */
    if (!SlruScanDirectory(XactCtl, SlruScanDirCbReportPresence, &cutoffPage))
        return;                 /* nothing to remove */
 
    /*
     * Advance oldestClogXid before truncating clog, so concurrent xact status
     * lookups can ensure they don't attempt to access truncated-away clog.
     *
     * It's only necessary to do this if we will actually truncate away clog
     * pages.
     */
    AdvanceOldestClogXid(oldestXact);
 
    /*
     * Write XLOG record and flush XLOG to disk. We record the oldest xid
     * we're keeping information about here so we can ensure that it's always
     * ahead of clog truncation in case we crash, and so a standby finds out
     * the new valid xid before the next checkpoint.
     */
    WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
 
    /* Now we can remove the old CLOG segment(s) */
    SimpleLruTruncate(XactCtl, cutoffPage);
}
 
 
/*
 * Decide whether a CLOG page number is "older" for truncation purposes.
 *
 * We need to use comparison of TransactionIds here in order to do the right
 * thing with wraparound XID arithmetic.  However, TransactionIdPrecedes()
 * would get weird about permanent xact IDs.  So, offset both such that xid1,
 * xid2, and xid2 + CLOG_XACTS_PER_PAGE - 1 are all normal XIDs; this offset
 * is relevant to page 0 and to the page preceding page 0.
 *
 * The page containing oldestXact-2^31 is the important edge case.  The
 * portion of that page equaling or following oldestXact-2^31 is expendable,
 * but the portion preceding oldestXact-2^31 is not.  When oldestXact-2^31 is
 * the first XID of a page and segment, the entire page and segment is
 * expendable, and we could truncate the segment.  Recognizing that case would
 * require making oldestXact, not just the page containing oldestXact,
 * available to this callback.  The benefit would be rare and small, so we
 * don't optimize that edge case.
 */
static bool
CLOGPagePrecedes(int64 page1, int64 page2)
{
    TransactionId xid1;
    TransactionId xid2;
 
    xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
    xid1 += FirstNormalTransactionId + 1;
    xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
    xid2 += FirstNormalTransactionId + 1;
 
    return (TransactionIdPrecedes(xid1, xid2) &&
            TransactionIdPrecedes(xid1, xid2 + CLOG_XACTS_PER_PAGE - 1));
}
 
 
/*
 * Write a ZEROPAGE xlog record
 */
static void
WriteZeroPageXlogRec(int64 pageno)
{
    XLogBeginInsert();
    XLogRegisterData((char *) (&pageno), sizeof(pageno));
    (void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE);
}
 
/*
 * Write a TRUNCATE xlog record
 *
 * We must flush the xlog record to disk before returning --- see notes
 * in TruncateCLOG().
 */
static void
WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact, Oid oldestXactDb)
{
    XLogRecPtr  recptr;
    xl_clog_truncate xlrec;
 
    xlrec.pageno = pageno;
    xlrec.oldestXact = oldestXact;
    xlrec.oldestXactDb = oldestXactDb;
 
    XLogBeginInsert();
    XLogRegisterData((char *) (&xlrec), sizeof(xl_clog_truncate));
    recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
    XLogFlush(recptr);
}
 
/*
 * CLOG resource manager's routines
 */
void
clog_redo(XLogReaderState *record)
{
    uint8       info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
 
    /* Backup blocks are not used in clog records */
    Assert(!XLogRecHasAnyBlockRefs(record));
 
    if (info == CLOG_ZEROPAGE)
    {
        int64       pageno;
        int         slotno;
 
        memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));
 
        LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
 
        slotno = ZeroCLOGPage(pageno, false);
        SimpleLruWritePage(XactCtl, slotno);
        Assert(!XactCtl->shared->page_dirty[slotno]);
 
        LWLockRelease(XactSLRULock);
    }
    else if (info == CLOG_TRUNCATE)
    {
        xl_clog_truncate xlrec;
 
        memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
 
        AdvanceOldestClogXid(xlrec.oldestXact);
 
        SimpleLruTruncate(XactCtl, xlrec.pageno);
    }
    else
        elog(PANIC, "clog_redo: unknown op code %u", info);
}
 
/*
 * Entrypoint for sync.c to sync clog files.
 */
int
clogsyncfiletag(const FileTag *ftag, char *path)
{
    return SlruSyncFileTag(XactCtl, ftag, path);
}