xlogutils.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * xlogutils.c
 *
 * PostgreSQL write-ahead log manager utility routines
 *
 * This file contains support routines that are used by XLOG replay functions.
 * None of this code is used during normal system operation.
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/xlogutils.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>

#include "access/timeline.h"
#include "access/xlog.h"
#include "access/xlog_internal.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/smgr.h"
#include "utils/guc.h"
#include "utils/hsearch.h"
#include "utils/rel.h"


/* GUC variable */
bool        ignore_invalid_pages = false;

/*
 * During XLOG replay, we may see XLOG records for incremental updates of
 * pages that no longer exist, because their relation was later dropped or
 * truncated.  (Note: this is only possible when full_page_writes = OFF,
 * since when it's ON, the first reference we see to a page should always
 * be a full-page rewrite not an incremental update.)  Rather than simply
 * ignoring such records, we make a note of the referenced page, and then
 * complain if we don't actually see a drop or truncate covering the page
 * later in replay.
 */
typedef struct xl_invalid_page_key
{
    RelFileNode node;           /* the relation */
    ForkNumber  forkno;         /* the fork number */
    BlockNumber blkno;          /* the page */
} xl_invalid_page_key;

typedef struct xl_invalid_page
{
    xl_invalid_page_key key;    /* hash key ... must be first */
    bool        present;        /* page existed but contained zeroes */
} xl_invalid_page;

static HTAB *invalid_page_tab = NULL;


/* Report a reference to an invalid page */
static void
report_invalid_page(int elevel, RelFileNode node, ForkNumber forkno,
                    BlockNumber blkno, bool present)
{
    char       *path = relpathperm(node, forkno);

    if (present)
        elog(elevel, "page %u of relation %s is uninitialized",
             blkno, path);
    else
        elog(elevel, "page %u of relation %s does not exist",
             blkno, path);
    pfree(path);
}

/* Log a reference to an invalid page */
static void
log_invalid_page(RelFileNode node, ForkNumber forkno, BlockNumber blkno,
                 bool present)
{
    xl_invalid_page_key key;
    xl_invalid_page *hentry;
    bool        found;

    /*
     * Once recovery has reached a consistent state, the invalid-page table
     * should be empty and remain so. If a reference to an invalid page is
     * found after consistency is reached, PANIC immediately. This might seem
     * aggressive, but it's better than letting the invalid reference linger
     * in the hash table until the end of recovery and PANIC there, which
     * might come only much later if this is a standby server.
     */
    if (reachedConsistency)
    {
        report_invalid_page(WARNING, node, forkno, blkno, present);
        elog(ignore_invalid_pages ? WARNING : PANIC,
             "WAL contains references to invalid pages");
    }

    /*
     * Log references to invalid pages at DEBUG1 level.  This allows some
     * tracing of the cause (note the elog context mechanism will tell us
     * something about the XLOG record that generated the reference).
     */
    if (message_level_is_interesting(DEBUG1))
        report_invalid_page(DEBUG1, node, forkno, blkno, present);

    if (invalid_page_tab == NULL)
    {
        /* create hash table when first needed */
        HASHCTL     ctl;

        memset(&ctl, 0, sizeof(ctl));
        ctl.keysize = sizeof(xl_invalid_page_key);
        ctl.entrysize = sizeof(xl_invalid_page);

        invalid_page_tab = hash_create("XLOG invalid-page table",
                                       100,
                                       &ctl,
                                       HASH_ELEM | HASH_BLOBS);
    }

    /* we currently assume xl_invalid_page_key contains no padding */
    key.node = node;
    key.forkno = forkno;
    key.blkno = blkno;
    hentry = (xl_invalid_page *)
        hash_search(invalid_page_tab, (void *) &key, HASH_ENTER, &found);

    if (!found)
    {
        /* hash_search already filled in the key */
        hentry->present = present;
    }
    else
    {
        /* repeat reference ... leave "present" as it was */
    }
}

/* Forget any invalid pages >= minblkno, because they've been dropped */
static void
forget_invalid_pages(RelFileNode node, ForkNumber forkno, BlockNumber minblkno)
{
    HASH_SEQ_STATUS status;
    xl_invalid_page *hentry;

    if (invalid_page_tab == NULL)
        return;                 /* nothing to do */

    hash_seq_init(&status, invalid_page_tab);

    while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
    {
        if (RelFileNodeEquals(hentry->key.node, node) &&
            hentry->key.forkno == forkno &&
            hentry->key.blkno >= minblkno)
        {
            if (message_level_is_interesting(DEBUG2))
            {
                char       *path = relpathperm(hentry->key.node, forkno);

                elog(DEBUG2, "page %u of relation %s has been dropped",
                     hentry->key.blkno, path);
                pfree(path);
            }

            if (hash_search(invalid_page_tab,
                            (void *) &hentry->key,
                            HASH_REMOVE, NULL) == NULL)
                elog(ERROR, "hash table corrupted");
        }
    }
}

/* Forget any invalid pages in a whole database */
static void
forget_invalid_pages_db(Oid dbid)
{
    HASH_SEQ_STATUS status;
    xl_invalid_page *hentry;

    if (invalid_page_tab == NULL)
        return;                 /* nothing to do */

    hash_seq_init(&status, invalid_page_tab);

    while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
    {
        if (hentry->key.node.dbNode == dbid)
        {
            if (message_level_is_interesting(DEBUG2))
            {
                char       *path = relpathperm(hentry->key.node, hentry->key.forkno);

                elog(DEBUG2, "page %u of relation %s has been dropped",
                     hentry->key.blkno, path);
                pfree(path);
            }

            if (hash_search(invalid_page_tab,
                            (void *) &hentry->key,
                            HASH_REMOVE, NULL) == NULL)
                elog(ERROR, "hash table corrupted");
        }
    }
}

/* Are there any unresolved references to invalid pages? */
bool
XLogHaveInvalidPages(void)
{
    if (invalid_page_tab != NULL &&
        hash_get_num_entries(invalid_page_tab) > 0)
        return true;
    return false;
}

/* Complain about any remaining invalid-page entries */
void
XLogCheckInvalidPages(void)
{
    HASH_SEQ_STATUS status;
    xl_invalid_page *hentry;
    bool        foundone = false;

    if (invalid_page_tab == NULL)
        return;                 /* nothing to do */

    hash_seq_init(&status, invalid_page_tab);

    /*
     * Our strategy is to emit WARNING messages for all remaining entries and
     * only PANIC after we've dumped all the available info.
     */
    while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
    {
        report_invalid_page(WARNING, hentry->key.node, hentry->key.forkno,
                            hentry->key.blkno, hentry->present);
        foundone = true;
    }

    if (foundone)
        elog(ignore_invalid_pages ? WARNING : PANIC,
             "WAL contains references to invalid pages");

    hash_destroy(invalid_page_tab);
    invalid_page_tab = NULL;
}


/*
 * XLogReadBufferForRedo
 *      Read a page during XLOG replay
 *
 * Reads a block referenced by a WAL record into shared buffer cache, and
 * determines what needs to be done to redo the changes to it.  If the WAL
 * record includes a full-page image of the page, it is restored.
 *
 * 'record.EndRecPtr' is compared to the page's LSN to determine if the record
 * has already been replayed.  'block_id' is the ID number the block was
 * registered with, when the WAL record was created.
 *
 * Returns one of the following:
 *
 *  BLK_NEEDS_REDO  - changes from the WAL record need to be applied
 *  BLK_DONE        - block doesn't need replaying
 *  BLK_RESTORED    - block was restored from a full-page image included in
 *                    the record
 *  BLK_NOTFOUND    - block was not found (because it was truncated away by
 *                    an operation later in the WAL stream)
 *
 * On return, the buffer is locked in exclusive-mode, and returned in *buf.
 * Note that the buffer is locked and returned even if it doesn't need
 * replaying.  (Getting the buffer lock is not really necessary during
 * single-process crash recovery, but some subroutines such as MarkBufferDirty
 * will complain if we don't have the lock.  In hot standby mode it's
 * definitely necessary.)
 *
 * Note: when a backup block is available in XLOG with the BKPIMAGE_APPLY flag
 * set, we restore it, even if the page in the database appears newer.  This
 * is to protect ourselves against database pages that were partially or
 * incorrectly written during a crash.  We assume that the XLOG data must be
 * good because it has passed a CRC check, while the database page might not
 * be.  This will force us to replay all subsequent modifications of the page
 * that appear in XLOG, rather than possibly ignoring them as already
 * applied, but that's not a huge drawback.
 */
XLogRedoAction
XLogReadBufferForRedo(XLogReaderState *record, uint8 block_id,
                      Buffer *buf)
{
    return XLogReadBufferForRedoExtended(record, block_id, RBM_NORMAL,
                                         false, buf);
}

/*
 * Pin and lock a buffer referenced by a WAL record, for the purpose of
 * re-initializing it.
 */
Buffer
XLogInitBufferForRedo(XLogReaderState *record, uint8 block_id)
{
    Buffer      buf;

    XLogReadBufferForRedoExtended(record, block_id, RBM_ZERO_AND_LOCK, false,
                                  &buf);
    return buf;
}

/*
 * XLogReadBufferForRedoExtended
 *      Like XLogReadBufferForRedo, but with extra options.
 *
 * In RBM_ZERO_* modes, if the page doesn't exist, the relation is extended
 * with all-zeroes pages up to the referenced block number.  In
 * RBM_ZERO_AND_LOCK and RBM_ZERO_AND_CLEANUP_LOCK modes, the return value
 * is always BLK_NEEDS_REDO.
 *
 * (The RBM_ZERO_AND_CLEANUP_LOCK mode is redundant with the get_cleanup_lock
 * parameter. Do not use an inconsistent combination!)
 *
 * If 'get_cleanup_lock' is true, a "cleanup lock" is acquired on the buffer
 * using LockBufferForCleanup(), instead of a regular exclusive lock.
 */
XLogRedoAction
XLogReadBufferForRedoExtended(XLogReaderState *record,
                              uint8 block_id,
                              ReadBufferMode mode, bool get_cleanup_lock,
                              Buffer *buf)
{
    XLogRecPtr  lsn = record->EndRecPtr;
    RelFileNode rnode;
    ForkNumber  forknum;
    BlockNumber blkno;
    Page        page;
    bool        zeromode;
    bool        willinit;

    if (!XLogRecGetBlockTag(record, block_id, &rnode, &forknum, &blkno))
    {
        /* Caller specified a bogus block_id */
        elog(PANIC, "failed to locate backup block with ID %d", block_id);
    }

    /*
     * Make sure that if the block is marked with WILL_INIT, the caller is
     * going to initialize it. And vice versa.
     */
    zeromode = (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK);
    willinit = (record->blocks[block_id].flags & BKPBLOCK_WILL_INIT) != 0;
    if (willinit && !zeromode)
        elog(PANIC, "block with WILL_INIT flag in WAL record must be zeroed by redo routine");
    if (!willinit && zeromode)
        elog(PANIC, "block to be initialized in redo routine must be marked with WILL_INIT flag in the WAL record");

    /* If it has a full-page image and it should be restored, do it. */
    if (XLogRecBlockImageApply(record, block_id))
    {
        Assert(XLogRecHasBlockImage(record, block_id));
        *buf = XLogReadBufferExtended(rnode, forknum, blkno,
                                      get_cleanup_lock ? RBM_ZERO_AND_CLEANUP_LOCK : RBM_ZERO_AND_LOCK);
        page = BufferGetPage(*buf);
        if (!RestoreBlockImage(record, block_id, page))
            elog(ERROR, "failed to restore block image");

        /*
         * The page may be uninitialized. If so, we can't set the LSN because
         * that would corrupt the page.
         */
        if (!PageIsNew(page))
        {
            PageSetLSN(page, lsn);
        }

        MarkBufferDirty(*buf);

        /*
         * At the end of crash recovery the init forks of unlogged relations
         * are copied, without going through shared buffers. So we need to
         * force the on-disk state of init forks to always be in sync with the
         * state in shared buffers.
         */
        if (forknum == INIT_FORKNUM)
            FlushOneBuffer(*buf);

        return BLK_RESTORED;
    }
    else
    {
        *buf = XLogReadBufferExtended(rnode, forknum, blkno, mode);
        if (BufferIsValid(*buf))
        {
            if (mode != RBM_ZERO_AND_LOCK && mode != RBM_ZERO_AND_CLEANUP_LOCK)
            {
                if (get_cleanup_lock)
                    LockBufferForCleanup(*buf);
                else
                    LockBuffer(*buf, BUFFER_LOCK_EXCLUSIVE);
            }
            if (lsn <= PageGetLSN(BufferGetPage(*buf)))
                return BLK_DONE;
            else
                return BLK_NEEDS_REDO;
        }
        else
            return BLK_NOTFOUND;
    }
}

/*
 * XLogReadBufferExtended
 *      Read a page during XLOG replay
 *
 * This is functionally comparable to ReadBufferExtended. There's some
 * differences in the behavior wrt. the "mode" argument:
 *
 * In RBM_NORMAL mode, if the page doesn't exist, or contains all-zeroes, we
 * return InvalidBuffer. In this case the caller should silently skip the
 * update on this page. (In this situation, we expect that the page was later
 * dropped or truncated. If we don't see evidence of that later in the WAL
 * sequence, we'll complain at the end of WAL replay.)
 *
 * In RBM_ZERO_* modes, if the page doesn't exist, the relation is extended
 * with all-zeroes pages up to the given block number.
 *
 * In RBM_NORMAL_NO_LOG mode, we return InvalidBuffer if the page doesn't
 * exist, and we don't check for all-zeroes.  Thus, no log entry is made
 * to imply that the page should be dropped or truncated later.
 *
 * NB: A redo function should normally not call this directly. To get a page
 * to modify, use XLogReadBufferForRedoExtended instead. It is important that
 * all pages modified by a WAL record are registered in the WAL records, or
 * they will be invisible to tools that that need to know which pages are
 * modified.
 */
Buffer
XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,
                       BlockNumber blkno, ReadBufferMode mode)
{
    BlockNumber lastblock;
    Buffer      buffer;
    SMgrRelation smgr;

    Assert(blkno != P_NEW);

    /* Open the relation at smgr level */
    smgr = smgropen(rnode, InvalidBackendId);

    /*
     * Create the target file if it doesn't already exist.  This lets us cope
     * if the replay sequence contains writes to a relation that is later
     * deleted.  (The original coding of this routine would instead suppress
     * the writes, but that seems like it risks losing valuable data if the
     * filesystem loses an inode during a crash.  Better to write the data
     * until we are actually told to delete the file.)
     */
    smgrcreate(smgr, forknum, true);

    lastblock = smgrnblocks(smgr, forknum);

    if (blkno < lastblock)
    {
        /* page exists in file */
        buffer = ReadBufferWithoutRelcache(rnode, forknum, blkno,
                                           mode, NULL);
    }
    else
    {
        /* hm, page doesn't exist in file */
        if (mode == RBM_NORMAL)
        {
            log_invalid_page(rnode, forknum, blkno, false);
            return InvalidBuffer;
        }
        if (mode == RBM_NORMAL_NO_LOG)
            return InvalidBuffer;
        /* OK to extend the file */
        /* we do this in recovery only - no rel-extension lock needed */
        Assert(InRecovery);
        buffer = InvalidBuffer;
        do
        {
            if (buffer != InvalidBuffer)
            {
                if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
                    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
                ReleaseBuffer(buffer);
            }
            buffer = ReadBufferWithoutRelcache(rnode, forknum,
                                               P_NEW, mode, NULL);
        }
        while (BufferGetBlockNumber(buffer) < blkno);
        /* Handle the corner case that P_NEW returns non-consecutive pages */
        if (BufferGetBlockNumber(buffer) != blkno)
        {
            if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
                LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
            ReleaseBuffer(buffer);
            buffer = ReadBufferWithoutRelcache(rnode, forknum, blkno,
                                               mode, NULL);
        }
    }

    if (mode == RBM_NORMAL)
    {
        /* check that page has been initialized */
        Page        page = (Page) BufferGetPage(buffer);

        /*
         * We assume that PageIsNew is safe without a lock. During recovery,
         * there should be no other backends that could modify the buffer at
         * the same time.
         */
        if (PageIsNew(page))
        {
            ReleaseBuffer(buffer);
            log_invalid_page(rnode, forknum, blkno, true);
            return InvalidBuffer;
        }
    }

    return buffer;
}

/*
 * Struct actually returned by CreateFakeRelcacheEntry, though the declared
 * return type is Relation.
 */
typedef struct
{
    RelationData reldata;       /* Note: this must be first */
    FormData_pg_class pgc;
} FakeRelCacheEntryData;

typedef FakeRelCacheEntryData *FakeRelCacheEntry;

/*
 * Create a fake relation cache entry for a physical relation
 *
 * It's often convenient to use the same functions in XLOG replay as in the
 * main codepath, but those functions typically work with a relcache entry.
 * We don't have a working relation cache during XLOG replay, but this
 * function can be used to create a fake relcache entry instead. Only the
 * fields related to physical storage, like rd_rel, are initialized, so the
 * fake entry is only usable in low-level operations like ReadBuffer().
 *
 * This is also used for syncing WAL-skipped files.
 *
 * Caller must free the returned entry with FreeFakeRelcacheEntry().
 */
Relation
CreateFakeRelcacheEntry(RelFileNode rnode)
{
    FakeRelCacheEntry fakeentry;
    Relation    rel;

    /* Allocate the Relation struct and all related space in one block. */
    fakeentry = palloc0(sizeof(FakeRelCacheEntryData));
    rel = (Relation) fakeentry;

    rel->rd_rel = &fakeentry->pgc;
    rel->rd_node = rnode;

    /*
     * We will never be working with temp rels during recovery or while
     * syncing WAL-skipped files.
     */
    rel->rd_backend = InvalidBackendId;

    /* It must be a permanent table here */
    rel->rd_rel->relpersistence = RELPERSISTENCE_PERMANENT;

    /* We don't know the name of the relation; use relfilenode instead */
    sprintf(RelationGetRelationName(rel), "%u", rnode.relNode);

    /*
     * We set up the lockRelId in case anything tries to lock the dummy
     * relation.  Note that this is fairly bogus since relNode may be
     * different from the relation's OID.  It shouldn't really matter though.
     * In recovery, we are running by ourselves and can't have any lock
     * conflicts.  While syncing, we already hold AccessExclusiveLock.
     */
    rel->rd_lockInfo.lockRelId.dbId = rnode.dbNode;
    rel->rd_lockInfo.lockRelId.relId = rnode.relNode;

    rel->rd_smgr = NULL;

    return rel;
}

/*
 * Free a fake relation cache entry.
 */
void
FreeFakeRelcacheEntry(Relation fakerel)
{
    /* make sure the fakerel is not referenced by the SmgrRelation anymore */
    if (fakerel->rd_smgr != NULL)
        smgrclearowner(&fakerel->rd_smgr, fakerel->rd_smgr);
    pfree(fakerel);
}

/*
 * Drop a relation during XLOG replay
 *
 * This is called when the relation is about to be deleted; we need to remove
 * any open "invalid-page" records for the relation.
 */
void
XLogDropRelation(RelFileNode rnode, ForkNumber forknum)
{
    forget_invalid_pages(rnode, forknum, 0);
}

/*
 * Drop a whole database during XLOG replay
 *
 * As above, but for DROP DATABASE instead of dropping a single rel
 */
void
XLogDropDatabase(Oid dbid)
{
    /*
     * This is unnecessarily heavy-handed, as it will close SMgrRelation
     * objects for other databases as well. DROP DATABASE occurs seldom enough
     * that it's not worth introducing a variant of smgrclose for just this
     * purpose. XXX: Or should we rather leave the smgr entries dangling?
     */
    smgrcloseall();

    forget_invalid_pages_db(dbid);
}

/*
 * Truncate a relation during XLOG replay
 *
 * We need to clean up any open "invalid-page" records for the dropped pages.
 */
void
XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
                     BlockNumber nblocks)
{
    forget_invalid_pages(rnode, forkNum, nblocks);
}

/*
 * Determine which timeline to read an xlog page from and set the
 * XLogReaderState's currTLI to that timeline ID.
 *
 * We care about timelines in xlogreader when we might be reading xlog
 * generated prior to a promotion, either if we're currently a standby in
 * recovery or if we're a promoted primary reading xlogs generated by the old
 * primary before our promotion.
 *
 * wantPage must be set to the start address of the page to read and
 * wantLength to the amount of the page that will be read, up to
 * XLOG_BLCKSZ. If the amount to be read isn't known, pass XLOG_BLCKSZ.
 *
 * We switch to an xlog segment from the new timeline eagerly when on a
 * historical timeline, as soon as we reach the start of the xlog segment
 * containing the timeline switch.  The server copied the segment to the new
 * timeline so all the data up to the switch point is the same, but there's no
 * guarantee the old segment will still exist. It may have been deleted or
 * renamed with a .partial suffix so we can't necessarily keep reading from
 * the old TLI even though tliSwitchPoint says it's OK.
 *
 * We can't just check the timeline when we read a page on a different segment
 * to the last page. We could've received a timeline switch from a cascading
 * upstream, so the current segment ends abruptly (possibly getting renamed to
 * .partial) and we have to switch to a new one.  Even in the middle of reading
 * a page we could have to dump the cached page and switch to a new TLI.
 *
 * Because of this, callers MAY NOT assume that currTLI is the timeline that
 * will be in a page's xlp_tli; the page may begin on an older timeline or we
 * might be reading from historical timeline data on a segment that's been
 * copied to a new timeline.
 *
 * The caller must also make sure it doesn't read past the current replay
 * position (using GetXLogReplayRecPtr) if executing in recovery, so it
 * doesn't fail to notice that the current timeline became historical. The
 * caller must also update ThisTimeLineID with the result of
 * GetXLogReplayRecPtr and must check RecoveryInProgress().
 */
void
XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wantLength)
{
    const XLogRecPtr lastReadPage = (state->seg.ws_segno *
                                     state->segcxt.ws_segsize + state->segoff);

    Assert(wantPage != InvalidXLogRecPtr && wantPage % XLOG_BLCKSZ == 0);
    Assert(wantLength <= XLOG_BLCKSZ);
    Assert(state->readLen == 0 || state->readLen <= XLOG_BLCKSZ);

    /*
     * If the desired page is currently read in and valid, we have nothing to
     * do.
     *
     * The caller should've ensured that it didn't previously advance readOff
     * past the valid limit of this timeline, so it doesn't matter if the
     * current TLI has since become historical.
     */
    if (lastReadPage == wantPage &&
        state->readLen != 0 &&
        lastReadPage + state->readLen >= wantPage + Min(wantLength, XLOG_BLCKSZ - 1))
        return;

    /*
     * If we're reading from the current timeline, it hasn't become historical
     * and the page we're reading is after the last page read, we can again
     * just carry on. (Seeking backwards requires a check to make sure the
     * older page isn't on a prior timeline).
     *
     * ThisTimeLineID might've become historical since we last looked, but the
     * caller is required not to read past the flush limit it saw at the time
     * it looked up the timeline. There's nothing we can do about it if
     * StartupXLOG() renames it to .partial concurrently.
     */
    if (state->currTLI == ThisTimeLineID && wantPage >= lastReadPage)
    {
        Assert(state->currTLIValidUntil == InvalidXLogRecPtr);
        return;
    }

    /*
     * If we're just reading pages from a previously validated historical
     * timeline and the timeline we're reading from is valid until the end of
     * the current segment we can just keep reading.
     */
    if (state->currTLIValidUntil != InvalidXLogRecPtr &&
        state->currTLI != ThisTimeLineID &&
        state->currTLI != 0 &&
        ((wantPage + wantLength) / state->segcxt.ws_segsize) <
        (state->currTLIValidUntil / state->segcxt.ws_segsize))
        return;

    /*
     * If we reach this point we're either looking up a page for random
     * access, the current timeline just became historical, or we're reading
     * from a new segment containing a timeline switch. In all cases we need
     * to determine the newest timeline on the segment.
     *
     * If it's the current timeline we can just keep reading from here unless
     * we detect a timeline switch that makes the current timeline historical.
     * If it's a historical timeline we can read all the segment on the newest
     * timeline because it contains all the old timelines' data too. So only
     * one switch check is required.
     */
    {
        /*
         * We need to re-read the timeline history in case it's been changed
         * by a promotion or replay from a cascaded replica.
         */
        List       *timelineHistory = readTimeLineHistory(ThisTimeLineID);
        XLogRecPtr  endOfSegment;

        endOfSegment = ((wantPage / state->segcxt.ws_segsize) + 1) *
            state->segcxt.ws_segsize - 1;
        Assert(wantPage / state->segcxt.ws_segsize ==
               endOfSegment / state->segcxt.ws_segsize);

        /*
         * Find the timeline of the last LSN on the segment containing
         * wantPage.
         */
        state->currTLI = tliOfPointInHistory(endOfSegment, timelineHistory);
        state->currTLIValidUntil = tliSwitchPoint(state->currTLI, timelineHistory,
                                                  &state->nextTLI);

        Assert(state->currTLIValidUntil == InvalidXLogRecPtr ||
               wantPage + wantLength < state->currTLIValidUntil);

        list_free_deep(timelineHistory);

        elog(DEBUG3, "switched to timeline %u valid until %X/%X",
             state->currTLI,
             (uint32) (state->currTLIValidUntil >> 32),
             (uint32) (state->currTLIValidUntil));
    }
}

/* XLogReaderRoutine->segment_open callback for local pg_wal files */
void
wal_segment_open(XLogReaderState *state, XLogSegNo nextSegNo,
                 TimeLineID *tli_p)
{
    TimeLineID  tli = *tli_p;
    char        path[MAXPGPATH];

    XLogFilePath(path, tli, nextSegNo, state->segcxt.ws_segsize);
    state->seg.ws_file = BasicOpenFile(path, O_RDONLY | PG_BINARY);
    if (state->seg.ws_file >= 0)
        return;

    if (errno == ENOENT)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("requested WAL segment %s has already been removed",
                        path)));
    else
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m",
                        path)));
}

/* stock XLogReaderRoutine->segment_close callback */
void
wal_segment_close(XLogReaderState *state)
{
    close(state->seg.ws_file);
    /* need to check errno? */
    state->seg.ws_file = -1;
}

/*
 * XLogReaderRoutine->page_read callback for reading local xlog files
 *
 * Public because it would likely be very helpful for someone writing another
 * output method outside walsender, e.g. in a bgworker.
 *
 * TODO: The walsender has its own version of this, but it relies on the
 * walsender's latch being set whenever WAL is flushed. No such infrastructure
 * exists for normal backends, so we have to do a check/sleep/repeat style of
 * loop for now.
 */
int
read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
                     int reqLen, XLogRecPtr targetRecPtr, char *cur_page)
{
    XLogRecPtr  read_upto,
                loc;
    TimeLineID  tli;
    int         count;
    WALReadError errinfo;

    loc = targetPagePtr + reqLen;

    /* Loop waiting for xlog to be available if necessary */
    while (1)
    {
        /*
         * Determine the limit of xlog we can currently read to, and what the
         * most recent timeline is.
         *
         * RecoveryInProgress() will update ThisTimeLineID when it first
         * notices recovery finishes, so we only have to maintain it for the
         * local process until recovery ends.
         */
        if (!RecoveryInProgress())
            read_upto = GetFlushRecPtr();
        else
            read_upto = GetXLogReplayRecPtr(&ThisTimeLineID);
        tli = ThisTimeLineID;

        /*
         * Check which timeline to get the record from.
         *
         * We have to do it each time through the loop because if we're in
         * recovery as a cascading standby, the current timeline might've
         * become historical. We can't rely on RecoveryInProgress() because in
         * a standby configuration like
         *
         * A => B => C
         *
         * if we're a logical decoding session on C, and B gets promoted, our
         * timeline will change while we remain in recovery.
         *
         * We can't just keep reading from the old timeline as the last WAL
         * archive in the timeline will get renamed to .partial by
         * StartupXLOG().
         *
         * If that happens after our caller updated ThisTimeLineID but before
         * we actually read the xlog page, we might still try to read from the
         * old (now renamed) segment and fail. There's not much we can do
         * about this, but it can only happen when we're a leaf of a cascading
         * standby whose primary gets promoted while we're decoding, so a
         * one-off ERROR isn't too bad.
         */
        XLogReadDetermineTimeline(state, targetPagePtr, reqLen);

        if (state->currTLI == ThisTimeLineID)
        {

            if (loc <= read_upto)
                break;

            CHECK_FOR_INTERRUPTS();
            pg_usleep(1000L);
        }
        else
        {
            /*
             * We're on a historical timeline, so limit reading to the switch
             * point where we moved to the next timeline.
             *
             * We don't need to GetFlushRecPtr or GetXLogReplayRecPtr. We know
             * about the new timeline, so we must've received past the end of
             * it.
             */
            read_upto = state->currTLIValidUntil;

            /*
             * Setting tli to our wanted record's TLI is slightly wrong; the
             * page might begin on an older timeline if it contains a timeline
             * switch, since its xlog segment will have been copied from the
             * prior timeline. This is pretty harmless though, as nothing
             * cares so long as the timeline doesn't go backwards.  We should
             * read the page header instead; FIXME someday.
             */
            tli = state->currTLI;

            /* No need to wait on a historical timeline */
            break;
        }
    }

    if (targetPagePtr + XLOG_BLCKSZ <= read_upto)
    {
        /*
         * more than one block available; read only that block, have caller
         * come back if they need more.
         */
        count = XLOG_BLCKSZ;
    }
    else if (targetPagePtr + reqLen > read_upto)
    {
        /* not enough data there */
        return -1;
    }
    else
    {
        /* enough bytes available to satisfy the request */
        count = read_upto - targetPagePtr;
    }

    /*
     * Even though we just determined how much of the page can be validly read
     * as 'count', read the whole page anyway. It's guaranteed to be
     * zero-padded up to the page boundary if it's incomplete.
     */
    if (!WALRead(state, cur_page, targetPagePtr, XLOG_BLCKSZ, tli,
                 &errinfo))
        WALReadRaiseError(&errinfo);

    /* number of valid bytes in the buffer */
    return count;
}

/*
 * Backend-specific convenience code to handle read errors encountered by
 * WALRead().
 */
void
WALReadRaiseError(WALReadError *errinfo)
{
    WALOpenSegment *seg = &errinfo->wre_seg;
    char        fname[MAXFNAMELEN];

    XLogFileName(fname, seg->ws_tli, seg->ws_segno, wal_segment_size);

    if (errinfo->wre_read < 0)
    {
        errno = errinfo->wre_errno;
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not read from log segment %s, offset %u: %m",
                        fname, errinfo->wre_off)));
    }
    else if (errinfo->wre_read == 0)
    {
        ereport(ERROR,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("could not read from log segment %s, offset %u: read %d of %zu",
                        fname, errinfo->wre_off, errinfo->wre_read,
                        (Size) errinfo->wre_req)));
    }
}