xlogreader.c

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/*-------------------------------------------------------------------------
 *
 * xlogreader.c
 *      Generic XLog reading facility
 *
 * Portions Copyright (c) 2013-2024, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *      src/backend/access/transam/xlogreader.c
 *
 * NOTES
 *      See xlogreader.h for more notes on this facility.
 *
 *      This file is compiled as both front-end and backend code, so it
 *      may not use ereport, server-defined static variables, etc.
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <unistd.h>
#ifdef USE_LZ4
#include <lz4.h>
#endif
#ifdef USE_ZSTD
#include <zstd.h>
#endif
 
#include "access/transam.h"
#include "access/xlog_internal.h"
#include "access/xlogreader.h"
#include "access/xlogrecord.h"
#include "catalog/pg_control.h"
#include "common/pg_lzcompress.h"
#include "replication/origin.h"
 
#ifndef FRONTEND
#include "pgstat.h"
#else
#include "common/logging.h"
#endif
 
static void report_invalid_record(XLogReaderState *state, const char *fmt,...)
            pg_attribute_printf(2, 3);
static void allocate_recordbuf(XLogReaderState *state, uint32 reclength);
static int  ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr,
                             int reqLen);
static void XLogReaderInvalReadState(XLogReaderState *state);
static XLogPageReadResult XLogDecodeNextRecord(XLogReaderState *state, bool nonblocking);
static bool ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
                                  XLogRecPtr PrevRecPtr, XLogRecord *record, bool randAccess);
static bool ValidXLogRecord(XLogReaderState *state, XLogRecord *record,
                            XLogRecPtr recptr);
static void ResetDecoder(XLogReaderState *state);
static void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
                               int segsize, const char *waldir);
 
/* size of the buffer allocated for error message. */
#define MAX_ERRORMSG_LEN 1000
 
/*
 * Default size; large enough that typical users of XLogReader won't often need
 * to use the 'oversized' memory allocation code path.
 */
#define DEFAULT_DECODE_BUFFER_SIZE (64 * 1024)
 
/*
 * Construct a string in state->errormsg_buf explaining what's wrong with
 * the current record being read.
 */
static void
report_invalid_record(XLogReaderState *state, const char *fmt,...)
{
    va_list     args;
 
    fmt = _(fmt);
 
    va_start(args, fmt);
    vsnprintf(state->errormsg_buf, MAX_ERRORMSG_LEN, fmt, args);
    va_end(args);
 
    state->errormsg_deferred = true;
}
 
/*
 * Set the size of the decoding buffer.  A pointer to a caller supplied memory
 * region may also be passed in, in which case non-oversized records will be
 * decoded there.
 */
void
XLogReaderSetDecodeBuffer(XLogReaderState *state, void *buffer, size_t size)
{
    Assert(state->decode_buffer == NULL);
 
    state->decode_buffer = buffer;
    state->decode_buffer_size = size;
    state->decode_buffer_tail = buffer;
    state->decode_buffer_head = buffer;
}
 
/*
 * Allocate and initialize a new XLogReader.
 *
 * Returns NULL if the xlogreader couldn't be allocated.
 */
XLogReaderState *
XLogReaderAllocate(int wal_segment_size, const char *waldir,
                   XLogReaderRoutine *routine, void *private_data)
{
    XLogReaderState *state;
 
    state = (XLogReaderState *)
        palloc_extended(sizeof(XLogReaderState),
                        MCXT_ALLOC_NO_OOM | MCXT_ALLOC_ZERO);
    if (!state)
        return NULL;
 
    /* initialize caller-provided support functions */
    state->routine = *routine;
 
    /*
     * Permanently allocate readBuf.  We do it this way, rather than just
     * making a static array, for two reasons: (1) no need to waste the
     * storage in most instantiations of the backend; (2) a static char array
     * isn't guaranteed to have any particular alignment, whereas
     * palloc_extended() will provide MAXALIGN'd storage.
     */
    state->readBuf = (char *) palloc_extended(XLOG_BLCKSZ,
                                              MCXT_ALLOC_NO_OOM);
    if (!state->readBuf)
    {
        pfree(state);
        return NULL;
    }
 
    /* Initialize segment info. */
    WALOpenSegmentInit(&state->seg, &state->segcxt, wal_segment_size,
                       waldir);
 
    /* system_identifier initialized to zeroes above */
    state->private_data = private_data;
    /* ReadRecPtr, EndRecPtr and readLen initialized to zeroes above */
    state->errormsg_buf = palloc_extended(MAX_ERRORMSG_LEN + 1,
                                          MCXT_ALLOC_NO_OOM);
    if (!state->errormsg_buf)
    {
        pfree(state->readBuf);
        pfree(state);
        return NULL;
    }
    state->errormsg_buf[0] = '\0';
 
    /*
     * Allocate an initial readRecordBuf of minimal size, which can later be
     * enlarged if necessary.
     */
    allocate_recordbuf(state, 0);
    return state;
}
 
void
XLogReaderFree(XLogReaderState *state)
{
    if (state->seg.ws_file != -1)
        state->routine.segment_close(state);
 
    if (state->decode_buffer && state->free_decode_buffer)
        pfree(state->decode_buffer);
 
    pfree(state->errormsg_buf);
    if (state->readRecordBuf)
        pfree(state->readRecordBuf);
    pfree(state->readBuf);
    pfree(state);
}
 
/*
 * Allocate readRecordBuf to fit a record of at least the given length.
 *
 * readRecordBufSize is set to the new buffer size.
 *
 * To avoid useless small increases, round its size to a multiple of
 * XLOG_BLCKSZ, and make sure it's at least 5*Max(BLCKSZ, XLOG_BLCKSZ) to start
 * with.  (That is enough for all "normal" records, but very large commit or
 * abort records might need more space.)
 *
 * Note: This routine should *never* be called for xl_tot_len until the header
 * of the record has been fully validated.
 */
static void
allocate_recordbuf(XLogReaderState *state, uint32 reclength)
{
    uint32      newSize = reclength;
 
    newSize += XLOG_BLCKSZ - (newSize % XLOG_BLCKSZ);
    newSize = Max(newSize, 5 * Max(BLCKSZ, XLOG_BLCKSZ));
 
    if (state->readRecordBuf)
        pfree(state->readRecordBuf);
    state->readRecordBuf = (char *) palloc(newSize);
    state->readRecordBufSize = newSize;
}
 
/*
 * Initialize the passed segment structs.
 */
static void
WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
                   int segsize, const char *waldir)
{
    seg->ws_file = -1;
    seg->ws_segno = 0;
    seg->ws_tli = 0;
 
    segcxt->ws_segsize = segsize;
    if (waldir)
        snprintf(segcxt->ws_dir, MAXPGPATH, "%s", waldir);
}
 
/*
 * Begin reading WAL at 'RecPtr'.
 *
 * 'RecPtr' should point to the beginning of a valid WAL record.  Pointing at
 * the beginning of a page is also OK, if there is a new record right after
 * the page header, i.e. not a continuation.
 *
 * This does not make any attempt to read the WAL yet, and hence cannot fail.
 * If the starting address is not correct, the first call to XLogReadRecord()
 * will error out.
 */
void
XLogBeginRead(XLogReaderState *state, XLogRecPtr RecPtr)
{
    Assert(!XLogRecPtrIsInvalid(RecPtr));
 
    ResetDecoder(state);
 
    /* Begin at the passed-in record pointer. */
    state->EndRecPtr = RecPtr;
    state->NextRecPtr = RecPtr;
    state->ReadRecPtr = InvalidXLogRecPtr;
    state->DecodeRecPtr = InvalidXLogRecPtr;
}
 
/*
 * Release the last record that was returned by XLogNextRecord(), if any, to
 * free up space.  Returns the LSN past the end of the record.
 */
XLogRecPtr
XLogReleasePreviousRecord(XLogReaderState *state)
{
    DecodedXLogRecord *record;
    XLogRecPtr  next_lsn;
 
    if (!state->record)
        return InvalidXLogRecPtr;
 
    /*
     * Remove it from the decoded record queue.  It must be the oldest item
     * decoded, decode_queue_head.
     */
    record = state->record;
    next_lsn = record->next_lsn;
    Assert(record == state->decode_queue_head);
    state->record = NULL;
    state->decode_queue_head = record->next;
 
    /* It might also be the newest item decoded, decode_queue_tail. */
    if (state->decode_queue_tail == record)
        state->decode_queue_tail = NULL;
 
    /* Release the space. */
    if (unlikely(record->oversized))
    {
        /* It's not in the decode buffer, so free it to release space. */
        pfree(record);
    }
    else
    {
        /* It must be the head (oldest) record in the decode buffer. */
        Assert(state->decode_buffer_head == (char *) record);
 
        /*
         * We need to update head to point to the next record that is in the
         * decode buffer, if any, being careful to skip oversized ones
         * (they're not in the decode buffer).
         */
        record = record->next;
        while (unlikely(record && record->oversized))
            record = record->next;
 
        if (record)
        {
            /* Adjust head to release space up to the next record. */
            state->decode_buffer_head = (char *) record;
        }
        else
        {
            /*
             * Otherwise we might as well just reset head and tail to the
             * start of the buffer space, because we're empty.  This means
             * we'll keep overwriting the same piece of memory if we're not
             * doing any prefetching.
             */
            state->decode_buffer_head = state->decode_buffer;
            state->decode_buffer_tail = state->decode_buffer;
        }
    }
 
    return next_lsn;
}
 
/*
 * Attempt to read an XLOG record.
 *
 * XLogBeginRead() or XLogFindNextRecord() and then XLogReadAhead() must be
 * called before the first call to XLogNextRecord().  This functions returns
 * records and errors that were put into an internal queue by XLogReadAhead().
 *
 * On success, a record is returned.
 *
 * The returned record (or *errormsg) points to an internal buffer that's
 * valid until the next call to XLogNextRecord.
 */
DecodedXLogRecord *
XLogNextRecord(XLogReaderState *state, char **errormsg)
{
    /* Release the last record returned by XLogNextRecord(). */
    XLogReleasePreviousRecord(state);
 
    if (state->decode_queue_head == NULL)
    {
        *errormsg = NULL;
        if (state->errormsg_deferred)
        {
            if (state->errormsg_buf[0] != '\0')
                *errormsg = state->errormsg_buf;
            state->errormsg_deferred = false;
        }
 
        /*
         * state->EndRecPtr is expected to have been set by the last call to
         * XLogBeginRead() or XLogNextRecord(), and is the location of the
         * error.
         */
        Assert(!XLogRecPtrIsInvalid(state->EndRecPtr));
 
        return NULL;
    }
 
    /*
     * Record this as the most recent record returned, so that we'll release
     * it next time.  This also exposes it to the traditional
     * XLogRecXXX(xlogreader) macros, which work with the decoder rather than
     * the record for historical reasons.
     */
    state->record = state->decode_queue_head;
 
    /*
     * Update the pointers to the beginning and one-past-the-end of this
     * record, again for the benefit of historical code that expected the
     * decoder to track this rather than accessing these fields of the record
     * itself.
     */
    state->ReadRecPtr = state->record->lsn;
    state->EndRecPtr = state->record->next_lsn;
 
    *errormsg = NULL;
 
    return state->record;
}
 
/*
 * Attempt to read an XLOG record.
 *
 * XLogBeginRead() or XLogFindNextRecord() must be called before the first call
 * to XLogReadRecord().
 *
 * If the page_read callback fails to read the requested data, NULL is
 * returned.  The callback is expected to have reported the error; errormsg
 * is set to NULL.
 *
 * If the reading fails for some other reason, NULL is also returned, and
 * *errormsg is set to a string with details of the failure.
 *
 * The returned pointer (or *errormsg) points to an internal buffer that's
 * valid until the next call to XLogReadRecord.
 */
XLogRecord *
XLogReadRecord(XLogReaderState *state, char **errormsg)
{
    DecodedXLogRecord *decoded;
 
    /*
     * Release last returned record, if there is one.  We need to do this so
     * that we can check for empty decode queue accurately.
     */
    XLogReleasePreviousRecord(state);
 
    /*
     * Call XLogReadAhead() in blocking mode to make sure there is something
     * in the queue, though we don't use the result.
     */
    if (!XLogReaderHasQueuedRecordOrError(state))
        XLogReadAhead(state, false /* nonblocking */ );
 
    /* Consume the head record or error. */
    decoded = XLogNextRecord(state, errormsg);
    if (decoded)
    {
        /*
         * This function returns a pointer to the record's header, not the
         * actual decoded record.  The caller will access the decoded record
         * through the XLogRecGetXXX() macros, which reach the decoded
         * recorded as xlogreader->record.
         */
        Assert(state->record == decoded);
        return &decoded->header;
    }
 
    return NULL;
}
 
/*
 * Allocate space for a decoded record.  The only member of the returned
 * object that is initialized is the 'oversized' flag, indicating that the
 * decoded record wouldn't fit in the decode buffer and must eventually be
 * freed explicitly.
 *
 * The caller is responsible for adjusting decode_buffer_tail with the real
 * size after successfully decoding a record into this space.  This way, if
 * decoding fails, then there is nothing to undo unless the 'oversized' flag
 * was set and pfree() must be called.
 *
 * Return NULL if there is no space in the decode buffer and allow_oversized
 * is false, or if memory allocation fails for an oversized buffer.
 */
static DecodedXLogRecord *
XLogReadRecordAlloc(XLogReaderState *state, size_t xl_tot_len, bool allow_oversized)
{
    size_t      required_space = DecodeXLogRecordRequiredSpace(xl_tot_len);
    DecodedXLogRecord *decoded = NULL;
 
    /* Allocate a circular decode buffer if we don't have one already. */
    if (unlikely(state->decode_buffer == NULL))
    {
        if (state->decode_buffer_size == 0)
            state->decode_buffer_size = DEFAULT_DECODE_BUFFER_SIZE;
        state->decode_buffer = palloc(state->decode_buffer_size);
        state->decode_buffer_head = state->decode_buffer;
        state->decode_buffer_tail = state->decode_buffer;
        state->free_decode_buffer = true;
    }
 
    /* Try to allocate space in the circular decode buffer. */
    if (state->decode_buffer_tail >= state->decode_buffer_head)
    {
        /* Empty, or tail is to the right of head. */
        if (required_space <=
            state->decode_buffer_size -
            (state->decode_buffer_tail - state->decode_buffer))
        {
            /*-
             * There is space between tail and end.
             *
             * +-----+--------------------+-----+
             * |     |////////////////////|here!|
             * +-----+--------------------+-----+
             *       ^                    ^
             *       |                    |
             *       h                    t
             */
            decoded = (DecodedXLogRecord *) state->decode_buffer_tail;
            decoded->oversized = false;
            return decoded;
        }
        else if (required_space <
                 state->decode_buffer_head - state->decode_buffer)
        {
            /*-
             * There is space between start and head.
             *
             * +-----+--------------------+-----+
             * |here!|////////////////////|     |
             * +-----+--------------------+-----+
             *       ^                    ^
             *       |                    |
             *       h                    t
             */
            decoded = (DecodedXLogRecord *) state->decode_buffer;
            decoded->oversized = false;
            return decoded;
        }
    }
    else
    {
        /* Tail is to the left of head. */
        if (required_space <
            state->decode_buffer_head - state->decode_buffer_tail)
        {
            /*-
             * There is space between tail and head.
             *
             * +-----+--------------------+-----+
             * |/////|here!               |/////|
             * +-----+--------------------+-----+
             *       ^                    ^
             *       |                    |
             *       t                    h
             */
            decoded = (DecodedXLogRecord *) state->decode_buffer_tail;
            decoded->oversized = false;
            return decoded;
        }
    }
 
    /* Not enough space in the decode buffer.  Are we allowed to allocate? */
    if (allow_oversized)
    {
        decoded = palloc(required_space);
        decoded->oversized = true;
        return decoded;
    }
 
    return NULL;
}
 
static XLogPageReadResult
XLogDecodeNextRecord(XLogReaderState *state, bool nonblocking)
{
    XLogRecPtr  RecPtr;
    XLogRecord *record;
    XLogRecPtr  targetPagePtr;
    bool        randAccess;
    uint32      len,
                total_len;
    uint32      targetRecOff;
    uint32      pageHeaderSize;
    bool        assembled;
    bool        gotheader;
    int         readOff;
    DecodedXLogRecord *decoded;
    char       *errormsg;       /* not used */
 
    /*
     * randAccess indicates whether to verify the previous-record pointer of
     * the record we're reading.  We only do this if we're reading
     * sequentially, which is what we initially assume.
     */
    randAccess = false;
 
    /* reset error state */
    state->errormsg_buf[0] = '\0';
    decoded = NULL;
 
    state->abortedRecPtr = InvalidXLogRecPtr;
    state->missingContrecPtr = InvalidXLogRecPtr;
 
    RecPtr = state->NextRecPtr;
 
    if (state->DecodeRecPtr != InvalidXLogRecPtr)
    {
        /* read the record after the one we just read */
 
        /*
         * NextRecPtr is pointing to end+1 of the previous WAL record.  If
         * we're at a page boundary, no more records can fit on the current
         * page. We must skip over the page header, but we can't do that until
         * we've read in the page, since the header size is variable.
         */
    }
    else
    {
        /*
         * Caller supplied a position to start at.
         *
         * In this case, NextRecPtr should already be pointing either to a
         * valid record starting position or alternatively to the beginning of
         * a page. See the header comments for XLogBeginRead.
         */
        Assert(RecPtr % XLOG_BLCKSZ == 0 || XRecOffIsValid(RecPtr));
        randAccess = true;
    }
 
restart:
    state->nonblocking = nonblocking;
    state->currRecPtr = RecPtr;
    assembled = false;
 
    targetPagePtr = RecPtr - (RecPtr % XLOG_BLCKSZ);
    targetRecOff = RecPtr % XLOG_BLCKSZ;
 
    /*
     * Read the page containing the record into state->readBuf. Request enough
     * byte to cover the whole record header, or at least the part of it that
     * fits on the same page.
     */
    readOff = ReadPageInternal(state, targetPagePtr,
                               Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
    if (readOff == XLREAD_WOULDBLOCK)
        return XLREAD_WOULDBLOCK;
    else if (readOff < 0)
        goto err;
 
    /*
     * ReadPageInternal always returns at least the page header, so we can
     * examine it now.
     */
    pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
    if (targetRecOff == 0)
    {
        /*
         * At page start, so skip over page header.
         */
        RecPtr += pageHeaderSize;
        targetRecOff = pageHeaderSize;
    }
    else if (targetRecOff < pageHeaderSize)
    {
        report_invalid_record(state, "invalid record offset at %X/%X: expected at least %u, got %u",
                              LSN_FORMAT_ARGS(RecPtr),
                              pageHeaderSize, targetRecOff);
        goto err;
    }
 
    if ((((XLogPageHeader) state->readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) &&
        targetRecOff == pageHeaderSize)
    {
        report_invalid_record(state, "contrecord is requested by %X/%X",
                              LSN_FORMAT_ARGS(RecPtr));
        goto err;
    }
 
    /* ReadPageInternal has verified the page header */
    Assert(pageHeaderSize <= readOff);
 
    /*
     * Read the record length.
     *
     * NB: Even though we use an XLogRecord pointer here, the whole record
     * header might not fit on this page. xl_tot_len is the first field of the
     * struct, so it must be on this page (the records are MAXALIGNed), but we
     * cannot access any other fields until we've verified that we got the
     * whole header.
     */
    record = (XLogRecord *) (state->readBuf + RecPtr % XLOG_BLCKSZ);
    total_len = record->xl_tot_len;
 
    /*
     * If the whole record header is on this page, validate it immediately.
     * Otherwise do just a basic sanity check on xl_tot_len, and validate the
     * rest of the header after reading it from the next page.  The xl_tot_len
     * check is necessary here to ensure that we enter the "Need to reassemble
     * record" code path below; otherwise we might fail to apply
     * ValidXLogRecordHeader at all.
     */
    if (targetRecOff <= XLOG_BLCKSZ - SizeOfXLogRecord)
    {
        if (!ValidXLogRecordHeader(state, RecPtr, state->DecodeRecPtr, record,
                                   randAccess))
            goto err;
        gotheader = true;
    }
    else
    {
        /* There may be no next page if it's too small. */
        if (total_len < SizeOfXLogRecord)
        {
            report_invalid_record(state,
                                  "invalid record length at %X/%X: expected at least %u, got %u",
                                  LSN_FORMAT_ARGS(RecPtr),
                                  (uint32) SizeOfXLogRecord, total_len);
            goto err;
        }
        /* We'll validate the header once we have the next page. */
        gotheader = false;
    }
 
    /*
     * Try to find space to decode this record, if we can do so without
     * calling palloc.  If we can't, we'll try again below after we've
     * validated that total_len isn't garbage bytes from a recycled WAL page.
     */
    decoded = XLogReadRecordAlloc(state,
                                  total_len,
                                  false /* allow_oversized */ );
    if (decoded == NULL && nonblocking)
    {
        /*
         * There is no space in the circular decode buffer, and the caller is
         * only reading ahead.  The caller should consume existing records to
         * make space.
         */
        return XLREAD_WOULDBLOCK;
    }
 
    len = XLOG_BLCKSZ - RecPtr % XLOG_BLCKSZ;
    if (total_len > len)
    {
        /* Need to reassemble record */
        char       *contdata;
        XLogPageHeader pageHeader;
        char       *buffer;
        uint32      gotlen;
 
        assembled = true;
 
        /*
         * We always have space for a couple of pages, enough to validate a
         * boundary-spanning record header.
         */
        Assert(state->readRecordBufSize >= XLOG_BLCKSZ * 2);
        Assert(state->readRecordBufSize >= len);
 
        /* Copy the first fragment of the record from the first page. */
        memcpy(state->readRecordBuf,
               state->readBuf + RecPtr % XLOG_BLCKSZ, len);
        buffer = state->readRecordBuf + len;
        gotlen = len;
 
        do
        {
            /* Calculate pointer to beginning of next page */
            targetPagePtr += XLOG_BLCKSZ;
 
            /* Wait for the next page to become available */
            readOff = ReadPageInternal(state, targetPagePtr,
                                       Min(total_len - gotlen + SizeOfXLogShortPHD,
                                           XLOG_BLCKSZ));
 
            if (readOff == XLREAD_WOULDBLOCK)
                return XLREAD_WOULDBLOCK;
            else if (readOff < 0)
                goto err;
 
            Assert(SizeOfXLogShortPHD <= readOff);
 
            pageHeader = (XLogPageHeader) state->readBuf;
 
            /*
             * If we were expecting a continuation record and got an
             * "overwrite contrecord" flag, that means the continuation record
             * was overwritten with a different record.  Restart the read by
             * assuming the address to read is the location where we found
             * this flag; but keep track of the LSN of the record we were
             * reading, for later verification.
             */
            if (pageHeader->xlp_info & XLP_FIRST_IS_OVERWRITE_CONTRECORD)
            {
                state->overwrittenRecPtr = RecPtr;
                RecPtr = targetPagePtr;
                goto restart;
            }
 
            /* Check that the continuation on next page looks valid */
            if (!(pageHeader->xlp_info & XLP_FIRST_IS_CONTRECORD))
            {
                report_invalid_record(state,
                                      "there is no contrecord flag at %X/%X",
                                      LSN_FORMAT_ARGS(RecPtr));
                goto err;
            }
 
            /*
             * Cross-check that xlp_rem_len agrees with how much of the record
             * we expect there to be left.
             */
            if (pageHeader->xlp_rem_len == 0 ||
                total_len != (pageHeader->xlp_rem_len + gotlen))
            {
                report_invalid_record(state,
                                      "invalid contrecord length %u (expected %lld) at %X/%X",
                                      pageHeader->xlp_rem_len,
                                      ((long long) total_len) - gotlen,
                                      LSN_FORMAT_ARGS(RecPtr));
                goto err;
            }
 
            /* Append the continuation from this page to the buffer */
            pageHeaderSize = XLogPageHeaderSize(pageHeader);
 
            if (readOff < pageHeaderSize)
                readOff = ReadPageInternal(state, targetPagePtr,
                                           pageHeaderSize);
 
            Assert(pageHeaderSize <= readOff);
 
            contdata = (char *) state->readBuf + pageHeaderSize;
            len = XLOG_BLCKSZ - pageHeaderSize;
            if (pageHeader->xlp_rem_len < len)
                len = pageHeader->xlp_rem_len;
 
            if (readOff < pageHeaderSize + len)
                readOff = ReadPageInternal(state, targetPagePtr,
                                           pageHeaderSize + len);
 
            memcpy(buffer, (char *) contdata, len);
            buffer += len;
            gotlen += len;
 
            /* If we just reassembled the record header, validate it. */
            if (!gotheader)
            {
                record = (XLogRecord *) state->readRecordBuf;
                if (!ValidXLogRecordHeader(state, RecPtr, state->DecodeRecPtr,
                                           record, randAccess))
                    goto err;
                gotheader = true;
            }
 
            /*
             * We might need a bigger buffer.  We have validated the record
             * header, in the case that it split over a page boundary.  We've
             * also cross-checked total_len against xlp_rem_len on the second
             * page, and verified xlp_pageaddr on both.
             */
            if (total_len > state->readRecordBufSize)
            {
                char        save_copy[XLOG_BLCKSZ * 2];
 
                /*
                 * Save and restore the data we already had.  It can't be more
                 * than two pages.
                 */
                Assert(gotlen <= lengthof(save_copy));
                Assert(gotlen <= state->readRecordBufSize);
                memcpy(save_copy, state->readRecordBuf, gotlen);
                allocate_recordbuf(state, total_len);
                memcpy(state->readRecordBuf, save_copy, gotlen);
                buffer = state->readRecordBuf + gotlen;
            }
        } while (gotlen < total_len);
        Assert(gotheader);
 
        record = (XLogRecord *) state->readRecordBuf;
        if (!ValidXLogRecord(state, record, RecPtr))
            goto err;
 
        pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
        state->DecodeRecPtr = RecPtr;
        state->NextRecPtr = targetPagePtr + pageHeaderSize
            + MAXALIGN(pageHeader->xlp_rem_len);
    }
    else
    {
        /* Wait for the record data to become available */
        readOff = ReadPageInternal(state, targetPagePtr,
                                   Min(targetRecOff + total_len, XLOG_BLCKSZ));
        if (readOff == XLREAD_WOULDBLOCK)
            return XLREAD_WOULDBLOCK;
        else if (readOff < 0)
            goto err;
 
        /* Record does not cross a page boundary */
        if (!ValidXLogRecord(state, record, RecPtr))
            goto err;
 
        state->NextRecPtr = RecPtr + MAXALIGN(total_len);
 
        state->DecodeRecPtr = RecPtr;
    }
 
    /*
     * Special processing if it's an XLOG SWITCH record
     */
    if (record->xl_rmid == RM_XLOG_ID &&
        (record->xl_info & ~XLR_INFO_MASK) == XLOG_SWITCH)
    {
        /* Pretend it extends to end of segment */
        state->NextRecPtr += state->segcxt.ws_segsize - 1;
        state->NextRecPtr -= XLogSegmentOffset(state->NextRecPtr, state->segcxt.ws_segsize);
    }
 
    /*
     * If we got here without a DecodedXLogRecord, it means we needed to
     * validate total_len before trusting it, but by now we've done that.
     */
    if (decoded == NULL)
    {
        Assert(!nonblocking);
        decoded = XLogReadRecordAlloc(state,
                                      total_len,
                                      true /* allow_oversized */ );
        /* allocation should always happen under allow_oversized */
        Assert(decoded != NULL);
    }
 
    if (DecodeXLogRecord(state, decoded, record, RecPtr, &errormsg))
    {
        /* Record the location of the next record. */
        decoded->next_lsn = state->NextRecPtr;
 
        /*
         * If it's in the decode buffer, mark the decode buffer space as
         * occupied.
         */
        if (!decoded->oversized)
        {
            /* The new decode buffer head must be MAXALIGNed. */
            Assert(decoded->size == MAXALIGN(decoded->size));
            if ((char *) decoded == state->decode_buffer)
                state->decode_buffer_tail = state->decode_buffer + decoded->size;
            else
                state->decode_buffer_tail += decoded->size;
        }
 
        /* Insert it into the queue of decoded records. */
        Assert(state->decode_queue_tail != decoded);
        if (state->decode_queue_tail)
            state->decode_queue_tail->next = decoded;
        state->decode_queue_tail = decoded;
        if (!state->decode_queue_head)
            state->decode_queue_head = decoded;
        return XLREAD_SUCCESS;
    }
 
err:
    if (assembled)
    {
        /*
         * We get here when a record that spans multiple pages needs to be
         * assembled, but something went wrong -- perhaps a contrecord piece
         * was lost.  If caller is WAL replay, it will know where the aborted
         * record was and where to direct followup WAL to be written, marking
         * the next piece with XLP_FIRST_IS_OVERWRITE_CONTRECORD, which will
         * in turn signal downstream WAL consumers that the broken WAL record
         * is to be ignored.
         */
        state->abortedRecPtr = RecPtr;
        state->missingContrecPtr = targetPagePtr;
 
        /*
         * If we got here without reporting an error, make sure an error is
         * queued so that XLogPrefetcherReadRecord() doesn't bring us back a
         * second time and clobber the above state.
         */
        state->errormsg_deferred = true;
    }
 
    if (decoded && decoded->oversized)
        pfree(decoded);
 
    /*
     * Invalidate the read state. We might read from a different source after
     * failure.
     */
    XLogReaderInvalReadState(state);
 
    /*
     * If an error was written to errormsg_buf, it'll be returned to the
     * caller of XLogReadRecord() after all successfully decoded records from
     * the read queue.
     */
 
    return XLREAD_FAIL;
}
 
/*
 * Try to decode the next available record, and return it.  The record will
 * also be returned to XLogNextRecord(), which must be called to 'consume'
 * each record.
 *
 * If nonblocking is true, may return NULL due to lack of data or WAL decoding
 * space.
 */
DecodedXLogRecord *
XLogReadAhead(XLogReaderState *state, bool nonblocking)
{
    XLogPageReadResult result;
 
    if (state->errormsg_deferred)
        return NULL;
 
    result = XLogDecodeNextRecord(state, nonblocking);
    if (result == XLREAD_SUCCESS)
    {
        Assert(state->decode_queue_tail != NULL);
        return state->decode_queue_tail;
    }
 
    return NULL;
}
 
/*
 * Read a single xlog page including at least [pageptr, reqLen] of valid data
 * via the page_read() callback.
 *
 * Returns XLREAD_FAIL if the required page cannot be read for some
 * reason; errormsg_buf is set in that case (unless the error occurs in the
 * page_read callback).
 *
 * Returns XLREAD_WOULDBLOCK if the requested data can't be read without
 * waiting.  This can be returned only if the installed page_read callback
 * respects the state->nonblocking flag, and cannot read the requested data
 * immediately.
 *
 * We fetch the page from a reader-local cache if we know we have the required
 * data and if there hasn't been any error since caching the data.
 */
static int
ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
{
    int         readLen;
    uint32      targetPageOff;
    XLogSegNo   targetSegNo;
    XLogPageHeader hdr;
 
    Assert((pageptr % XLOG_BLCKSZ) == 0);
 
    XLByteToSeg(pageptr, targetSegNo, state->segcxt.ws_segsize);
    targetPageOff = XLogSegmentOffset(pageptr, state->segcxt.ws_segsize);
 
    /* check whether we have all the requested data already */
    if (targetSegNo == state->seg.ws_segno &&
        targetPageOff == state->segoff && reqLen <= state->readLen)
        return state->readLen;
 
    /*
     * Invalidate contents of internal buffer before read attempt.  Just set
     * the length to 0, rather than a full XLogReaderInvalReadState(), so we
     * don't forget the segment we last successfully read.
     */
    state->readLen = 0;
 
    /*
     * Data is not in our buffer.
     *
     * Every time we actually read the segment, even if we looked at parts of
     * it before, we need to do verification as the page_read callback might
     * now be rereading data from a different source.
     *
     * Whenever switching to a new WAL segment, we read the first page of the
     * file and validate its header, even if that's not where the target
     * record is.  This is so that we can check the additional identification
     * info that is present in the first page's "long" header.
     */
    if (targetSegNo != state->seg.ws_segno && targetPageOff != 0)
    {
        XLogRecPtr  targetSegmentPtr = pageptr - targetPageOff;
 
        readLen = state->routine.page_read(state, targetSegmentPtr, XLOG_BLCKSZ,
                                           state->currRecPtr,
                                           state->readBuf);
        if (readLen == XLREAD_WOULDBLOCK)
            return XLREAD_WOULDBLOCK;
        else if (readLen < 0)
            goto err;
 
        /* we can be sure to have enough WAL available, we scrolled back */
        Assert(readLen == XLOG_BLCKSZ);
 
        if (!XLogReaderValidatePageHeader(state, targetSegmentPtr,
                                          state->readBuf))
            goto err;
    }
 
    /*
     * First, read the requested data length, but at least a short page header
     * so that we can validate it.
     */
    readLen = state->routine.page_read(state, pageptr, Max(reqLen, SizeOfXLogShortPHD),
                                       state->currRecPtr,
                                       state->readBuf);
    if (readLen == XLREAD_WOULDBLOCK)
        return XLREAD_WOULDBLOCK;
    else if (readLen < 0)
        goto err;
 
    Assert(readLen <= XLOG_BLCKSZ);
 
    /* Do we have enough data to check the header length? */
    if (readLen <= SizeOfXLogShortPHD)
        goto err;
 
    Assert(readLen >= reqLen);
 
    hdr = (XLogPageHeader) state->readBuf;
 
    /* still not enough */
    if (readLen < XLogPageHeaderSize(hdr))
    {
        readLen = state->routine.page_read(state, pageptr, XLogPageHeaderSize(hdr),
                                           state->currRecPtr,
                                           state->readBuf);
        if (readLen == XLREAD_WOULDBLOCK)
            return XLREAD_WOULDBLOCK;
        else if (readLen < 0)
            goto err;
    }
 
    /*
     * Now that we know we have the full header, validate it.
     */
    if (!XLogReaderValidatePageHeader(state, pageptr, (char *) hdr))
        goto err;
 
    /* update read state information */
    state->seg.ws_segno = targetSegNo;
    state->segoff = targetPageOff;
    state->readLen = readLen;
 
    return readLen;
 
err:
    XLogReaderInvalReadState(state);
 
    return XLREAD_FAIL;
}
 
/*
 * Invalidate the xlogreader's read state to force a re-read.
 */
static void
XLogReaderInvalReadState(XLogReaderState *state)
{
    state->seg.ws_segno = 0;
    state->segoff = 0;
    state->readLen = 0;
}
 
/*
 * Validate an XLOG record header.
 *
 * This is just a convenience subroutine to avoid duplicated code in
 * XLogReadRecord.  It's not intended for use from anywhere else.
 */
static bool
ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
                      XLogRecPtr PrevRecPtr, XLogRecord *record,
                      bool randAccess)
{
    if (record->xl_tot_len < SizeOfXLogRecord)
    {
        report_invalid_record(state,
                              "invalid record length at %X/%X: expected at least %u, got %u",
                              LSN_FORMAT_ARGS(RecPtr),
                              (uint32) SizeOfXLogRecord, record->xl_tot_len);
        return false;
    }
    if (!RmgrIdIsValid(record->xl_rmid))
    {
        report_invalid_record(state,
                              "invalid resource manager ID %u at %X/%X",
                              record->xl_rmid, LSN_FORMAT_ARGS(RecPtr));
        return false;
    }
    if (randAccess)
    {
        /*
         * We can't exactly verify the prev-link, but surely it should be less
         * than the record's own address.
         */
        if (!(record->xl_prev < RecPtr))
        {
            report_invalid_record(state,
                                  "record with incorrect prev-link %X/%X at %X/%X",
                                  LSN_FORMAT_ARGS(record->xl_prev),
                                  LSN_FORMAT_ARGS(RecPtr));
            return false;
        }
    }
    else
    {
        /*
         * Record's prev-link should exactly match our previous location. This
         * check guards against torn WAL pages where a stale but valid-looking
         * WAL record starts on a sector boundary.
         */
        if (record->xl_prev != PrevRecPtr)
        {
            report_invalid_record(state,
                                  "record with incorrect prev-link %X/%X at %X/%X",
                                  LSN_FORMAT_ARGS(record->xl_prev),
                                  LSN_FORMAT_ARGS(RecPtr));
            return false;
        }
    }
 
    return true;
}
 
 
/*
 * CRC-check an XLOG record.  We do not believe the contents of an XLOG
 * record (other than to the minimal extent of computing the amount of
 * data to read in) until we've checked the CRCs.
 *
 * We assume all of the record (that is, xl_tot_len bytes) has been read
 * into memory at *record.  Also, ValidXLogRecordHeader() has accepted the
 * record's header, which means in particular that xl_tot_len is at least
 * SizeOfXLogRecord.
 */
static bool
ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr)
{
    pg_crc32c   crc;
 
    Assert(record->xl_tot_len >= SizeOfXLogRecord);
 
    /* Calculate the CRC */
    INIT_CRC32C(crc);
    COMP_CRC32C(crc, ((char *) record) + SizeOfXLogRecord, record->xl_tot_len - SizeOfXLogRecord);
    /* include the record header last */
    COMP_CRC32C(crc, (char *) record, offsetof(XLogRecord, xl_crc));
    FIN_CRC32C(crc);
 
    if (!EQ_CRC32C(record->xl_crc, crc))
    {
        report_invalid_record(state,
                              "incorrect resource manager data checksum in record at %X/%X",
                              LSN_FORMAT_ARGS(recptr));
        return false;
    }
 
    return true;
}
 
/*
 * Validate a page header.
 *
 * Check if 'phdr' is valid as the header of the XLog page at position
 * 'recptr'.
 */
bool
XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
                             char *phdr)
{
    XLogSegNo   segno;
    int32       offset;
    XLogPageHeader hdr = (XLogPageHeader) phdr;
 
    Assert((recptr % XLOG_BLCKSZ) == 0);
 
    XLByteToSeg(recptr, segno, state->segcxt.ws_segsize);
    offset = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
 
    if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
    {
        char        fname[MAXFNAMELEN];
 
        XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
 
        report_invalid_record(state,
                              "invalid magic number %04X in WAL segment %s, LSN %X/%X, offset %u",
                              hdr->xlp_magic,
                              fname,
                              LSN_FORMAT_ARGS(recptr),
                              offset);
        return false;
    }
 
    if ((hdr->xlp_info & ~XLP_ALL_FLAGS) != 0)
    {
        char        fname[MAXFNAMELEN];
 
        XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
 
        report_invalid_record(state,
                              "invalid info bits %04X in WAL segment %s, LSN %X/%X, offset %u",
                              hdr->xlp_info,
                              fname,
                              LSN_FORMAT_ARGS(recptr),
                              offset);
        return false;
    }
 
    if (hdr->xlp_info & XLP_LONG_HEADER)
    {
        XLogLongPageHeader longhdr = (XLogLongPageHeader) hdr;
 
        if (state->system_identifier &&
            longhdr->xlp_sysid != state->system_identifier)
        {
            report_invalid_record(state,
                                  "WAL file is from different database system: WAL file database system identifier is %llu, pg_control database system identifier is %llu",
                                  (unsigned long long) longhdr->xlp_sysid,
                                  (unsigned long long) state->system_identifier);
            return false;
        }
        else if (longhdr->xlp_seg_size != state->segcxt.ws_segsize)
        {
            report_invalid_record(state,
                                  "WAL file is from different database system: incorrect segment size in page header");
            return false;
        }
        else if (longhdr->xlp_xlog_blcksz != XLOG_BLCKSZ)
        {
            report_invalid_record(state,
                                  "WAL file is from different database system: incorrect XLOG_BLCKSZ in page header");
            return false;
        }
    }
    else if (offset == 0)
    {
        char        fname[MAXFNAMELEN];
 
        XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
 
        /* hmm, first page of file doesn't have a long header? */
        report_invalid_record(state,
                              "invalid info bits %04X in WAL segment %s, LSN %X/%X, offset %u",
                              hdr->xlp_info,
                              fname,
                              LSN_FORMAT_ARGS(recptr),
                              offset);
        return false;
    }
 
    /*
     * Check that the address on the page agrees with what we expected. This
     * check typically fails when an old WAL segment is recycled, and hasn't
     * yet been overwritten with new data yet.
     */
    if (hdr->xlp_pageaddr != recptr)
    {
        char        fname[MAXFNAMELEN];
 
        XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
 
        report_invalid_record(state,
                              "unexpected pageaddr %X/%X in WAL segment %s, LSN %X/%X, offset %u",
                              LSN_FORMAT_ARGS(hdr->xlp_pageaddr),
                              fname,
                              LSN_FORMAT_ARGS(recptr),
                              offset);
        return false;
    }
 
    /*
     * Since child timelines are always assigned a TLI greater than their
     * immediate parent's TLI, we should never see TLI go backwards across
     * successive pages of a consistent WAL sequence.
     *
     * Sometimes we re-read a segment that's already been (partially) read. So
     * we only verify TLIs for pages that are later than the last remembered
     * LSN.
     */
    if (recptr > state->latestPagePtr)
    {
        if (hdr->xlp_tli < state->latestPageTLI)
        {
            char        fname[MAXFNAMELEN];
 
            XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
 
            report_invalid_record(state,
                                  "out-of-sequence timeline ID %u (after %u) in WAL segment %s, LSN %X/%X, offset %u",
                                  hdr->xlp_tli,
                                  state->latestPageTLI,
                                  fname,
                                  LSN_FORMAT_ARGS(recptr),
                                  offset);
            return false;
        }
    }
    state->latestPagePtr = recptr;
    state->latestPageTLI = hdr->xlp_tli;
 
    return true;
}
 
/*
 * Forget about an error produced by XLogReaderValidatePageHeader().
 */
void
XLogReaderResetError(XLogReaderState *state)
{
    state->errormsg_buf[0] = '\0';
    state->errormsg_deferred = false;
}
 
/*
 * Find the first record with an lsn >= RecPtr.
 *
 * This is different from XLogBeginRead() in that RecPtr doesn't need to point
 * to a valid record boundary.  Useful for checking whether RecPtr is a valid
 * xlog address for reading, and to find the first valid address after some
 * address when dumping records for debugging purposes.
 *
 * This positions the reader, like XLogBeginRead(), so that the next call to
 * XLogReadRecord() will read the next valid record.
 */
XLogRecPtr
XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr)
{
    XLogRecPtr  tmpRecPtr;
    XLogRecPtr  found = InvalidXLogRecPtr;
    XLogPageHeader header;
    char       *errormsg;
 
    Assert(!XLogRecPtrIsInvalid(RecPtr));
 
    /* Make sure ReadPageInternal() can't return XLREAD_WOULDBLOCK. */
    state->nonblocking = false;
 
    /*
     * skip over potential continuation data, keeping in mind that it may span
     * multiple pages
     */
    tmpRecPtr = RecPtr;
    while (true)
    {
        XLogRecPtr  targetPagePtr;
        int         targetRecOff;
        uint32      pageHeaderSize;
        int         readLen;
 
        /*
         * Compute targetRecOff. It should typically be equal or greater than
         * short page-header since a valid record can't start anywhere before
         * that, except when caller has explicitly specified the offset that
         * falls somewhere there or when we are skipping multi-page
         * continuation record. It doesn't matter though because
         * ReadPageInternal() is prepared to handle that and will read at
         * least short page-header worth of data
         */
        targetRecOff = tmpRecPtr % XLOG_BLCKSZ;
 
        /* scroll back to page boundary */
        targetPagePtr = tmpRecPtr - targetRecOff;
 
        /* Read the page containing the record */
        readLen = ReadPageInternal(state, targetPagePtr, targetRecOff);
        if (readLen < 0)
            goto err;
 
        header = (XLogPageHeader) state->readBuf;
 
        pageHeaderSize = XLogPageHeaderSize(header);
 
        /* make sure we have enough data for the page header */
        readLen = ReadPageInternal(state, targetPagePtr, pageHeaderSize);
        if (readLen < 0)
            goto err;
 
        /* skip over potential continuation data */
        if (header->xlp_info & XLP_FIRST_IS_CONTRECORD)
        {
            /*
             * If the length of the remaining continuation data is more than
             * what can fit in this page, the continuation record crosses over
             * this page. Read the next page and try again. xlp_rem_len in the
             * next page header will contain the remaining length of the
             * continuation data
             *
             * Note that record headers are MAXALIGN'ed
             */
            if (MAXALIGN(header->xlp_rem_len) >= (XLOG_BLCKSZ - pageHeaderSize))
                tmpRecPtr = targetPagePtr + XLOG_BLCKSZ;
            else
            {
                /*
                 * The previous continuation record ends in this page. Set
                 * tmpRecPtr to point to the first valid record
                 */
                tmpRecPtr = targetPagePtr + pageHeaderSize
                    + MAXALIGN(header->xlp_rem_len);
                break;
            }
        }
        else
        {
            tmpRecPtr = targetPagePtr + pageHeaderSize;
            break;
        }
    }
 
    /*
     * we know now that tmpRecPtr is an address pointing to a valid XLogRecord
     * because either we're at the first record after the beginning of a page
     * or we just jumped over the remaining data of a continuation.
     */
    XLogBeginRead(state, tmpRecPtr);
    while (XLogReadRecord(state, &errormsg) != NULL)
    {
        /* past the record we've found, break out */
        if (RecPtr <= state->ReadRecPtr)
        {
            /* Rewind the reader to the beginning of the last record. */
            found = state->ReadRecPtr;
            XLogBeginRead(state, found);
            return found;
        }
    }
 
err:
    XLogReaderInvalReadState(state);
 
    return InvalidXLogRecPtr;
}
 
/*
 * Helper function to ease writing of XLogReaderRoutine->page_read callbacks.
 * If this function is used, caller must supply a segment_open callback in
 * 'state', as that is used here.
 *
 * Read 'count' bytes into 'buf', starting at location 'startptr', from WAL
 * fetched from timeline 'tli'.
 *
 * Returns true if succeeded, false if an error occurs, in which case
 * 'errinfo' receives error details.
 */
bool
WALRead(XLogReaderState *state,
        char *buf, XLogRecPtr startptr, Size count, TimeLineID tli,
        WALReadError *errinfo)
{
    char       *p;
    XLogRecPtr  recptr;
    Size        nbytes;
 
    p = buf;
    recptr = startptr;
    nbytes = count;
 
    while (nbytes > 0)
    {
        uint32      startoff;
        int         segbytes;
        int         readbytes;
 
        startoff = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
 
        /*
         * If the data we want is not in a segment we have open, close what we
         * have (if anything) and open the next one, using the caller's
         * provided segment_open callback.
         */
        if (state->seg.ws_file < 0 ||
            !XLByteInSeg(recptr, state->seg.ws_segno, state->segcxt.ws_segsize) ||
            tli != state->seg.ws_tli)
        {
            XLogSegNo   nextSegNo;
 
            if (state->seg.ws_file >= 0)
                state->routine.segment_close(state);
 
            XLByteToSeg(recptr, nextSegNo, state->segcxt.ws_segsize);
            state->routine.segment_open(state, nextSegNo, &tli);
 
            /* This shouldn't happen -- indicates a bug in segment_open */
            Assert(state->seg.ws_file >= 0);
 
            /* Update the current segment info. */
            state->seg.ws_tli = tli;
            state->seg.ws_segno = nextSegNo;
        }
 
        /* How many bytes are within this segment? */
        if (nbytes > (state->segcxt.ws_segsize - startoff))
            segbytes = state->segcxt.ws_segsize - startoff;
        else
            segbytes = nbytes;
 
#ifndef FRONTEND
        pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
#endif
 
        /* Reset errno first; eases reporting non-errno-affecting errors */
        errno = 0;
        readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
 
#ifndef FRONTEND
        pgstat_report_wait_end();
#endif
 
        if (readbytes <= 0)
        {
            errinfo->wre_errno = errno;
            errinfo->wre_req = segbytes;
            errinfo->wre_read = readbytes;
            errinfo->wre_off = startoff;
            errinfo->wre_seg = state->seg;
            return false;
        }
 
        /* Update state for read */
        recptr += readbytes;
        nbytes -= readbytes;
        p += readbytes;
    }
 
    return true;
}
 
/* ----------------------------------------
 * Functions for decoding the data and block references in a record.
 * ----------------------------------------
 */
 
/*
 * Private function to reset the state, forgetting all decoded records, if we
 * are asked to move to a new read position.
 */
static void
ResetDecoder(XLogReaderState *state)
{
    DecodedXLogRecord *r;
 
    /* Reset the decoded record queue, freeing any oversized records. */
    while ((r = state->decode_queue_head) != NULL)
    {
        state->decode_queue_head = r->next;
        if (r->oversized)
            pfree(r);
    }
    state->decode_queue_tail = NULL;
    state->decode_queue_head = NULL;
    state->record = NULL;
 
    /* Reset the decode buffer to empty. */
    state->decode_buffer_tail = state->decode_buffer;
    state->decode_buffer_head = state->decode_buffer;
 
    /* Clear error state. */
    state->errormsg_buf[0] = '\0';
    state->errormsg_deferred = false;
}
 
/*
 * Compute the maximum possible amount of padding that could be required to
 * decode a record, given xl_tot_len from the record's header.  This is the
 * amount of output buffer space that we need to decode a record, though we
 * might not finish up using it all.
 *
 * This computation is pessimistic and assumes the maximum possible number of
 * blocks, due to lack of better information.
 */
size_t
DecodeXLogRecordRequiredSpace(size_t xl_tot_len)
{
    size_t      size = 0;
 
    /* Account for the fixed size part of the decoded record struct. */
    size += offsetof(DecodedXLogRecord, blocks[0]);
    /* Account for the flexible blocks array of maximum possible size. */
    size += sizeof(DecodedBkpBlock) * (XLR_MAX_BLOCK_ID + 1);
    /* Account for all the raw main and block data. */
    size += xl_tot_len;
    /* We might insert padding before main_data. */
    size += (MAXIMUM_ALIGNOF - 1);
    /* We might insert padding before each block's data. */
    size += (MAXIMUM_ALIGNOF - 1) * (XLR_MAX_BLOCK_ID + 1);
    /* We might insert padding at the end. */
    size += (MAXIMUM_ALIGNOF - 1);
 
    return size;
}
 
/*
 * Decode a record.  "decoded" must point to a MAXALIGNed memory area that has
 * space for at least DecodeXLogRecordRequiredSpace(record) bytes.  On
 * success, decoded->size contains the actual space occupied by the decoded
 * record, which may turn out to be less.
 *
 * Only decoded->oversized member must be initialized already, and will not be
 * modified.  Other members will be initialized as required.
 *
 * On error, a human-readable error message is returned in *errormsg, and
 * the return value is false.
 */
bool
DecodeXLogRecord(XLogReaderState *state,
                 DecodedXLogRecord *decoded,
                 XLogRecord *record,
                 XLogRecPtr lsn,
                 char **errormsg)
{
    /*
     * read next _size bytes from record buffer, but check for overrun first.
     */
#define COPY_HEADER_FIELD(_dst, _size)          \
    do {                                        \
        if (remaining < _size)                  \
            goto shortdata_err;                 \
        memcpy(_dst, ptr, _size);               \
        ptr += _size;                           \
        remaining -= _size;                     \
    } while(0)
 
    char       *ptr;
    char       *out;
    uint32      remaining;
    uint32      datatotal;
    RelFileLocator *rlocator = NULL;
    uint8       block_id;
 
    decoded->header = *record;
    decoded->lsn = lsn;
    decoded->next = NULL;
    decoded->record_origin = InvalidRepOriginId;
    decoded->toplevel_xid = InvalidTransactionId;
    decoded->main_data = NULL;
    decoded->main_data_len = 0;
    decoded->max_block_id = -1;
    ptr = (char *) record;
    ptr += SizeOfXLogRecord;
    remaining = record->xl_tot_len - SizeOfXLogRecord;
 
    /* Decode the headers */
    datatotal = 0;
    while (remaining > datatotal)
    {
        COPY_HEADER_FIELD(&block_id, sizeof(uint8));
 
        if (block_id == XLR_BLOCK_ID_DATA_SHORT)
        {
            /* XLogRecordDataHeaderShort */
            uint8       main_data_len;
 
            COPY_HEADER_FIELD(&main_data_len, sizeof(uint8));
 
            decoded->main_data_len = main_data_len;
            datatotal += main_data_len;
            break;              /* by convention, the main data fragment is
                                 * always last */
        }
        else if (block_id == XLR_BLOCK_ID_DATA_LONG)
        {
            /* XLogRecordDataHeaderLong */
            uint32      main_data_len;
 
            COPY_HEADER_FIELD(&main_data_len, sizeof(uint32));
            decoded->main_data_len = main_data_len;
            datatotal += main_data_len;
            break;              /* by convention, the main data fragment is
                                 * always last */
        }
        else if (block_id == XLR_BLOCK_ID_ORIGIN)
        {
            COPY_HEADER_FIELD(&decoded->record_origin, sizeof(RepOriginId));
        }
        else if (block_id == XLR_BLOCK_ID_TOPLEVEL_XID)
        {
            COPY_HEADER_FIELD(&decoded->toplevel_xid, sizeof(TransactionId));
        }
        else if (block_id <= XLR_MAX_BLOCK_ID)
        {
            /* XLogRecordBlockHeader */
            DecodedBkpBlock *blk;
            uint8       fork_flags;
 
            /* mark any intervening block IDs as not in use */
            for (int i = decoded->max_block_id + 1; i < block_id; ++i)
                decoded->blocks[i].in_use = false;
 
            if (block_id <= decoded->max_block_id)
            {
                report_invalid_record(state,
                                      "out-of-order block_id %u at %X/%X",
                                      block_id,
                                      LSN_FORMAT_ARGS(state->ReadRecPtr));
                goto err;
            }
            decoded->max_block_id = block_id;
 
            blk = &decoded->blocks[block_id];
            blk->in_use = true;
            blk->apply_image = false;
 
            COPY_HEADER_FIELD(&fork_flags, sizeof(uint8));
            blk->forknum = fork_flags & BKPBLOCK_FORK_MASK;
            blk->flags = fork_flags;
            blk->has_image = ((fork_flags & BKPBLOCK_HAS_IMAGE) != 0);
            blk->has_data = ((fork_flags & BKPBLOCK_HAS_DATA) != 0);
 
            blk->prefetch_buffer = InvalidBuffer;
 
            COPY_HEADER_FIELD(&blk->data_len, sizeof(uint16));
            /* cross-check that the HAS_DATA flag is set iff data_length > 0 */
            if (blk->has_data && blk->data_len == 0)
            {
                report_invalid_record(state,
                                      "BKPBLOCK_HAS_DATA set, but no data included at %X/%X",
                                      LSN_FORMAT_ARGS(state->ReadRecPtr));
                goto err;
            }
            if (!blk->has_data && blk->data_len != 0)
            {
                report_invalid_record(state,
                                      "BKPBLOCK_HAS_DATA not set, but data length is %u at %X/%X",
                                      (unsigned int) blk->data_len,
                                      LSN_FORMAT_ARGS(state->ReadRecPtr));
                goto err;
            }
            datatotal += blk->data_len;
 
            if (blk->has_image)
            {
                COPY_HEADER_FIELD(&blk->bimg_len, sizeof(uint16));
                COPY_HEADER_FIELD(&blk->hole_offset, sizeof(uint16));
                COPY_HEADER_FIELD(&blk->bimg_info, sizeof(uint8));
 
                blk->apply_image = ((blk->bimg_info & BKPIMAGE_APPLY) != 0);
 
                if (BKPIMAGE_COMPRESSED(blk->bimg_info))
                {
                    if (blk->bimg_info & BKPIMAGE_HAS_HOLE)
                        COPY_HEADER_FIELD(&blk->hole_length, sizeof(uint16));
                    else
                        blk->hole_length = 0;
                }
                else
                    blk->hole_length = BLCKSZ - blk->bimg_len;
                datatotal += blk->bimg_len;
 
                /*
                 * cross-check that hole_offset > 0, hole_length > 0 and
                 * bimg_len < BLCKSZ if the HAS_HOLE flag is set.
                 */
                if ((blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
                    (blk->hole_offset == 0 ||
                     blk->hole_length == 0 ||
                     blk->bimg_len == BLCKSZ))
                {
                    report_invalid_record(state,
                                          "BKPIMAGE_HAS_HOLE set, but hole offset %u length %u block image length %u at %X/%X",
                                          (unsigned int) blk->hole_offset,
                                          (unsigned int) blk->hole_length,
                                          (unsigned int) blk->bimg_len,
                                          LSN_FORMAT_ARGS(state->ReadRecPtr));
                    goto err;
                }
 
                /*
                 * cross-check that hole_offset == 0 and hole_length == 0 if
                 * the HAS_HOLE flag is not set.
                 */
                if (!(blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
                    (blk->hole_offset != 0 || blk->hole_length != 0))
                {
                    report_invalid_record(state,
                                          "BKPIMAGE_HAS_HOLE not set, but hole offset %u length %u at %X/%X",
                                          (unsigned int) blk->hole_offset,
                                          (unsigned int) blk->hole_length,
                                          LSN_FORMAT_ARGS(state->ReadRecPtr));
                    goto err;
                }
 
                /*
                 * Cross-check that bimg_len < BLCKSZ if it is compressed.
                 */
                if (BKPIMAGE_COMPRESSED(blk->bimg_info) &&
                    blk->bimg_len == BLCKSZ)
                {
                    report_invalid_record(state,
                                          "BKPIMAGE_COMPRESSED set, but block image length %u at %X/%X",
                                          (unsigned int) blk->bimg_len,
                                          LSN_FORMAT_ARGS(state->ReadRecPtr));
                    goto err;
                }
 
                /*
                 * cross-check that bimg_len = BLCKSZ if neither HAS_HOLE is
                 * set nor COMPRESSED().
                 */
                if (!(blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
                    !BKPIMAGE_COMPRESSED(blk->bimg_info) &&
                    blk->bimg_len != BLCKSZ)
                {
                    report_invalid_record(state,
                                          "neither BKPIMAGE_HAS_HOLE nor BKPIMAGE_COMPRESSED set, but block image length is %u at %X/%X",
                                          (unsigned int) blk->data_len,
                                          LSN_FORMAT_ARGS(state->ReadRecPtr));
                    goto err;
                }
            }
            if (!(fork_flags & BKPBLOCK_SAME_REL))
            {
                COPY_HEADER_FIELD(&blk->rlocator, sizeof(RelFileLocator));
                rlocator = &blk->rlocator;
            }
            else
            {
                if (rlocator == NULL)
                {
                    report_invalid_record(state,
                                          "BKPBLOCK_SAME_REL set but no previous rel at %X/%X",
                                          LSN_FORMAT_ARGS(state->ReadRecPtr));
                    goto err;
                }
 
                blk->rlocator = *rlocator;
            }
            COPY_HEADER_FIELD(&blk->blkno, sizeof(BlockNumber));
        }
        else
        {
            report_invalid_record(state,
                                  "invalid block_id %u at %X/%X",
                                  block_id, LSN_FORMAT_ARGS(state->ReadRecPtr));
            goto err;
        }
    }
 
    if (remaining != datatotal)
        goto shortdata_err;
 
    /*
     * Ok, we've parsed the fragment headers, and verified that the total
     * length of the payload in the fragments is equal to the amount of data
     * left.  Copy the data of each fragment to contiguous space after the
     * blocks array, inserting alignment padding before the data fragments so
     * they can be cast to struct pointers by REDO routines.
     */
    out = ((char *) decoded) +
        offsetof(DecodedXLogRecord, blocks) +
        sizeof(decoded->blocks[0]) * (decoded->max_block_id + 1);
 
    /* block data first */
    for (block_id = 0; block_id <= decoded->max_block_id; block_id++)
    {
        DecodedBkpBlock *blk = &decoded->blocks[block_id];
 
        if (!blk->in_use)
            continue;
 
        Assert(blk->has_image || !blk->apply_image);
 
        if (blk->has_image)
        {
            /* no need to align image */
            blk->bkp_image = out;
            memcpy(out, ptr, blk->bimg_len);
            ptr += blk->bimg_len;
            out += blk->bimg_len;
        }
        if (blk->has_data)
        {
            out = (char *) MAXALIGN(out);
            blk->data = out;
            memcpy(blk->data, ptr, blk->data_len);
            ptr += blk->data_len;
            out += blk->data_len;
        }
    }
 
    /* and finally, the main data */
    if (decoded->main_data_len > 0)
    {
        out = (char *) MAXALIGN(out);
        decoded->main_data = out;
        memcpy(decoded->main_data, ptr, decoded->main_data_len);
        ptr += decoded->main_data_len;
        out += decoded->main_data_len;
    }
 
    /* Report the actual size we used. */
    decoded->size = MAXALIGN(out - (char *) decoded);
    Assert(DecodeXLogRecordRequiredSpace(record->xl_tot_len) >=
           decoded->size);
 
    return true;
 
shortdata_err:
    report_invalid_record(state,
                          "record with invalid length at %X/%X",
                          LSN_FORMAT_ARGS(state->ReadRecPtr));
err:
    *errormsg = state->errormsg_buf;
 
    return false;
}
 
/*
 * Returns information about the block that a block reference refers to.
 *
 * This is like XLogRecGetBlockTagExtended, except that the block reference
 * must exist and there's no access to prefetch_buffer.
 */
void
XLogRecGetBlockTag(XLogReaderState *record, uint8 block_id,
                   RelFileLocator *rlocator, ForkNumber *forknum,
                   BlockNumber *blknum)
{
    if (!XLogRecGetBlockTagExtended(record, block_id, rlocator, forknum,
                                    blknum, NULL))
    {
#ifndef FRONTEND
        elog(ERROR, "could not locate backup block with ID %d in WAL record",
             block_id);
#else
        pg_fatal("could not locate backup block with ID %d in WAL record",
                 block_id);
#endif
    }
}
 
/*
 * Returns information about the block that a block reference refers to,
 * optionally including the buffer that the block may already be in.
 *
 * If the WAL record contains a block reference with the given ID, *rlocator,
 * *forknum, *blknum and *prefetch_buffer are filled in (if not NULL), and
 * returns true.  Otherwise returns false.
 */
bool
XLogRecGetBlockTagExtended(XLogReaderState *record, uint8 block_id,
                           RelFileLocator *rlocator, ForkNumber *forknum,
                           BlockNumber *blknum,
                           Buffer *prefetch_buffer)
{
    DecodedBkpBlock *bkpb;
 
    if (!XLogRecHasBlockRef(record, block_id))
        return false;
 
    bkpb = &record->record->blocks[block_id];
    if (rlocator)
        *rlocator = bkpb->rlocator;
    if (forknum)
        *forknum = bkpb->forknum;
    if (blknum)
        *blknum = bkpb->blkno;
    if (prefetch_buffer)
        *prefetch_buffer = bkpb->prefetch_buffer;
    return true;
}
 
/*
 * Returns the data associated with a block reference, or NULL if there is
 * no data (e.g. because a full-page image was taken instead). The returned
 * pointer points to a MAXALIGNed buffer.
 */
char *
XLogRecGetBlockData(XLogReaderState *record, uint8 block_id, Size *len)
{
    DecodedBkpBlock *bkpb;
 
    if (block_id > record->record->max_block_id ||
        !record->record->blocks[block_id].in_use)
        return NULL;
 
    bkpb = &record->record->blocks[block_id];
 
    if (!bkpb->has_data)
    {
        if (len)
            *len = 0;
        return NULL;
    }
    else
    {
        if (len)
            *len = bkpb->data_len;
        return bkpb->data;
    }
}
 
/*
 * Restore a full-page image from a backup block attached to an XLOG record.
 *
 * Returns true if a full-page image is restored, and false on failure with
 * an error to be consumed by the caller.
 */
bool
RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page)
{
    DecodedBkpBlock *bkpb;
    char       *ptr;
    PGAlignedBlock tmp;
 
    if (block_id > record->record->max_block_id ||
        !record->record->blocks[block_id].in_use)
    {
        report_invalid_record(record,
                              "could not restore image at %X/%X with invalid block %d specified",
                              LSN_FORMAT_ARGS(record->ReadRecPtr),
                              block_id);
        return false;
    }
    if (!record->record->blocks[block_id].has_image)
    {
        report_invalid_record(record, "could not restore image at %X/%X with invalid state, block %d",
                              LSN_FORMAT_ARGS(record->ReadRecPtr),
                              block_id);
        return false;
    }
 
    bkpb = &record->record->blocks[block_id];
    ptr = bkpb->bkp_image;
 
    if (BKPIMAGE_COMPRESSED(bkpb->bimg_info))
    {
        /* If a backup block image is compressed, decompress it */
        bool        decomp_success = true;
 
        if ((bkpb->bimg_info & BKPIMAGE_COMPRESS_PGLZ) != 0)
        {
            if (pglz_decompress(ptr, bkpb->bimg_len, tmp.data,
                                BLCKSZ - bkpb->hole_length, true) < 0)
                decomp_success = false;
        }
        else if ((bkpb->bimg_info & BKPIMAGE_COMPRESS_LZ4) != 0)
        {
#ifdef USE_LZ4
            if (LZ4_decompress_safe(ptr, tmp.data,
                                    bkpb->bimg_len, BLCKSZ - bkpb->hole_length) <= 0)
                decomp_success = false;
#else
            report_invalid_record(record, "could not restore image at %X/%X compressed with %s not supported by build, block %d",
                                  LSN_FORMAT_ARGS(record->ReadRecPtr),
                                  "LZ4",
                                  block_id);
            return false;
#endif
        }
        else if ((bkpb->bimg_info & BKPIMAGE_COMPRESS_ZSTD) != 0)
        {
#ifdef USE_ZSTD
            size_t      decomp_result = ZSTD_decompress(tmp.data,
                                                        BLCKSZ - bkpb->hole_length,
                                                        ptr, bkpb->bimg_len);
 
            if (ZSTD_isError(decomp_result))
                decomp_success = false;
#else
            report_invalid_record(record, "could not restore image at %X/%X compressed with %s not supported by build, block %d",
                                  LSN_FORMAT_ARGS(record->ReadRecPtr),
                                  "zstd",
                                  block_id);
            return false;
#endif
        }
        else
        {
            report_invalid_record(record, "could not restore image at %X/%X compressed with unknown method, block %d",
                                  LSN_FORMAT_ARGS(record->ReadRecPtr),
                                  block_id);
            return false;
        }
 
        if (!decomp_success)
        {
            report_invalid_record(record, "could not decompress image at %X/%X, block %d",
                                  LSN_FORMAT_ARGS(record->ReadRecPtr),
                                  block_id);
            return false;
        }
 
        ptr = tmp.data;
    }
 
    /* generate page, taking into account hole if necessary */
    if (bkpb->hole_length == 0)
    {
        memcpy(page, ptr, BLCKSZ);
    }
    else
    {
        memcpy(page, ptr, bkpb->hole_offset);
        /* must zero-fill the hole */
        MemSet(page + bkpb->hole_offset, 0, bkpb->hole_length);
        memcpy(page + (bkpb->hole_offset + bkpb->hole_length),
               ptr + bkpb->hole_offset,
               BLCKSZ - (bkpb->hole_offset + bkpb->hole_length));
    }
 
    return true;
}
 
#ifndef FRONTEND
 
/*
 * Extract the FullTransactionId from a WAL record.
 */
FullTransactionId
XLogRecGetFullXid(XLogReaderState *record)
{
    TransactionId xid,
                next_xid;
    uint32      epoch;
 
    /*
     * This function is only safe during replay, because it depends on the
     * replay state.  See AdvanceNextFullTransactionIdPastXid() for more.
     */
    Assert(AmStartupProcess() || !IsUnderPostmaster);
 
    xid = XLogRecGetXid(record);
    next_xid = XidFromFullTransactionId(TransamVariables->nextXid);
    epoch = EpochFromFullTransactionId(TransamVariables->nextXid);
 
    /*
     * If xid is numerically greater than next_xid, it has to be from the last
     * epoch.
     */
    if (unlikely(xid > next_xid))
        --epoch;
 
    return FullTransactionIdFromEpochAndXid(epoch, xid);
}
 
#endif