xlogreader.c

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/*-------------------------------------------------------------------------
 *
 * xlogreader.c
 *      Generic XLog reading facility
 *
 * Portions Copyright (c) 2013, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *      src/backend/access/transam/xlogreader.c
 *
 * NOTES
 *      See xlogreader.h for more notes on this facility.
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/transam.h"
#include "access/xlog.h"
#include "access/xlog_internal.h"
#include "access/xlogreader.h"
#include "catalog/pg_control.h"

static bool allocate_recordbuf(XLogReaderState *state, uint32 reclength);

static bool ValidXLogPageHeader(XLogReaderState *state, XLogRecPtr recptr,
                    XLogPageHeader hdr);
static bool ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
                 XLogRecPtr PrevRecPtr, XLogRecord *record, bool randAccess);
static bool ValidXLogRecord(XLogReaderState *state, XLogRecord *record,
                XLogRecPtr recptr);
static int ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr,
                 int reqLen);
static void
report_invalid_record(XLogReaderState *state, const char *fmt,...)
/* This extension allows gcc to check the format string for consistency with
   the supplied arguments. */
__attribute__((format(PG_PRINTF_ATTRIBUTE, 2, 3)));

/* size of the buffer allocated for error message. */
#define MAX_ERRORMSG_LEN 1000

/*
 * 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);
}

/*
 * Allocate and initialize a new XLogReader.
 *
 * Returns NULL if the xlogreader couldn't be allocated.
 */
XLogReaderState *
XLogReaderAllocate(XLogPageReadCB pagereadfunc, void *private_data)
{
    XLogReaderState *state;

    AssertArg(pagereadfunc != NULL);

    state = (XLogReaderState *) malloc(sizeof(XLogReaderState));
    if (!state)
        return NULL;
    MemSet(state, 0, sizeof(XLogReaderState));

    /*
     * 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 malloc()
     * will provide MAXALIGN'd storage.
     */
    state->readBuf = (char *) malloc(XLOG_BLCKSZ);
    if (!state->readBuf)
    {
        free(state);
        return NULL;
    }

    state->read_page = pagereadfunc;
    /* system_identifier initialized to zeroes above */
    state->private_data = private_data;
    /* ReadRecPtr and EndRecPtr initialized to zeroes above */
    /* readSegNo, readOff, readLen, readPageTLI initialized to zeroes above */
    state->errormsg_buf = malloc(MAX_ERRORMSG_LEN + 1);
    if (!state->errormsg_buf)
    {
        free(state->readBuf);
        free(state);
        return NULL;
    }
    state->errormsg_buf[0] = '\0';

    /*
     * Allocate an initial readRecordBuf of minimal size, which can later be
     * enlarged if necessary.
     */
    if (!allocate_recordbuf(state, 0))
    {
        free(state->errormsg_buf);
        free(state->readBuf);
        free(state);
        return NULL;
    }

    return state;
}

void
XLogReaderFree(XLogReaderState *state)
{
    free(state->errormsg_buf);
    if (state->readRecordBuf)
        free(state->readRecordBuf);
    free(state->readBuf);
    free(state);
}

/*
 * Allocate readRecordBuf to fit a record of at least the given length.
 * Returns true if successful, false if out of memory.
 *
 * 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.)
 */
static bool
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)
        free(state->readRecordBuf);
    state->readRecordBuf = (char *) malloc(newSize);
    if (!state->readRecordBuf)
    {
        state->readRecordBufSize = 0;
        return false;
    }

    state->readRecordBufSize = newSize;
    return true;
}

/*
 * Attempt to read an XLOG record.
 *
 * If RecPtr is not NULL, try to read a record at that position.  Otherwise
 * try to read a record just after the last one previously read.
 *
 * If the read_page 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, XLogRecPtr RecPtr, char **errormsg)
{
    XLogRecord *record;
    XLogRecPtr  targetPagePtr;
    bool        randAccess = false;
    uint32      len,
                total_len;
    uint32      targetRecOff;
    uint32      pageHeaderSize;
    bool        gotheader;
    int         readOff;

    /* reset error state */
    *errormsg = NULL;
    state->errormsg_buf[0] = '\0';

    if (RecPtr == InvalidXLogRecPtr)
    {
        RecPtr = state->EndRecPtr;

        if (state->ReadRecPtr == InvalidXLogRecPtr)
            randAccess = true;

        /*
         * RecPtr 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
    {
        /*
         * In this case, the passed-in record pointer should already be
         * pointing to a valid record starting position.
         */
        Assert(XRecOffIsValid(RecPtr));
        randAccess = true;      /* allow readPageTLI to go backwards too */
    }

    state->currRecPtr = RecPtr;

    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 < 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",
                              (uint32) (RecPtr >> 32), (uint32) RecPtr);
        goto err;
    }

    if ((((XLogPageHeader) state->readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) &&
        targetRecOff == pageHeaderSize)
    {
        report_invalid_record(state, "contrecord is requested by %X/%X",
                              (uint32) (RecPtr >> 32), (uint32) 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->ReadRecPtr, record,
                                   randAccess))
            goto err;
        gotheader = true;
    }
    else
    {
        /* XXX: more validation should be done here */
        if (total_len < SizeOfXLogRecord)
        {
            report_invalid_record(state, "invalid record length at %X/%X",
                                  (uint32) (RecPtr >> 32), (uint32) RecPtr);
            goto err;
        }
        gotheader = false;
    }

    /*
     * Enlarge readRecordBuf as needed.
     */
    if (total_len > state->readRecordBufSize &&
        !allocate_recordbuf(state, total_len))
    {
        /* We treat this as a "bogus data" condition */
        report_invalid_record(state, "record length %u at %X/%X too long",
                              total_len,
                              (uint32) (RecPtr >> 32), (uint32) RecPtr);
        goto err;
    }

    len = XLOG_BLCKSZ - RecPtr % XLOG_BLCKSZ;
    if (total_len > len)
    {
        /* Need to reassemble record */
        char       *contdata;
        XLogPageHeader pageHeader;
        char       *buffer;
        uint32      gotlen;

        /* 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 < 0)
                goto err;

            Assert(SizeOfXLogShortPHD <= readOff);

            /* Check that the continuation on next page looks valid */
            pageHeader = (XLogPageHeader) state->readBuf;
            if (!(pageHeader->xlp_info & XLP_FIRST_IS_CONTRECORD))
            {
                report_invalid_record(state,
                                      "there is no contrecord flag at %X/%X",
                                   (uint32) (RecPtr >> 32), (uint32) 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 at %X/%X",
                                      pageHeader->xlp_rem_len,
                                   (uint32) (RecPtr >> 32), (uint32) 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->ReadRecPtr,
                                           record, randAccess))
                    goto err;
                gotheader = true;
            }
        } while (gotlen < total_len);

        Assert(gotheader);

        record = (XLogRecord *) state->readRecordBuf;
        if (!ValidXLogRecord(state, record, RecPtr))
            goto err;

        pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
        state->ReadRecPtr = RecPtr;
        state->EndRecPtr = 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 < 0)
            goto err;

        /* Record does not cross a page boundary */
        if (!ValidXLogRecord(state, record, RecPtr))
            goto err;

        state->EndRecPtr = RecPtr + MAXALIGN(total_len);

        state->ReadRecPtr = RecPtr;
        memcpy(state->readRecordBuf, record, total_len);
    }

    /*
     * Special processing if it's an XLOG SWITCH record
     */
    if (record->xl_rmid == RM_XLOG_ID && record->xl_info == XLOG_SWITCH)
    {
        /* Pretend it extends to end of segment */
        state->EndRecPtr += XLogSegSize - 1;
        state->EndRecPtr -= state->EndRecPtr % XLogSegSize;
    }

    return record;

err:

    /*
     * Invalidate the xlog page we've cached. We might read from a different
     * source after failure.
     */
    state->readSegNo = 0;
    state->readOff = 0;
    state->readLen = 0;

    if (state->errormsg_buf[0] != '\0')
        *errormsg = state->errormsg_buf;

    return NULL;
}

/*
 * Read a single xlog page including at least [pageptr, reqLen] of valid data
 * via the read_page() callback.
 *
 * Returns -1 if the required page cannot be read for some reason; errormsg_buf
 * is set in that case (unless the error occurs in the read_page callback).
 *
 * 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);
    targetPageOff = (pageptr % XLogSegSize);

    /* check whether we have all the requested data already */
    if (targetSegNo == state->readSegNo && targetPageOff == state->readOff &&
        reqLen < state->readLen)
        return state->readLen;

    /*
     * Data is not in our buffer.
     *
     * Every time we actually read the page, even if we looked at parts of it
     * before, we need to do verification as the read_page 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->readSegNo && targetPageOff != 0)
    {
        XLogPageHeader hdr;
        XLogRecPtr  targetSegmentPtr = pageptr - targetPageOff;

        readLen = state->read_page(state, targetSegmentPtr, XLOG_BLCKSZ,
                                   state->currRecPtr,
                                   state->readBuf, &state->readPageTLI);
        if (readLen < 0)
            goto err;

        /* we can be sure to have enough WAL available, we scrolled back */
        Assert(readLen == XLOG_BLCKSZ);

        hdr = (XLogPageHeader) state->readBuf;

        if (!ValidXLogPageHeader(state, targetSegmentPtr, hdr))
            goto err;
    }

    /*
     * First, read the requested data length, but at least a short page header
     * so that we can validate it.
     */
    readLen = state->read_page(state, pageptr, Max(reqLen, SizeOfXLogShortPHD),
                               state->currRecPtr,
                               state->readBuf, &state->readPageTLI);
    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->read_page(state, pageptr, XLogPageHeaderSize(hdr),
                                   state->currRecPtr,
                                   state->readBuf, &state->readPageTLI);
        if (readLen < 0)
            goto err;
    }

    /*
     * Now that we know we have the full header, validate it.
     */
    if (!ValidXLogPageHeader(state, pageptr, hdr))
        goto err;

    /* update cache information */
    state->readSegNo = targetSegNo;
    state->readOff = targetPageOff;
    state->readLen = readLen;

    return readLen;

err:
    state->readSegNo = 0;
    state->readOff = 0;
    state->readLen = 0;
    return -1;
}

/*
 * 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)
{
    /*
     * xl_len == 0 is bad data for everything except XLOG SWITCH, where it is
     * required.
     */
    if (record->xl_rmid == RM_XLOG_ID && record->xl_info == XLOG_SWITCH)
    {
        if (record->xl_len != 0)
        {
            report_invalid_record(state,
                                  "invalid xlog switch record at %X/%X",
                                  (uint32) (RecPtr >> 32), (uint32) RecPtr);
            return false;
        }
    }
    else if (record->xl_len == 0)
    {
        report_invalid_record(state,
                              "record with zero length at %X/%X",
                              (uint32) (RecPtr >> 32), (uint32) RecPtr);
        return false;
    }
    if (record->xl_tot_len < SizeOfXLogRecord + record->xl_len ||
        record->xl_tot_len > SizeOfXLogRecord + record->xl_len +
        XLR_MAX_BKP_BLOCKS * (sizeof(BkpBlock) + BLCKSZ))
    {
        report_invalid_record(state,
                              "invalid record length at %X/%X",
                              (uint32) (RecPtr >> 32), (uint32) RecPtr);
        return false;
    }
    if (record->xl_rmid > RM_MAX_ID)
    {
        report_invalid_record(state,
                              "invalid resource manager ID %u at %X/%X",
                              record->xl_rmid, (uint32) (RecPtr >> 32),
                              (uint32) 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",
                                  (uint32) (record->xl_prev >> 32),
                                  (uint32) record->xl_prev,
                                  (uint32) (RecPtr >> 32), (uint32) 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",
                                  (uint32) (record->xl_prev >> 32),
                                  (uint32) record->xl_prev,
                                  (uint32) (RecPtr >> 32), (uint32) 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, so it is safe to fetch xl_len.
 */
static bool
ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr)
{
    pg_crc32    crc;
    int         i;
    uint32      len = record->xl_len;
    BkpBlock    bkpb;
    char       *blk;
    size_t      remaining = record->xl_tot_len;

    /* First the rmgr data */
    if (remaining < SizeOfXLogRecord + len)
    {
        /* ValidXLogRecordHeader() should've caught this already... */
        report_invalid_record(state, "invalid record length at %X/%X",
                              (uint32) (recptr >> 32), (uint32) recptr);
        return false;
    }
    remaining -= SizeOfXLogRecord + len;
    INIT_CRC32(crc);
    COMP_CRC32(crc, XLogRecGetData(record), len);

    /* Add in the backup blocks, if any */
    blk = (char *) XLogRecGetData(record) + len;
    for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
    {
        uint32      blen;

        if (!(record->xl_info & XLR_BKP_BLOCK(i)))
            continue;

        if (remaining < sizeof(BkpBlock))
        {
            report_invalid_record(state,
                              "invalid backup block size in record at %X/%X",
                                  (uint32) (recptr >> 32), (uint32) recptr);
            return false;
        }
        memcpy(&bkpb, blk, sizeof(BkpBlock));

        if (bkpb.hole_offset + bkpb.hole_length > BLCKSZ)
        {
            report_invalid_record(state,
                                  "incorrect hole size in record at %X/%X",
                                  (uint32) (recptr >> 32), (uint32) recptr);
            return false;
        }
        blen = sizeof(BkpBlock) + BLCKSZ - bkpb.hole_length;

        if (remaining < blen)
        {
            report_invalid_record(state,
                              "invalid backup block size in record at %X/%X",
                                  (uint32) (recptr >> 32), (uint32) recptr);
            return false;
        }
        remaining -= blen;
        COMP_CRC32(crc, blk, blen);
        blk += blen;
    }

    /* Check that xl_tot_len agrees with our calculation */
    if (remaining != 0)
    {
        report_invalid_record(state,
                              "incorrect total length in record at %X/%X",
                              (uint32) (recptr >> 32), (uint32) recptr);
        return false;
    }

    /* Finally include the record header */
    COMP_CRC32(crc, (char *) record, offsetof(XLogRecord, xl_crc));
    FIN_CRC32(crc);

    if (!EQ_CRC32(record->xl_crc, crc))
    {
        report_invalid_record(state,
               "incorrect resource manager data checksum in record at %X/%X",
                              (uint32) (recptr >> 32), (uint32) recptr);
        return false;
    }

    return true;
}

/*
 * Validate a page header
 */
static bool
ValidXLogPageHeader(XLogReaderState *state, XLogRecPtr recptr,
                    XLogPageHeader hdr)
{
    XLogRecPtr  recaddr;
    XLogSegNo   segno;
    int32       offset;

    Assert((recptr % XLOG_BLCKSZ) == 0);

    XLByteToSeg(recptr, segno);
    offset = recptr % XLogSegSize;

    XLogSegNoOffsetToRecPtr(segno, offset, recaddr);

    if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
    {
        char        fname[MAXFNAMELEN];

        XLogFileName(fname, state->readPageTLI, segno);

        report_invalid_record(state,
                    "invalid magic number %04X in log segment %s, offset %u",
                              hdr->xlp_magic,
                              fname,
                              offset);
        return false;
    }

    if ((hdr->xlp_info & ~XLP_ALL_FLAGS) != 0)
    {
        char        fname[MAXFNAMELEN];

        XLogFileName(fname, state->readPageTLI, segno);

        report_invalid_record(state,
                       "invalid info bits %04X in log segment %s, offset %u",
                              hdr->xlp_info,
                              fname,
                              offset);
        return false;
    }

    if (hdr->xlp_info & XLP_LONG_HEADER)
    {
        XLogLongPageHeader longhdr = (XLogLongPageHeader) hdr;

        if (state->system_identifier &&
            longhdr->xlp_sysid != state->system_identifier)
        {
            char        fhdrident_str[32];
            char        sysident_str[32];

            /*
             * Format sysids separately to keep platform-dependent format code
             * out of the translatable message string.
             */
            snprintf(fhdrident_str, sizeof(fhdrident_str), UINT64_FORMAT,
                     longhdr->xlp_sysid);
            snprintf(sysident_str, sizeof(sysident_str), UINT64_FORMAT,
                     state->system_identifier);
            report_invalid_record(state,
                                  "WAL file is from different database system: WAL file database system identifier is %s, pg_control database system identifier is %s.",
                                  fhdrident_str, sysident_str);
            return false;
        }
        else if (longhdr->xlp_seg_size != XLogSegSize)
        {
            report_invalid_record(state,
                                  "WAL file is from different database system: Incorrect XLOG_SEG_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->readPageTLI, segno);

        /* hmm, first page of file doesn't have a long header? */
        report_invalid_record(state,
                       "invalid info bits %04X in log segment %s, offset %u",
                              hdr->xlp_info,
                              fname,
                              offset);
        return false;
    }

    if (hdr->xlp_pageaddr != recaddr)
    {
        char        fname[MAXFNAMELEN];

        XLogFileName(fname, state->readPageTLI, segno);

        report_invalid_record(state,
                    "unexpected pageaddr %X/%X in log segment %s, offset %u",
              (uint32) (hdr->xlp_pageaddr >> 32), (uint32) hdr->xlp_pageaddr,
                              fname,
                              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->readPageTLI, segno);

            report_invalid_record(state,
                                  "out-of-sequence timeline ID %u (after %u) in log segment %s, offset %u",
                                  hdr->xlp_tli,
                                  state->latestPageTLI,
                                  fname,
                                  offset);
            return false;
        }
    }
    state->latestPagePtr = recptr;
    state->latestPageTLI = hdr->xlp_tli;

    return true;
}

#ifdef FRONTEND
/*
 * Functions that are currently not needed in the backend, but are better
 * implemented inside xlogreader.c because of the internal facilities available
 * here.
 */

/*
 * Find the first record with at an lsn >= RecPtr.
 *
 * Useful for checking wether RecPtr is a valid xlog address for reading and to
 * find the first valid address after some address when dumping records for
 * debugging purposes.
 */
XLogRecPtr
XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr)
{
    XLogReaderState saved_state = *state;
    XLogRecPtr  targetPagePtr;
    XLogRecPtr  tmpRecPtr;
    int         targetRecOff;
    XLogRecPtr  found = InvalidXLogRecPtr;
    uint32      pageHeaderSize;
    XLogPageHeader header;
    XLogRecord *record;
    int         readLen;
    char       *errormsg;

    Assert(!XLogRecPtrIsInvalid(RecPtr));

    targetRecOff = RecPtr % XLOG_BLCKSZ;

    /* scroll back to page boundary */
    targetPagePtr = RecPtr - 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)
    {
        /* record headers are MAXALIGN'ed */
        tmpRecPtr = targetPagePtr + pageHeaderSize
            + MAXALIGN(header->xlp_rem_len);
    }
    else
    {
        tmpRecPtr = targetPagePtr + pageHeaderSize;
    }

    /*
     * 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.
     */
    while ((record = XLogReadRecord(state, tmpRecPtr, &errormsg)))
    {
        /* continue after the record */
        tmpRecPtr = InvalidXLogRecPtr;

        /* past the record we've found, break out */
        if (RecPtr <= state->ReadRecPtr)
        {
            found = state->ReadRecPtr;
            goto out;
        }
    }

err:
out:
    /* Reset state to what we had before finding the record */
    state->readSegNo = 0;
    state->readOff = 0;
    state->readLen = 0;
    state->ReadRecPtr = saved_state.ReadRecPtr;
    state->EndRecPtr = saved_state.EndRecPtr;

    return found;
}

#endif   /* FRONTEND */