ReadRecord(xlogreader, ControlFile->checkPoint, LOG, true)
 |--record = XLogReadRecord(xlogreader, ControlFile->checkPoint, &errormsg);
 |-- 2.1 record==NULL && ！StandbyMode
 |-- 2.2 record!=NULL && !tliInHistory(xlogreader->latestPageTLI, expectedTLEs)
 /*-----
 note:只要读取了一页xlog,就会赋值为该页第一个记录的时间线
 XLogReaderValidatePageHeader
  -->xlogreader->latestPageTLI=hdr->xlp_tli;
 ------*/

三、XlogReadRecord读取checkpoint返回NULL的条件？

XLogReadRecord(xlogreader, ControlFile->checkPoint, &errormsg)
    targetPagePtr = ControlFile->checkPoint - (ControlFile->checkPoint % XLOG_BLCKSZ);
    targetRecOff = ControlFile->checkPoint % XLOG_BLCKSZ;
    readOff = ReadPageInternal(state,targetPagePtr, Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
    pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
    record = (XLogRecord *) (state->readBuf + RecPtr % XLOG_BLCKSZ);
    total_len = record->xl_tot_len;
    -------------
    1、readOff < 0
    2、0< targetRecOff < pageHeaderSize
    3、(((XLogPageHeader) state->readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) && targetRecOff == pageHeaderSize
       page头有跨页的record并且checkpoint定位的偏移正好在页头尾部
    4、targetRecOff <= XLOG_BLCKSZ - SizeOfXLogRecord &&
       !ValidXLogRecordHeader(state, ControlFile->checkPoint, state->ReadRecPtr, record,randAccess)
       ---（record->xl_tot_len < SizeOfXLogRecord || record->xl_rmid > RM_MAX_ID || record->xl_prev != state->ReadRecPtr）
    5、targetRecOff > XLOG_BLCKSZ - SizeOfXLogRecord && total_len < SizeOfXLogRecord
    6、total_len > state->readRecordBufSize && !allocate_recordbuf(state, total_len)
       一旦该记录损坏，total_len的长度非常大的话，就需要allocate_recordbuf扩展state->readbuf，可能因此分配失败abort
       记录的checksum需要等待全部读取完整记录后才校验
    -------------

三、ReadPageInternal返回的readOff返回小于0的条件

ReadPageInternal(state,targetPagePtr, Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ))

    1、第一次read wal文件，readLen = state->read_page：读取第一页。readLen < 0

    2、readLen>0 && !XLogReaderValidatePageHeader(state, targetSegmentPtr, state->readBuf)
    --

    3、读取checkpoint所在页readLen = state->read_page: readLen < 0

    4、readLen > 0 && readLen <= SizeOfXLogShortPHD

    5、!XLogReaderValidatePageHeader(state, pageptr, (char *) hdr)

四、XLogPageRead何时返回值<0 "htmlcode">

/*
 1、WaitForWALToBecomeAvailable open失败
 2、lseek 失败 && ！StandbyMode
 3、read失败 && ！StandbyMode
 4、校验page头失败 && ！StandbyMode
 如果是StandbyMode，则会重新retry->WaitForWALToBecomeAvailable,切换日志源进行open
 */
 !WaitForWALToBecomeAvailable(targetPagePtr + reqLen,private->randAccess,1,targetRecPtr)//open
 |-- return -1
 readOff = targetPageOff;
 if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0){
  ！StandbyMode:: return -1
 }
 if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ){
  ！StandbyMode:: return -1
 }
 XLogReaderValidatePageHeader(xlogreader, targetPagePtr, readBuf)
 ！StandbyMode:: return -1