1、首先读取ControlFile->checkPoint指向的checkpoint
2、如果读取失败，slave直接abort退出，master再次读取ControlFile->prevCheckPoint指向的checkpoint
StartupXLOG->
    |--checkPointLoc = ControlFile->checkPoint;
    |--record = ReadCheckpointRecord(xlogreader,checkPointLoc,1,true):
    |-- if (record != NULL){
           ...
        }else if (StandbyMode){
            ereport(PANIC,(errmsg("could not locate a valid checkpoint record")));
        }else{
            checkPointLoc = ControlFile->prevCheckPoint;
            record = ReadCheckpointRecord(xlogreader,2,true);
            if (record != NULL){
                InRecovery = true;//标记下面进入recovery
            }else{
                ereport(PANIC,(errmsg("could not locate a valid checkpoint record")));
            }
        }

一、那么什么条件下读取的checkpoint记录record==NULL?

1、ControlFile->checkPoint % XLOG_BLCKSZ < SizeOfXLogShortPHD
2、ReadRecord(xlogreader,ControlFile->checkPoint,LOG,true)返回NULL
3、ReadRecord读到的record!=NULL && record->xl_rmid != RM_XLOG_ID
4、ReadRecord读到的record!=NULL && info != XLOG_CHECKPOINT_SHUTDOWN && info != XLOG_CHECKPOINT_ONLINE
5、ReadRecord读到的record!=NULL && record->xl_tot_len != SizeOfXLogRecord + SizeOfXLogRecordDataHeaderShort + sizeof(CheckPoint)

二、ReadRecord函数返回NULL的条件

ReadRecord(xlogreader,true)
    |--record = XLogReadRecord(xlogreader,&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,&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,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,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 ?

/*
    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,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,readBuf)
    ！StandbyMode:: return -1

五、WaitForWALToBecomeAvailable何时返回false？

--XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL
    1、先XLogFileReadAnyTLI open日志：
        1、遍历时间线列表里的每一个时间线，从最新的开始
        2、当读取checkpoint的时候，source是XLOG_FROM_ANY
        3、先找归档的日志进行open；如果open失败再找WAL日志进行open
        4、如果都没有open成功，则向前找时间线，open前一个时间线segno和文件号相同的文件进行open
        5、open成功后expectedTLEs被赋值为当前时间线列表的所有值
    2、如果open失败，则切换日志源：XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL -> XLOG_FROM_STREAM
    3、切换日志源后，XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL 则：
       slave && promote ：return false
       !StandbyMode：return false
    --XLOG_FROM_STREAM
    1、!WalRcvStreaming()即receiver进程挂了，切换日志源
    2、CheckForStandbyTrigger()切换日志源
    3、XLOG_FROM_STREAM->XLOG_FROM_ARCHIVE