1、首先讀取ControlFile->checkPoint指向的checkpoint

2、如果讀取失敗，slave直接abort退出，master再次讀取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;//標記下面進入recovery
   }else{
    ereport(PANIC,(errmsg("could not locate a valid checkpoint record")));
   }
  }

一、那麼什麼條件下讀取的checkpoint記錄record==NULL?

1、ControlFile->checkPoint % XLOG_BLCKSZ < SizeOfXLogShortPHD
2、ReadRecord(xlogreader,ControlFile->checkPoint,LOG,true)返回NULL
3、ReadRecord讀到的record!=NULL && record->xl_rmid != RM_XLOG_ID
4、ReadRecord讀到的record!=NULL && info != XLOG_CHECKPOINT_SHUTDOWN && info != XLOG_CHECKPOINT_ONLINE

5、ReadRecord讀到的record!=NULL && record->xl_tot_len != SizeOfXLogRecord + SizeOfXLogRecordDataHeaderShort + sizeof(CheckPoint)

二、ReadRecord函式返回NULL的條件

ReadRecord(xlogreader,true)
 |--record = XLogReadRecord(xlogreader,&errormsg);
 |-- 2.1 record==NULL && ！StandbyMode
 |-- 2.2 record!=NULL && !tliInHistory(xlogreader->latestPageTLI,expectedTLEs)
 /*-----
 note:只要讀取了一頁xlog,就會賦值為該頁第一個記錄的時間線
 XLogReaderValidatePageHeader
  -->xlogreader->latestPageTLI=hdr->xlp_tli;
 ------*/

三、XlogReadRecord讀取checkpoint返回NULL的條件？

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頭有跨頁的record並且checkpoint定位的偏移正好在頁頭尾部
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)
一旦該記錄損壞，total_len的長度非常大的話，就需要allocate_recordbuf擴充套件state->readbuf，可能因此分配失敗abort
記錄的checksum需要等待全部讀取完整記錄後才校驗
-------------

三、ReadPageInternal返回的readOff返回小於0的條件

ReadPageInternal(state,XLOG_BLCKSZ))

1、第一次read wal檔案，readLen = state->read_page：讀取第一頁。readLen < 0

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

3、讀取checkpoint所在頁readLen = state->read_page: readLen < 0

4、readLen > 0 && readLen <= SizeOfXLogShortPHD

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

四、XLogPageRead何時返回值<0 ?

/*
 1、WaitForWALToBecomeAvailable open失敗
 2、lseek 失敗 && ！StandbyMode
 3、read失敗 && ！StandbyMode
 4、校驗page頭失敗 && ！StandbyMode
 如果是StandbyMode，則會重新retry->WaitForWALToBecomeAvailable,切換日誌源進行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何時返回false？

--XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL
1、先XLogFileReadAnyTLI open日誌：
1、遍歷時間線列表裡的每一個時間線，從最新的開始
2、當讀取checkpoint的時候，source是XLOG_FROM_ANY
3、先找歸檔的日誌進行open；如果open失敗再找WAL日誌進行open
4、如果都沒有open成功，則向前找時間線，open前一個時間線segno和檔案號相同的檔案進行open
5、open成功後expectedTLEs被賦值為當前時間線列表的所有值
2、如果open失敗，則切換日誌源：XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL -> XLOG_FROM_STREAM
3、切換日誌源後，XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL 則：
slave && promote ：return false
!StandbyMode：return false
--XLOG_FROM_STREAM
1、!WalRcvStreaming()即receiver程序掛了，切換日誌源
2、CheckForStandbyTrigger()切換日誌源
3、XLOG_FROM_STREAM->XLOG_FROM_ARCHIVE

總結

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