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, checkPointLoc, 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, 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 ?
/*
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
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
总结
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