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”)));
}
}