PostgreSQL启动恢复过程中日志源的切换

日志源会在XLOG_FROM_ARCHIVE->XLOG_FROM_STREAM->XLOG_FROM_ARCHIVE直接切换，只有读取过程中出错，就会切换到另外一个日志源。但实际执行过程中，XLOG_FROM_ARCHIVE出错后会到XLOG_FROM_PG_WAL读取，但是日志源的变量并不会改变。这个需要注意。

static intXLogFileReadAnyTLI(XLogSegNo segno, int emode, int source){         if (expectedTLEs)                tles = expectedTLEs;        else                tles = readTimeLineHistory(recoveryTargetTLI);        /*        1、WaitForWALToBecomeAvailable调用时，source是XLOG_FROM_ARCHIVE时，会使用XLOG_FROM_ANY        2、使用XLOG_FROM_ANY，会首先从归档中读取xlog，如果open失败，则会使用XLOG_FROM_PG_WAL        3、外部日志源变量并没有切换        */        foreach(cell, tles){                TimeLineID      tli = ((TimeLineHistoryEntry *) lfirst(cell))->tli;                if (tli < curFileTLI)                        break;                          /* don't bother looking at too-old TLIs */                if (source == XLOG_FROM_ANY || source == XLOG_FROM_ARCHIVE){                        fd = XLogFileRead(segno, emode, tli,XLOG_FROM_ARCHIVE, true);                        if (fd != -1){                                if (!expectedTLEs)                                        expectedTLEs = tles;                                return fd;                        }                }                if (source == XLOG_FROM_ANY || source == XLOG_FROM_PG_WAL){                        fd = XLogFileRead(segno, emode, tli, XLOG_FROM_PG_WAL, true);                        if (fd != -1){                                if (!expectedTLEs)                                        expectedTLEs = tles;                                return fd;                        }                }        }        return -1;}