本文接上文介绍函数WaitForWALToBecomeAvailable打开WAL日志文件并切换日志源的实现。该函数不涉及关于NVM WAL BUFFER的修改。
1)主机重启恢复时:日志源currentSource = XLOG_FROM_PG_WAL,之后由于第一次进来,调用函数XLogFileReadAnyTLI打开日志所在WAL文件。如果open成功则返回TRUE,否则lastSourceFailed=TRUE,然后跳到1,因为是主机直接返回false
2)备机重启恢复时:日志源currentSource = XLOG_FROM_ARCHIVE,之后由于第一次进来,调用函数XLogFileReadAnyTLI打开日志所在WAL文件。如果open成功则返回TRUE,否则lastSourceFailed=TRUE,然后跳到1,切换日志源:currentSource = XLOG_FROM_STREAM;请求的日志已接收到则返回TRUE,未接收到则sleep一会,重新返回1循环,切到XLOG_FROM_ARCHIVE往次循环
关于函数XLogFileReadAnyTLI的实现参考我之前总结的:https://blog.51cto.com/yanzongshuai/2306988:XLOG_FROM_ARCHIVE出错后还会切到XLOG_FROM_PG_WAL进行open的,只是currentSource 状态机没有改变而已。
static int XLogFileReadAnyTLI(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; }