非易失性WAL buffer实现解析（三）

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;
}