Function XLogFileRead

src/backend/access/transam/xlog.c:3786–3862

* Open a logfile segment for reading (during recovery). * * If source == XLOG_FROM_ARCHIVE, the segment is retrieved from archive. * Otherwise, it's assumed to be already available in pg_wal. */

 * Otherwise, it's assumed to be already available in pg_wal.
 */
static int
XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
			 XLogSource source, bool notfoundOk)
{
	char		xlogfname[MAXFNAMELEN];
	char		activitymsg[MAXFNAMELEN + 16];
	char		path[MAXPGPATH];
	int			fd;

	XLogFileName(xlogfname, tli, segno, wal_segment_size);

	switch (source)
	{
		case XLOG_FROM_ARCHIVE:
			/* Report recovery progress in PS display */
			snprintf(activitymsg, sizeof(activitymsg), "waiting for %s",
					 xlogfname);
			set_ps_display(activitymsg);

			restoredFromArchive = RestoreArchivedFile(path, xlogfname,
													  "RECOVERYXLOG",
													  wal_segment_size,
													  InRedo);
			if (!restoredFromArchive)
				return -1;
			break;

		case XLOG_FROM_PG_WAL:
		case XLOG_FROM_STREAM:
			XLogFilePath(path, tli, segno, wal_segment_size);
			restoredFromArchive = false;
			break;

		default:
			elog(ERROR, "invalid XLogFileRead source %d", source);
	}

	/*
	 * If the segment was fetched from archival storage, replace the existing
	 * xlog segment (if any) with the archival version.
	 */
	if (source == XLOG_FROM_ARCHIVE)
	{
		KeepFileRestoredFromArchive(path, xlogfname);

		/*
		 * Set path to point at the new file in pg_wal.
		 */
		snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlogfname);
	}

	fd = BasicOpenFile(path, O_RDONLY | PG_BINARY);
	if (fd >= 0)
	{
		/* Success! */
		curFileTLI = tli;

		/* Report recovery progress in PS display */