Function load_relmap_file

src/backend/utils/cache/relmapper.c:704–786

* load_relmap_file -- load data from the shared or local map file * * Because the map file is essential for access to core system catalogs, * failure to read it is a fatal error. * * Note that the local case requires DatabasePath to be set up. */

 * Note that the local case requires DatabasePath to be set up.
 */
static void
load_relmap_file(bool shared, bool lock_held)
{
	RelMapFile *map;
	char		mapfilename[MAXPGPATH];
	pg_crc32c	crc;
	int			fd;
	int			r;

	if (shared)
	{
		snprintf(mapfilename, sizeof(mapfilename), "global/%s",
				 RELMAPPER_FILENAME);
		map = &shared_map;
	}
	else
	{
		snprintf(mapfilename, sizeof(mapfilename), "%s/%s",
				 DatabasePath, RELMAPPER_FILENAME);
		map = &local_map;
	}

	/* Read data ... */
	fd = OpenTransientFile(mapfilename, O_RDONLY | PG_BINARY);
	if (fd < 0)
		ereport(FATAL,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m",
						mapfilename)));

	/*
	 * Grab the lock to prevent the file from being updated while we read it,
	 * unless the caller is already holding the lock.  If the file is updated
	 * shortly after we look, the sinval signaling mechanism will make us
	 * re-read it before we are able to access any relation that's affected by
	 * the change.
	 */
	if (!lock_held)
		LWLockAcquire(RelationMappingLock, LW_SHARED);

	pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_READ);
	r = read(fd, map, sizeof(RelMapFile));
	if (r != sizeof(RelMapFile))
	{
		if (r < 0)
			ereport(FATAL,
					(errcode_for_file_access(),
					 errmsg("could not read file \"%s\": %m", mapfilename)));
		else
			ereport(FATAL,
					(errcode(ERRCODE_DATA_CORRUPTED),
					 errmsg("could not read file \"%s\": read %d of %zu",
							mapfilename, r, sizeof(RelMapFile))));
	}
	pgstat_report_wait_end();

	if (!lock_held)
		LWLockRelease(RelationMappingLock);