Function AddToDataDirLockFile

src/backend/utils/init/miscinit.c:1447–1560

* Add (or replace) a line in the data directory lock file. * The given string should not include a trailing newline. * * Note: because we don't truncate the file, if we were to rewrite a line * with less data than it had before, there would be garbage after the last * line. While we could fix that by adding a truncate call, that would make * the file update non-atomic, which we'd rather avo

 * should endeavor never to shorten a line once it's been written.
 */
void
AddToDataDirLockFile(int target_line, const char *str)
{
	int			fd;
	int			len;
	int			lineno;
	char	   *srcptr;
	char	   *destptr;
	char		srcbuffer[BLCKSZ];
	char		destbuffer[BLCKSZ];

	fd = open(DIRECTORY_LOCK_FILE, O_RDWR | PG_BINARY, 0);
	if (fd < 0)
	{
		ereport(LOG,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m",
						DIRECTORY_LOCK_FILE)));
		return;
	}
	pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_ADDTODATADIR_READ);
	len = read(fd, srcbuffer, sizeof(srcbuffer) - 1);
	pgstat_report_wait_end();
	if (len < 0)
	{
		ereport(LOG,
				(errcode_for_file_access(),
				 errmsg("could not read from file \"%s\": %m",
						DIRECTORY_LOCK_FILE)));
		close(fd);
		return;
	}
	srcbuffer[len] = '\0';

	/*
	 * Advance over lines we are not supposed to rewrite, then copy them to
	 * destbuffer.
	 */
	srcptr = srcbuffer;
	for (lineno = 1; lineno < target_line; lineno++)
	{
		char	   *eol = strchr(srcptr, '\n');

		if (eol == NULL)
			break;				/* not enough lines in file yet */
		srcptr = eol + 1;
	}
	memcpy(destbuffer, srcbuffer, srcptr - srcbuffer);
	destptr = destbuffer + (srcptr - srcbuffer);

	/*
	 * Fill in any missing lines before the target line, in case lines are
	 * added to the file out of order.
	 */
	for (; lineno < target_line; lineno++)
	{
		if (destptr < destbuffer + sizeof(destbuffer))
			*destptr++ = '\n';