Try to handle torn reads of pg_control in frontend.

macdice · macdice · commit 63a582222c6b · 2023-10-16T17:33:08.000+13:00
Some of our src/bin tools read the control file without any kind of interlocking against concurrent writes from the server. At least ext4 and ntfs can expose partially modified contents when you do that. For now, we'll try to tolerate this by retrying up to 10 times if the checksum doesn't match, until we get two reads in a row with the same bad checksum. This is not guaranteed to reach the right conclusion, but it seems very likely to. Thanks to Tom Lane for this suggestion. Various ideas for interlocking or atomicity were considered too complicated, unportable or expensive given the lack of field reports, but remain open for future reconsideration. Back-patch as far as 12. It doesn't seem like a good idea to put a heuristic change for a very rare problem into the final release of 11. Reviewed-by: Anton A. Melnikov <aamelnikov@inbox.ru> Reviewed-by: David Steele <david@pgmasters.net> Reviewed-by: Michael Paquier <michael@paquier.xyz> Discussion: https://postgr.es/m/20221123014224.xisi44byq3cf5psi%40awork3.anarazel.de
diff --git a/src/common/controldata_utils.c b/src/common/controldata_utils.c
@@ -56,12 +56,22 @@ get_controlfile(const char *DataDir, bool *crc_ok_p)
 	char		ControlFilePath[MAXPGPATH];
 	pg_crc32c	crc;
 	int			r;
+#ifdef FRONTEND
+	pg_crc32c	last_crc;
+	int			retries = 0;
+#endif
 
 	Assert(crc_ok_p);
 
 	ControlFile = palloc_object(ControlFileData);
 	snprintf(ControlFilePath, MAXPGPATH, "%s/global/pg_control", DataDir);
 
+#ifdef FRONTEND
+	INIT_CRC32C(last_crc);
+
+retry:
+#endif
+
 #ifndef FRONTEND
 	if ((fd = OpenTransientFile(ControlFilePath, O_RDONLY | PG_BINARY)) == -1)
 		ereport(ERROR,
@@ -117,6 +127,26 @@ get_controlfile(const char *DataDir, bool *crc_ok_p)
 
 	*crc_ok_p = EQ_CRC32C(crc, ControlFile->crc);
 
+#ifdef FRONTEND
+
+	/*
+	 * If the server was writing at the same time, it is possible that we read
+	 * partially updated contents on some systems.  If the CRC doesn't match,
+	 * retry a limited number of times until we compute the same bad CRC twice
+	 * in a row with a short sleep in between.  Then the failure is unlikely
+	 * to be due to a concurrent write.
+	 */
+	if (!*crc_ok_p &&
+		(retries == 0 || !EQ_CRC32C(crc, last_crc)) &&
+		retries < 10)
+	{
+		retries++;
+		last_crc = crc;
+		pg_usleep(10000);
+		goto retry;
+	}
+#endif
+
 	/* Make sure the control file is valid byte order. */
 	if (ControlFile->pg_control_version % 65536 == 0 &&
 		ControlFile->pg_control_version / 65536 != 0)
