Prevent WAL corruption after a standby promotion.

robertmhaas · robertmhaas · commit 3c0ef0832748 · 2022-08-29T11:55:49.000-04:00
When a PostgreSQL instance performing archive recovery but not using standby mode is promoted, and the last WAL segment that it attempted to read ended in a partial record, the previous code would create invalid WAL on the new timeline. The WAL from the previously timeline would be copied to the new timeline up until the end of the last valid record, but instead of beginning to write WAL at immediately afterwards, the promoted server would write an overwrite contrecord at the beginning of the next segment. The end of the previous segment would be left as all-zeroes, resulting in failures if anything tried to read WAL from that file. The root of the issue is that ReadRecord() decides whether to set abortedRecPtr and missingContrecPtr based on the value of StandbyMode, but ReadRecord() switches to a new timeline based on the value of ArchiveRecoveryRequested. We shouldn't try to write an overwrite contrecord if we're switching to a new timeline, so change the test in ReadRecod() to check ArchiveRecoveryRequested instead. Code fix by Dilip Kumar. Comments by me incorporating suggested language from Álvaro Herrera. Further review from Kyotaro Horiguchi and Sami Imseih. Discussion: http://postgr.es/m/CAFiTN-t7umki=PK8dT1tcPV=mOUe2vNhHML6b3T7W7qqvvajjg@mail.gmail.com Discussion: http://postgr.es/m/FB0DEA0B-E14E-43A0-811F-C1AE93D00FF3%40amazon.com
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
@@ -4321,12 +4321,18 @@ ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr, int emode,
 		if (record == NULL)
 		{
 			/*
-			 * When not in standby mode we find that WAL ends in an incomplete
-			 * record, keep track of that record.  After recovery is done,
-			 * we'll write a record to indicate downstream WAL readers that
-			 * that portion is to be ignored.
+			 * When we find that WAL ends in an incomplete record, keep track
+			 * of that record.  After recovery is done, we'll write a record to
+			 * indicate to downstream WAL readers that that portion is to be
+			 * ignored.
+			 *
+			 * However, when ArchiveRecoveryRequested = true, we're going to
+			 * switch to a new timeline at the end of recovery. We will only
+			 * copy WAL over to the new timeline up to the end of the last
+			 * complete record, so if we did this, we would later create an
+			 * overwrite contrecord in the wrong place, breaking everything.
 			 */
-			if (!StandbyMode &&
+			if (!ArchiveRecoveryRequested &&
 				!XLogRecPtrIsInvalid(xlogreader->abortedRecPtr))
 			{
 				abortedRecPtr = xlogreader->abortedRecPtr;
@@ -7612,6 +7618,14 @@ StartupXLOG(void)
 	 */
 	if (!XLogRecPtrIsInvalid(missingContrecPtr))
 	{
+		/*
+		 * We should only have a missingContrecPtr if we're not switching to
+		 * a new timeline. When a timeline switch occurs, WAL is copied from
+		 * the old timeline to the new only up to the end of the last complete
+		 * record, so there can't be an incomplete WAL record that we need to
+		 * disregard.
+		 */
+		Assert(ThisTimeLineID == PrevTimeLineID);
 		Assert(!XLogRecPtrIsInvalid(abortedRecPtr));
 		EndOfLog = missingContrecPtr;
 	}
