postgres.git / summary / log / commit / refs

commit    0b3ff539dd341b52b697c535aae4c270052f7e8e
Author:   Robert Haas <rhaas@postgresql.org>
Date:     Mon Aug 29 14:47:12 2022 +0000
First released in: 16.0, 10.23, 11.18, 12.13, 13.9, 14.6, 15.0

    Prevent WAL corruption after a standby promotion.
    
    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


 src/backend/access/transam/xlog.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8e483e226a9..85227654898 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -4260,12 +4260,18 @@ 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;
@@ -7667,6 +7673,14 @@ 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;
 	}


[parent: 2efeb0437cce]