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commit    4d689a17693ed65461b2f3a02c24c98f30d930d0
Author:   Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:     Fri Dec 05 09:32:38 2025 +0000
First released in: 14.21, 15.16, 16.12, 17.8, 18.2

    Fix setting next multixid's offset at offset wraparound
    
    In commit 789d65364c, we started updating the next multixid's offset
    too when recording a multixid, so that it can always be used to
    calculate the number of members. I got it wrong at offset wraparound:
    we need to skip over offset 0. Fix that.
    
    Discussion: https://www.postgresql.org/message-id/d9996478-389a-4340-8735-bfad456b313c@iki.fi
    Backpatch-through: 14


 src/backend/access/transam/multixact.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 6ce5202a078..a0ffa513e94 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -876,6 +876,7 @@ RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
 	int			next_pageno;
 	int			next_entryno;
 	MultiXactOffset *next_offptr;
+	MultiXactOffset next_offset;
 
 	LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
 
@@ -962,11 +963,15 @@ RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
 		next_offptr += next_entryno;
 	}
 
-	if (*next_offptr != offset + nmembers)
+	/* Like in GetNewMultiXactId(), skip over offset 0 */
+	next_offset = offset + nmembers;
+	if (next_offset == 0)
+		next_offset = 1;
+	if (*next_offptr != next_offset)
 	{
 		/* should already be set to the correct value, or not at all */
 		Assert(*next_offptr == 0);
-		*next_offptr = offset + nmembers;
+		*next_offptr = next_offset;
 		MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
 	}
 


[parent: b38feca1ce00]