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commit    fcc0e27f45e2d8dc6046435908a48626c91d3d84
Author:   Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:     Wed Sep 16 11:52:12 2026 +0000

    Fix multixact members truncation at page boundary
    
    Like in PerformOffsetsTruncation() and e.g. in TruncateSUBTRANS(),
    need to step back to previous page at page boundary, to avoid passing
    a cutoff page to SimpleLruTruncate() that hasn't been created
    yet. Otherwise you can get a bogus 'LOG: could not truncate directory
    "pg_multixact/members": apparent wraparound' message in the log.
    
    Reported-by: Noah Misch <noah@leadboat.com>
    Discussion: https://www.postgresql.org/message-id/20260827231757.78.noahmisch@microsoft.com
    Backpatch-through: 19


 src/backend/access/transam/multixact.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index d688815083c..70a4ea69486 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -2642,8 +2642,16 @@ MultiXactMemberFreezeThreshold(void)
 static void
 PerformMembersTruncation(MultiXactOffset newOldestOffset)
 {
+	/*
+	 * We step back one entry to avoid passing a cutoff page that hasn't been
+	 * created yet in the rare case that oldestOffset would be the first item
+	 * on a page and oldestOffset == nextOffset.  In that case, if we didn't
+	 * subtract one, we'd trigger SimpleLruTruncate's wraparound detection.
+	 */
+	if (newOldestOffset <= 1)
+		return;
 	SimpleLruTruncate(MultiXactMemberCtl,
-					  MXOffsetToMemberPage(newOldestOffset));
+					  MXOffsetToMemberPage(newOldestOffset - 1));
 }
 
 /*
@@ -2653,11 +2661,8 @@ static void
 PerformOffsetsTruncation(MultiXactId newOldestMulti)
 {
 	/*
-	 * We step back one multixact to avoid passing a cutoff page that hasn't
-	 * been created yet in the rare case that oldestMulti would be the first
-	 * item on a page and oldestMulti == nextMulti.  In that case, if we
-	 * didn't subtract one, we'd trigger SimpleLruTruncate's wraparound
-	 * detection.
+	 * Like in PerformMembersTruncation(), step back one entry to avoid
+	 * passing a cutoff page that hasn't been created yet.
 	 */
 	SimpleLruTruncate(MultiXactOffsetCtl,
 					  MultiXactIdToOffsetPage(PreviousMultiXactId(newOldestMulti)));