Clarify comment on multixid offset wraparound check

hlinnaka · hlinnaka · commit 7d42e2367c6b · 2025-12-15T11:48:02.000+02:00
Coverity complained that offset cannot be 0 here because there's an explicit check for "offset == 0" earlier in the function, but it didn't see the possibility that offset could've wrapped around to 0. The code is correct, but clarify the comment about it. The same code exists in backbranches in the server GetMultiXactIdMembers() function and in 'master' in the pg_upgrade GetOldMultiXactIdSingleMember function. In backbranches Coverity didn't complain about it because the check was merely an assertion, but change the comment in all supported branches for consistency. Per Tom Lane's suggestion. Discussion: https://www.postgresql.org/message-id/1827755.1765752936@sss.pgh.pa.us
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
@@ -1504,7 +1504,10 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
 
 		if (!TransactionIdIsValid(*xactptr))
 		{
-			/* Corner case 2: we must be looking at unused slot zero */
+			/*
+			 * Corner case 2: offset must have wrapped around to unused slot
+			 * zero.
+			 */
 			Assert(offset == 0);
 			continue;
 		}

Original file line number	Diff line number	Diff line change
`@@ -1504,7 +1504,10 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,`
`1504`	`1504`
`1505`	`1505`	`if (!TransactionIdIsValid(*xactptr))`
`1506`	`1506`	`{`
`1507`		`- /* Corner case 2: we must be looking at unused slot zero */`
	`1507`	`+ /*`
	`1508`	`+ * Corner case 2: offset must have wrapped around to unused slot`
	`1509`	`+ * zero.`
	`1510`	`+ */`
`1508`	`1511`	`Assert(offset == 0);`
`1509`	`1512`	`continue;`
`1510`	`1513`	`}`