Clarify comment on multixid offset wraparound check

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
index a0ffa513e94efdd067e75b3f6f6b56b608fb875e..8385abb0a4dbe0bf297446778dc4fe7287599383 100644 (file)
--- 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;
        }