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Fix incorrect logic in HaveRegisteredOrActiveSnapshot().

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This function gave the wrong answer when there's more than one
RegisteredSnapshots entry, whether or not any of them is the
CatalogSnapshot.  This leads to assertion failure in some scenarios
involving fetching toasted data using a cursor.  (As per discussion,
I'm dubious that this is the right contract to be enforcing at all;
but it surely doesn't help to be enforcing it incorrectly.)

Fetching toasted data using a cursor is evidently under-tested,
so add a test case too.

Per report from Erik Rijkers.  This is new code, so no need for
back-patch.

Discussion: https://postgr.es/m/dc9dd229-ed30-6c62-4c41-d733ffff776b@xs4all.nl
This commit is contained in:
Tom Lane
2022-04-16 16:04:50 -04:00
parent a17fd67d2f
commit 9f4f0a0dad
3 changed files with 52 additions and 3 deletions
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6
src/backend/utils/time/snapmgr.c
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@@ -1646,11 +1646,11 @@ HaveRegisteredOrActiveSnapshot(void)
* removed at any time due to invalidation processing. If explicitly
* registered more than one snapshot has to be in RegisteredSnapshots.
*/
if (pairingheap_is_empty(&RegisteredSnapshots) ||
!pairingheap_is_singular(&RegisteredSnapshots))
if (CatalogSnapshot != NULL &&
pairingheap_is_singular(&RegisteredSnapshots))
return false;
return CatalogSnapshot == NULL;
return !pairingheap_is_empty(&RegisteredSnapshots);
}