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Fix incorrect logic in HaveRegisteredOrActiveSnapshot().
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
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‎src/backend/utils/time/snapmgr.c

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1646,11 +1646,11 @@ HaveRegisteredOrActiveSnapshot(void)
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* removed at any time due to invalidation processing. If explicitly
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* registered more than one snapshot has to be in RegisteredSnapshots.
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*/
1649-
if (pairingheap_is_empty(&RegisteredSnapshots) ||
1650-
!pairingheap_is_singular(&RegisteredSnapshots))
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if (CatalogSnapshot != NULL &&
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pairingheap_is_singular(&RegisteredSnapshots))
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return false;
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1653-
return CatalogSnapshot == NULL;
1653+
return !pairingheap_is_empty(&RegisteredSnapshots);
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}
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