Restore the portal-level snapshot for simple expressions, too.
Commits
84f5c2908 et al missed the need to cover plpgsql's "simple
expression" code path. If the first thing we execute after a
COMMIT/ROLLBACK is one of those, rather than a full-fledged SPI command,
we must explicitly do EnsurePortalSnapshotExists() to make sure we have
an outer snapshot. Note that it wouldn't be good enough to just push a
snapshot for the duration of the expression execution: what comes back
might be toasted, so we'd better have a snapshot protecting it.
The test case demonstrating this fact cheats a bit by marking a SQL
function immutable even though it fetches from a table. That's
nothing that users haven't been seen to do, though.
Per report from Jim Nasby. Back-patch to v11, like the previous fix.
Discussion: https://postgr.es/m/
378885e4-f85f-fc28-6c91-
c4d1c080bf26@amazon.com