TransactionXmin != MyProc->xmin

The comment in GetSnapshotData() defines transactionXmin like this:

> TransactionXmin: the oldest xmin of any snapshot in use in the
> current transaction (this is the same as MyProc->xmin).
However, we don't update TransactionXmin when we update MyProc->xmin in
SnapshotResetXmin(). So TransactionXmin can be older than MyProc->xmin.

I browsed around to see if that might cause trouble, and found one such
case: SubTransGetTopmostTransaction() uses TransactionXmin as the
cut-off point so that it doesn't try to perform lookups of old XIDs that
might've already been truncated away from pg_subtrans. When
TransactionXmin is older than MyProc->xmin, then it might do that. The
attached isolation test demonstrates that case and produces an error:

ERROR: could not access status of transaction 17190290
DETAIL: Could not open file "pg_subtrans/0106": No such file or directory.

A straightforward fix is to ensure that TransactionXmin is updated
whenever MyProc->xmin is:

diff --git a/src/backend/utils/time/snapmgr.c
b/src/backend/utils/time/snapmgr.c
index a1a0c2adeb6..f59830abd21 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -883,7 +883,7 @@ SnapshotResetXmin(void)
pairingheap_first(&RegisteredSnapshots));

if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
- MyProc->xmin = minSnapshot->xmin;
+ MyProc->xmin = TransactionXmin = minSnapshot->xmin;
}

/*

Anyone see a reason not to do that?

There are a two other places where we set MyProc->xmin without updating
TransactionXmin:

1. In ProcessStandbyHSFeedbackMessage(). AFAICS that's OK because
walsender doesn't use TransactionXmin for anything.

2. In SnapBuildInitialSnapshot(). I suppose that's also OK because the
TransactionXmin isn't used. I don't quite remember when that function
might be called though.

In any case, I propose that we set TransactionXmin in all of those cases
as well, so that TransactionXmin is always the equal to MyProc->xmin.
Maybe even rename it to MyProcXmin to make that more clear.

--
Heikki Linnakangas
Neon (https://neon.tech)