Re: Possible G2-item at SERIALIZABLE

+ /*
+ * Mark ourselves doomed before raising the error. Otherwise a
+ * subtransaction abort (ROLLBACK TO SAVEPOINT) could swallow this
+ * error and let the transaction commit anyway, defeating SSI.
+ */
+ MySerializableXact->flags |= SXACT_FLAG_DOOMED;
LWLockRelease(SerializableXactHashLock);
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),

I wonder if it would make sense to introduce a
"DoomMyselfAndRaiseSerializationFailure" helper for this, while fixing
all occurrences we can find? That would make it more explicit, and
also less repeated.

> PFA attached isolation tester and hand-wavy fix. But I suspect there are more G-items
> around.

There are at least 2 more reproducible with the isolation tester in
CheckForSerializableConflictOut ("conflict out to old pivot %u", and
"conflict out to old committed transaction %u" directly below it).
Probably "Canceled on conflict out to old pivot." also should have the
same changes?