Actually ... wait a minute. The proposed hack covers the case of
SELECT FOR SHARE followed by SELECT FOR UPDATE within a subtransaction.
But what about SELECT FOR SHARE followed by an actual UPDATE (or DELETE)?
We certainly don't want to mark the UPDATE/DELETE as having been carried
out by the upper transaction, but there's no way we can record the
UPDATE while still remembering the previous share-lock.
So I think I'm back to the position that we should throw an error here.
regards, tom lane
