c7edaeec50
When ExecBRUpdateTriggers switches to a new target tuple as a result of the EvalPlanQual logic, it must form a new proposed update tuple. Since commit 86dc90056, that tuple (the result of ExecGetUpdateNewTuple) has been a virtual tuple that might contain pointers to by-ref fields of the new target tuple (in "oldslot"). However, immediately after that we materialize oldslot, causing it to drop its buffer pin, whereupon the by-ref pointers are unsafe to use. This is a live bug only when the new target tuple is in a different page than the original target tuple, since we do still hold a pin on the original one. (Before 86dc90056, there was no bug because the EPQ plantree would hold a pin on the new target tuple; but now that's not assured.) To fix, forcibly materialize the new tuple before we materialize oldslot. This costs nothing since we would have done that shortly anyway. The real-world impact of this is probably minimal. A visible failure could occur if the new target tuple's buffer were recycled for some other page in the short interval before we materialize newslot within the trigger-calling loop; but that's quite unlikely given that we'd just touched that page. There's a larger hazard that some other process could prune and repack that page within the window. We have lock on the new target tuple, but that wouldn't prevent it being moved on the page. Alexander Lakhin and Tom Lane, per bug #17798 from Alexander Lakhin. Back-patch to v14 where 86dc90056 came in. Discussion: https://postgr.es/m/17798-0907404928dcf0dd@postgresql.org
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