Re: BUG #17893: Assert failed in heap_update()/_delete() when FK modiified by RI trigger in non-read-committed xact

Hello Heikki,

27.11.2023 22:23, Heikki Linnakangas wrote:
> I propose to attached slight variation, which moves the assertions before the crosscheck snapshot check. The
> assertions are correct as they are, if you don't need to take the crosscheck into account.
>

Thank you for paying attention to this!
I agree with your variation, it's more accurate.

> I'm a little worried if the callers of tuple_update() might also get confused when tuple_update() returns TM_Updated
> but xmax is invalid. As you said:
> ...
> I think you only get that error with REPEATABLE READ or SERIALIZABLE isolation level. I don't quite understand how
> this works. In READ COMMITTED level, ISTM that ExecUpdate() or ExecDelete() will proceed with EvalPlanQualSlot() and
> locking the row with table_tuple_lock(). Or do we never use the cross-check snapshot in READ COMMITTED mode?
>

I think so, as I see the following in ri_PerformCheck():
    /*
     * In READ COMMITTED mode, we just need to use an up-to-date regular
     * snapshot, and we will see all rows that could be interesting. But in
     * transaction-snapshot mode, we can't change the transaction snapshot. If
     * the caller passes detectNewRows == false then it's okay to do the query
     * with the transaction snapshot; otherwise we use a current snapshot, and
     * tell the executor to error out if it finds any rows under the current
     * snapshot that wouldn't be visible per the transaction snapshot.  Note
     * that SPI_execute_snapshot will register the snapshots, so we don't need
     * to bother here.
     */
    if (IsolationUsesXactSnapshot() && detectNewRows)
    {
        CommandCounterIncrement();  /* be sure all my own work is visible */
        test_snapshot = GetLatestSnapshot();
        crosscheck_snapshot = GetTransactionSnapshot();
    }

IIUC, that's the only place where crosscheck_snapshot (and then
es_crosscheck_snapshot) may get a non-invalid value.

Best regards,
Alexander