Re: Bug in asynchronous Append

Alexander Korotkov писал(а) 2026-07-04 01:00:
> Hi!
>
> ExecReScanAppend() unconditionally resets callback_pending for all
> AsyncRequests. The problem is that postgres_fdw keeps its own
> knowledge for the same fact: PgFdwConnState.pendingAreq – a pointer
> to "pending async request" for a given connection. That connection
> can be shared by several partitions/foreign tables (postgres_fdw
> caches one connection per server+usermapping pair). The blind reset in
> nodeAppend.c only touches the local AsyncRequest.callback_pending; it
> never touches PgFdwConnState.pendingAreq, which correctly points to
> the still-dangling request.
>
> Later, when another partition sharing that same connection gets its
> own ReScan (for instance, its chgParam changed because of the LATERAL
> parameter, and it already has a cursor open), it sends "CLOSE cursor"
> via pgfdw_exec_query(). Before sending any new command on the
> connection, that function first drains whatever request is still
> outstanding on it:
>
> if (state && state->pendingAreq)
> process_pending_request(state->pendingAreq);
>
> And process_pending_request() starts with:
>
> Assert(areq->callback_pending);
>
> – which fails, because the flag was corrupted some rounds earlier.
>
> The attached patch contains both the reproduction case and the fix.
> The fix postpones the reset of the callback_pending flag to
> ExecAppendAsyncBegin(). ExecAppendAsyncBegin() performs this cleanup
> along with ExecReScan(), which completes the async fetch.
>

Hi. The analysis seems correct to me as well as fix.
--
Best regards,
Alexander Pyhalov,
Postgres Professional