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.
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Regards,
Alexander Korotkov
Supabase
