Bug in asynchronous Append

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