Re: Bug in asynchronous Append

Hi,

On Mon, Aug 3, 2026 at 6:45 PM Alexander Pyhalov
<a(dot)pyhalov(at)postgrespro(dot)ru> wrote:
> I've looked on the suggested patch more attentively in Async Merge
> Append thread[1].
> There it's a bit more obvious that ExecReScanAppend() behavior in
> ExecAppendAsyncProcessPending()
> depends on node->as_syncdone. It either sleeps on latch in
> ExecAppendAsyncEventWait() or busy loops.
> The first behavior seems to be more appropriate, as we still should wait
> for all requests
> with set callback_pending to complete. Or perhaps, it's not a big
> problem, given that this case should
> be rare?

Good catch! Incorporated. Attached is an updated version of the
patch. Other changes are:

* Add CHECK_FOR_INTERRUPTS() to the for loop in ExecAppendAsyncProcessPending.
* Merge the async-state-reset code in ExecReScanAppend into
ExecAppendAsyncProcessPending, for readability, and rename that
function to ExecAppendAsyncReset.
* Remove this bit from postgresReScanForeignScan:

/*
* If the node is async-capable, and an asynchronous fetch for it has
* begun, the asynchronous fetch might not have yet completed. Check if
* the node is async-capable, and an asynchronous fetch for it is still in
* progress; if so, complete the asynchronous fetch before restarting the
* scan.
*/
if (fsstate->async_capable &&
fsstate->conn_state->pendingAreq &&
fsstate->conn_state->pendingAreq->requestee == (PlanState *) node)
fetch_more_data(node);

and instead add an assertion, as it's no longer needed due to the
handling in ExecAppendAsyncReset.

* As a test case causing an issue (infinite loop!) on a
non-assert-enabled build, add (a modified version of) Gleb's test case
as well. (The original test case only causes an assertion failure.)

Best regards,
Etsuro Fujita