BUG #19494: Error on transaction commit inside pipeline triggers psql's Assert

On Tue, May 26, 2026 at 07:00:01PM +0000, PG Bug reporting form wrote:

The following psql script:
CREATE TABLE t(a INTEGER PRIMARY KEY DEFERRABLE INITIALLY DEFERRED);
\startpipeline
INSERT INTO t VALUES ($1), ($1) RETURNING * \bind 1 \sendpipeline
\endpipeline

psql: common.c:1503: discardAbortedPipelineResults: Assertion
`pset.available_results > 0' failed.

That's one assertion, and with a bit more imagination for a deferred
constraint, I can trigger a second one for the number of syncs when at
the end of a pipeline:
\startpipeline
INSERT INTO pipeline_defer_tab VALUES ($1), ($1) \bind 1 \sendpipeline
\syncpipeline
SELECT 1 \bind \sendpipeline
\endpipeline

#5 0x000000000041bb0a in ExecQueryAndProcessResults (query=0x5db560
"SELECT 1 ", elapsed_msec=0x7fffffffc660, svpt_gone_p=0x7fffffffc65f,
is_watch=false, min_rows=0, opt=0x0, printQueryFout=0x0) at
common.c:2190
2190 Assert(pset.piped_syncs == 0);
(gdb) p pset.piped_syncs
$1 = 1

I am completely sure yet, but it looks like we will need to be smarter
with the handling of the number of piped commands by tracking them
across the syncs in the shape of a queue, or something like that? So
it feels like we need to think harder about the tracking of this
activity depending on the state of the pipeline we're in. Or we could
lift some of these assertions, but that would not be right to me.
--
Michael

On Thu, May 28, 2026 at 12:51:38PM +0900, Michael Paquier wrote:

I am completely sure yet, but it looks like we will need to be smarter
with the handling of the number of piped commands by tracking them
across the syncs in the shape of a queue, or something like that? So
it feels like we need to think harder about the tracking of this
activity depending on the state of the pipeline we're in. Or we could
lift some of these assertions, but that would not be right to me.

Hmm. Taking a step back this would be overcomplicating things. As
long as we are careful to consume the synced results still in a
pipeline, it looks like we should be fine. While digging into it, I
have found a third assertion that was triggerable with
available_results at the end of the pipeline, once I began mixing
\getresults with a deferred error.

This stuff is tricky enough that I may not have overseen all the
patterns possible, of course, at least this is progress.

Alexander, what do you think?
--
Michael

Hello Michael,

28.05.2026 08:26, Michael Paquier wrote:

On Thu, May 28, 2026 at 12:51:38PM +0900, Michael Paquier wrote:

I am completely sure yet, but it looks like we will need to be smarter
with the handling of the number of piped commands by tracking them
across the syncs in the shape of a queue, or something like that? So
it feels like we need to think harder about the tracking of this
activity depending on the state of the pipeline we're in. Or we could
lift some of these assertions, but that would not be right to me.

Hmm. Taking a step back this would be overcomplicating things. As
long as we are careful to consume the synced results still in a
pipeline, it looks like we should be fine. While digging into it, I
have found a third assertion that was triggerable with
available_results at the end of the pipeline, once I began mixing
\getresults with a deferred error.

This stuff is tricky enough that I may not have overseen all the
patterns possible, of course, at least this is progress.

Alexander, what do you think?

While testing the patch, I've observed apparently new anomaly. psql got
stuck inside this loop:
    if (end_pipeline)
    {
        /*
         * Reset available/requested results.  Normally these are already 0,
         * but an error generated by Sync processing itself can leave some of
         * them behind.  Consume them before exiting pipeline mode.
         */
        while (pset.piped_syncs > 0)
        {
            PGresult   *remaining = PQgetResult(pset.db);

            if (remaining == NULL)
                continue;
...
        }

it's happening upon/after postgres process termination, so PQgetResult()
returns NULL, pset.piped_syncs == 1. I need more time to look deeper and
to come with a reproducer, but maybe you can already see what's wrong.

Best regards,
Alexander

On Fri, May 29, 2026 at 07:00:01AM +0300, Alexander Lakhin wrote:

it's happening upon/after postgres process termination, so PQgetResult()
returns NULL, pset.piped_syncs == 1. I need more time to look deeper and
to come with a reproducer, but maybe you can already see what's wrong.

Yeah, I do. Nice catch. See this sequence to reproduce the problem:
\startpipeline
INSERT INTO psql_pipeline_defer VALUES (1), (1) \bind \sendpipeline
\syncpipeline
SELECT pg_terminate_backend(pg_backend_pid()) \bind \sendpipeline
SELECT 1 \bind \sendpipeline
\endpipeline

When ending the pipeline the loop consuming the results is stuck, so
we could check the connection state. We are going to enter in a
freeze of the branches due to beta1 next week, so let's take our time.

Please feel to use the v2 attached for your tests. I am also testing
it more on my side.
--
Michael