Re: Using PQexecQuery in pipeline mode produces unexpected Close messages

At Wed, 15 Jun 2022 14:56:42 -0400, Tom Lane <tgl@sss.pgh.pa.us> wrote in 
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> > So, git archaeology led me to this thread
> > https://postgr.es/m/202106072107.d4i55hdscxqj@alvherre.pgsql
> > which is why we added that message in the first place.
> 
> Um.  Good thing you looked.  I doubt we want to revert that change now.
> 
> > Alternatives:
> > - Have the client not complain if it gets CloseComplete in idle state.
> >   (After all, it's a pretty useless message, since we already do nothing
> >   with it if we get it in BUSY state.)
> 
> ISTM the actual problem here is that we're reverting to IDLE state too
> soon.  I didn't try to trace down exactly where that's happening, but

Yes. I once visited that fact but also I thought that in the
comparison with non-pipelined PQsendQuery, the three messages look
extra.  Thus I concluded (at the time) that removing Close is enough
here.

> I notice that in the non-pipeline case we don't go to IDLE till we've
> seen 'Z' (Sync).  Something in the pipeline logic must be jumping the
> gun on that state transition.

PQgetResult() resets the state to IDLE when not in pipeline mode.

fe-exec.c:2171

>			if (conn->pipelineStatus != PQ_PIPELINE_OFF)
>			{
>				/*
>				 * We're about to send the results of the current query.  Set
>				 * us idle now, and ...
>				 */
>				conn->asyncStatus = PGASYNC_IDLE;

And actually that code let the connection state enter to IDLE before
CloseComplete.  In the test case I posted, the following happens.

  PQsendQuery(conn, "SELECT 1;");
  PQsendFlushRequest(conn);
  PQgetResult(conn);      // state enters IDLE, reads down to <CommandComplete>
  PQgetResult(conn);      // reads <CloseComplete comes>
  PQpipelineSync(conn);   // sync too late

Pipeline feature seems intending to allow PQgetResult called before
PQpipelineSync. And also seems allowing to call QPpipelineSync() after
PQgetResult().

I haven't come up with a valid *fix* of this flow..

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center