Re: Suggestion to add --continue-client-on-abort option to pgbench

On Thu, 25 Sep 2025 02:19:27 +0900
Fujii Masao <masao.fujii@gmail.com> wrote:

> On Tue, Sep 23, 2025 at 11:58 AM Rintaro Ikeda
> <ikedarintarof@oss.nttdata.com> wrote:
> > I think the issue is a new bug because we have transitioned to CSTATE_ABORT
> > immediately after queries failed, without executing discardUntilSync().

Agreed. 
 
> If so, the fix in discardUntilSync() should be part of the continue-on-error
> patch. The assertion failure fix should be a separate patch, since only
> that needs to be backpatched (the failure can also occur in back branches).

+1

> 
> > I've attached a patch that fixes the assertion error. The content of v1 patch by
> > Mr. Nagata is also included. I would appreciate it if you review my patch.

> + switch (PQresultStatus(res))
> + {
> + case PGRES_PIPELINE_SYNC:
> + received_sync = true;
> 
> In the PGRES_PIPELINE_SYNC case, PQclear(res) isn't called but should be.
> 
> + case PGRES_FATAL_ERROR:
> + PQclear(res);
> + goto done;
> 
> I don't think we need goto here. How about this instead?
> 
> -----------------------
> @@ -3525,11 +3525,18 @@ discardUntilSync(CState *st)
>                          * results have been discarded.
>                          */
>                         st->num_syncs = 0;
> -                       PQclear(res);
>                         break;
>                 }
> +               /*
> +                * Stop receiving further results if PGRES_FATAL_ERROR
> is returned
> +                * (e.g., due to a connection failure) before
> PGRES_PIPELINE_SYNC,
> +                * since PGRES_PIPELINE_SYNC will never arrive.
> +                */
> +               else if (PQresultStatus(res) == PGRES_FATAL_ERROR)
> +                       break;
>                 PQclear(res);
>         }
> +       PQclear(res);
> 
>         /* exit pipeline */
>         if (PQexitPipelineMode(st->con) != 1)
> -----------------------

I think Fujii-san's version is better because Ikeda-san's version doesn't
consider the case where PGRES_PIPELINE_SYNC is followed by another one.
In that situation, the loop would terminate without getting NULL, which would
causes an error in PQexitPipelineMode().

However, I would like to suggest an alternative solution: checking the connection
status when readCommandResponse() returns false. This seems more straightforwad,
since the cause of the error can be investigated immediately after it is detected.

@@ -4024,7 +4043,10 @@ advanceConnectionState(TState *thread, CState *st, StatsData *agg)
                                        if (PQpipelineStatus(st->con) != PQ_PIPELINE_ON)
                                                st->state = CSTATE_END_COMMAND;
                                }
-                               else if (canRetryError(st->estatus))
+                               else if (PQstatus(st->con) == CONNECTION_BAD)
+                                       st->state = CSTATE_ABORTED;
+                               else if ((st->estatus == ESTATUS_OTHER_SQL_ERROR && continue_on_error) ||
+                                                canRetryError(st->estatus))
                                        st->state = CSTATE_ERROR;
                                else
                                        st->state = CSTATE_ABORTED;

What do you think?


Additionally, I noticed that in pipeline mode, the error message reported in
readCommandResponse() is lost, because it is reset when PQgetResult() returned
NULL to indicate the end of query processing. For example:

 pgbench: client 0 got an error in command 3 (SQL) of script 0; 
 pgbench: client 1 got an error in command 3 (SQL) of script 0; 

This can be fixed this by saving the previous error message and using it
for the report. After the fix:

 pgbench: client 0 got an error in command 3 (SQL) of script 0; FATAL:  terminating connection due to administrator command

I've attached update patches.

0001 fixes the assersion failure in commandError() and error message lost
in readCommandResponse().

0002 was the previous 0001 that adds --continiue-on-error, including the
fix to handle connection termination errors.

0003 is for improving error messages for errors that cause client abortion.

Regareds,
Yugo Nagata



-- 
Yugo Nagata <nagata@sraoss.co.jp>