Re: Should we improve "PID XXXX is not a PostgreSQL server process" warning for pg_terminate_backend(<<postmaster_pid>>)?

On Mon, Mar 15, 2021 at 11:23 AM torikoshia <torikoshia(at)oss(dot)nttdata(dot)com> wrote:
>
> On 2021-03-07 19:16, Bharath Rupireddy wrote:
> > On Fri, Feb 5, 2021 at 5:15 PM Bharath Rupireddy
> > <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
> >>
> >> pg_terminate_backend and pg_cancel_backend with postmaster PID produce
> >> "PID XXXX is not a PostgresSQL server process" warning [1], which
> >> basically implies that the postmaster is not a PostgreSQL process at
> >> all. This is a bit misleading because the postmaster is the parent of
> >> all PostgreSQL processes. Should we improve the warning message if the
> >> given PID is postmasters' PID?
>
> +1. I felt it was a bit confusing when reviewing a thread[1].

Hmmm.

> > I'm attaching a small patch that emits a warning "signalling
> > postmaster with PID %d is not allowed" for postmaster and "signalling
> > PostgreSQL server process with PID %d is not allowed" for auxiliary
> > processes such as checkpointer, background writer, walwriter.
> >
> > However, for stats collector and sys logger processes, we still get
> > "PID XXXXX is not a PostgreSQL server process" warning because they
> > don't have PGPROC entries(??). So BackendPidGetProc and
> > AuxiliaryPidGetProc will not help and even pg_stat_activity is not
> > having these processes' pid.
>
> I also ran into the same problem while creating a patch in [2].

I have not gone through that thread though. Is there any way we can
detect those child processes(stats collector, sys logger) that are
forked by the postmaster from a backend process? Thoughts?

> I'm now wondering if changing the message to something like
> "PID XXXX is not a PostgreSQL backend process".
>
> "backend process' is now defined as "Process of an instance
> which acts on behalf of a client session and handles its
> requests." in Appendix.

Yeah, that looks good to me. IIUC, we can just change the message from
"PID XXXX is not a PostgreSQL server process" to "PID XXXX is not a
PostgreSQL backend process" and we don't need look for AuxiliaryProcs
or PostmasterPid.

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com