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

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].

>>
>> If yes, how about a generic message for both of the functions -
>> "signalling postmaster process is not allowed" or "cannot signal
>> postmaster process" or some other better suggestion?
>>
>> [1] 2471176 ---> is postmaster PID.
>> postgres=# select pg_terminate_backend(2471176);
>> WARNING: PID 2471176 is not a PostgreSQL server process
>> pg_terminate_backend
>> ----------------------
>> f
>> (1 row)
>> postgres=# select pg_cancel_backend(2471176);
>> WARNING: PID 2471176 is not a PostgreSQL server process
>> pg_cancel_backend
>> -------------------
>> f
>> (1 row)
>
> 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'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.

[1]
https://www.postgresql.org/message-id/CALDaNm3ZzmFS-%3Dr7oDUzj7y7BgQv%2BN06Kqyft6C3xZDoKnk_6w%40mail.gmail.com

[2]
https://www.postgresql.org/message-id/0271f440ac77f2a4180e0e56ebd944d1%40oss.nttdata.com

Regards,

--
Atsushi Torikoshi
NTT DATA CORPORATION