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Re: Refactoring postmaster's code to cleanup after child exit

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Michael Paquier <michael@paquier.xyz>, Andres Freund <andres@anarazel.de>
Cc: Tomas Vondra <tomas@vondra.me>, Thomas Munro <thomas.munro@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2025-03-06T20:18:10Z
Lists: pgsql-hackers 
On 05/03/2025 01:23, Michael Paquier wrote:
> On Tue, Mar 04, 2025 at 05:58:42PM -0500, Andres Freund wrote:
>> On 2024-12-10 12:00:12 +0200, Heikki Linnakangas wrote:
>>> 2. Move the pgstat_bestart() call earlier in the startup sequence, so that a
>>> backend shows up in pg_stat_activity before it acquires a PGPROC entry, and
>>> stays visible until after it has released its PGPROC entry. This would give
>>> more visibility to backends that are starting up.
>>
>> We don't necessarily *have* a PGPROC entry for that backend when we run out of
>> connections, no?
> 
> Exactly.  If I got this thread's argument right, you cannot have a
> PGPROC entry that could be plugged into pg_stat_activity that early
> during the startup process when collecting the startup packet.

That's true in general; once you start running out of connections, you 
can indeed run out PGPROC slots too. In this particular case, though, 
there were still PGPROC slots available, reserved for superuser 
connections, so it would've helped.

We could also have more pg_stat_activity slots than PGPROC slots, or 
just have a few more PGPROC slots than what is required by MaxBackends.

>> For this test, could we perhaps rely on the log messages postmaster logs when
>> child processes exit?
>>
>> 2025-03-04 17:56:12.528 EST [3509838][not initialized][:0][[unknown]] LOG:  connection received: host=[local]
>> 2025-03-04 17:56:12.528 EST [3509838][client backend][:0][[unknown]] FATAL:  sorry, too many clients already
>> 2025-03-04 17:56:12.529 EST [3509817][postmaster][:0][] DEBUG:  releasing pm child slot 2
>> 2025-03-04 17:56:12.529 EST [3509817][postmaster][:0][] DEBUG:  client backend (PID 3509838) exited with exit code 1
>>
>> I.e. the test could wait for the 'client backend exited' message using
>> ->wait_for_log()?
> 
> Matching expected contents in the server logs is a practice I've found
> to be rather reliable, with wait_for_log().  Why not adding an
> injection point with a WARNING or a LOG generated, then check the
> server logs for the code path taken based on the elog() generated with
> the point name?

Hmm, yeah, watching for "releasing pm child slot" or an explicit 
injection point would work.

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix test name and username used in failed connection attempts

  2. tests: Don't fail due to high default timeout in postmaster/003_start_stop

  3. tests: Fix race condition in postmaster/002_connection_limits

  4. Pass MyPMChildSlot as an explicit argument to child process

  5. Assign a child slot to every postmaster child process

  6. Kill dead-end children when there's nothing else left

  7. Replace postmaster.c's own backend type codes with BackendType

  8. Use an shmem_exit callback to remove backend from PMChildFlags on exit

  9. Add test for dead-end backends

  10. Add test for connection limits

  11. Fix garbled process name on backend crash

  12. Consolidate postmaster code to launch background processes

  13. Fix comment on processes being kept over a restart

  14. Refactor code to handle death of a backend or bgworker in postmaster

  15. Make BackgroundWorkerList doubly-linked

  16. Minor refactoring of assign_backendlist_entry()

  17. Fix outdated comment; all running bgworkers are in BackendList

  18. Allow building with MSVC and Strawberry perl

  19. Fix rare failure in LDAP tests.

  20. Allow notifications to bgworkers without database connections.