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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Tomas Vondra <tomas@vondra.me>, Thomas Munro <thomas.munro@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2025-03-04T22:58:42Z
Lists: pgsql-hackers 
Hi,

On 2024-12-10 12:00:12 +0200, Heikki Linnakangas wrote:
> On 09/12/2024 22:55, Heikki Linnakangas wrote:
> > Not sure how to fix this. A small sleep in the test would work, but in
> > principle there's no delay that's guaranteed to be enough. A more robust
> > solution would be to run a "select count(*) from pg_stat_activity" and
> > wait until the number of connections are what's expected. I'll try that
> > and see how complicated that gets..
> 
> Checking pg_stat_activity doesn't help, because the backend doesn't register
> itself in pg_stat_activity until later. A connection that's rejected due to
> connection limits never shows up in pg_stat_activity.
> 
> Some options:
> 
> 0. Do nothing
> 
> 1. Add a small sleep to the test
> 
> 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?



> 3. Rearrange the FATAL error handling so that the process removes itself
> from PGPROC before sending the error to the client. That would be kind of
> nice anyway. Currently, if sending the rejection error message to the client
> blocks, you are holding up a PGPROC slot until the message is sent. The
> error message packet is short, so it's highly unlikely to block, but still.

This is definitely a problem, there was even a recent thread about it. It can
be triggered even with just an ERROR message though :(


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()?

Greetings,

Andres Freund

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.