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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: Tomas Vondra <tomas@vondra.me>, Noah Misch <noah@leadboat.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Thomas Munro <thomas.munro@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2025-03-04T22:50:34Z
Lists: pgsql-hackers 
Hi,

On 2024-12-09 00:12:32 +0100, Tomas Vondra wrote:
> Hi, the TAP test 001_connection_limits.pl introduced by 6a1d0d470e84
> seems to have problems with valgrind :-( I reliably get this failure:
> 
> 
> t/001_connection_limits.pl .. 3/? # Tests were run but no plan was
> declared and done_testing() was not seen.
> # Looks like your test exited with 29 just after 4.
> t/001_connection_limits.pl .. Dubious, test returned 29 (wstat 7424, 0x1d00)
> All 4 subtests passed
> 
> 
> and tmp_check/log/regress_log_001_connection_limits says:
> 
> 
> [23:48:44.444](1.129s) ok 3 - reserved_connections limit
> [23:48:44.445](0.001s) ok 4 - reserved_connections limit: matches
> process ended prematurely at
> /home/user/work/postgres/src/test/postmaster/../../../src/test/perl/PostgreSQL/Test/BackgroundPsql.pm
> line 154.
> # Postmaster PID for node "primary" is 198592


I just saw this failure on skink in the BF:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2025-03-04%2015%3A43%3A23

[17:05:56.438](0.247s) ok 3 - reserved_connections limit
[17:05:56.438](0.000s) ok 4 - reserved_connections limit: matches
process ended prematurely at /home/bf/bf-build/skink-master/HEAD/pgsql/src/test/perl/PostgreSQL/Test/BackgroundPsql.pm line 160.


> That BackgroundPsql.pm line is this in wait_connect()
> 
>   $self->{run}->pump()
>     until $self->{stdout} =~ /$banner/ || $self->{timeout}->is_expired;

A big part of the problem here imo is the exception behaviour that
IPC::Run::pump() has:

  If pump() is called after all harnessed activities have completed, a "process
  ended prematurely" exception to be thrown.  This allows for simple scripting
  of external applications without having to add lots of error handling code at
  each step of the script:

Which is, uh, not very compatible with how we use IPC::Run (here and
elsewhere).  Just ending the test because a connection failed is pretty awful.


This behaviour makes it really hard to debug problems. It'd have been a lot
easier to understand the problem if we'd seen psql's stderr before the test
died.

I guess that mean at the very least we'd need to put an eval {} around the
->pump() call., print $self->{stdout}, ->{stderr} and reraise an error?
Presumably not just in in wait_connect(), but also at least in pump_until()?



Will respond downthread to a potential workaround for the issue.

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.