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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: Noah Misch <noah@leadboat.com>, Michael Paquier <michael@paquier.xyz>, Tomas Vondra <tomas@vondra.me>, Thomas Munro <thomas.munro@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2025-03-07T14:53:16Z
Lists: pgsql-hackers

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Hi,

On 2025-03-06 22:49:20 +0200, Heikki Linnakangas wrote:
> In short, all the 4 patches look good to me. Thanks for picking this up!
> 
> On 06/03/2025 22:16, Andres Freund wrote:
> > On 2025-03-05 20:49:33 -0800, Noah Misch wrote:
> > > > 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?
> > > 
> > > That sounds right.
> > 
> > In the attached patch I did that for wait_connect().  I did verify that it
> > works by implementing the wait_connect() fix before fixing
> > 002_connection_limits.pl, which fails if a sleep(1) is added just before the
> > proc_exit(1) for FATAL.
> 
> +1. For the archives sake, I just want to clarify that this pump stuff is
> all about getting better error messages on a test failure. It doesn't help
> with the original issue.

Agreed.


> This is all annoyingly complicated, but getting good error messages is worth
> it.

Yea. I really look forward to having a way to write stuff like this that
doesn't involve hackily driving psql from 100m away using rubber bands.


> > On 2025-03-05 08:23:32 +0900, Michael Paquier wrote:>> 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?
> > 
> > I think the log_min_messages approach is a lot simpler. If we need something
> > like this more widely we can reconsider injection points...
> 
> +1. It's a little annoying to depend on a detail like the "client backend
> process exited" debug message, but seems like the best fix for now.

We use the same message for LOG messages too, for other types of backends, so
I think it's not that likely to change.  But stilll not great.


> While we're at it, attached are a few more cleanups I noticed.

I assume you'll apply that yourself?


Commits with updated commit messages attached.


I wonder if we should apply the polishing of connect_ok()/connect_fails() and
the wait_connect() debuggability improvements to the backbranches? Keeping TAP
infrastructure as similar as possible between branches has proven worthwhile
IME.


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.