GNU bug report logs - #67132
[Shepherd] Bogus value from PID file can prevent service from being stopped

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Ludovic Courtès <ludo <at> gnu.org>
To: bug-guix <at> gnu.org
Subject: [Shepherd] Bogus value from PID file can prevent service from being
 stopped
Date: Sun, 12 Nov 2023 18:45:38 +0100

Hi,

‘terminate-process’ in (shepherd service) assumes that, once it has sent
SIGKILL to a given PID, it eventually gets a SIGCHLD notification for
that PID.

Unfortunately there are corner cases where it doesn’t hold.  Take PID 2:

--8<---------------cut here---------------start------------->8---
$ ps x 2
  PID TTY      STAT   TIME COMMAND
    2 ?        S      0:00 [kthreadd]
--8<---------------cut here---------------end--------------->8---

That’s not a “real” process, so when you (kill 2 SIGKILL), nothing
happens.  Thus, ‘terminate-process’ never returns; the corresponding
service stays in “stopping” state and you cannot halt your machine.

You may wonder how we end up calling ‘terminate-process’ with a “bogus”
PID.  It normally never happens, but it can happen with a combination of
‘make-forkexec-constructor’ with #:pid-file and
‘least-authority-wrapper’: the PID file of the wrapper may read “2” (the
PID inside the least-authority PID namespace), so ‘herd stop’ on that
service will end up doing (kill 2 SIGKILL).

Ludo’.

Message #10 received at 67132-done <at> debbugs.gnu.org (full text, mbox):

From: Ludovic Courtès <ludo <at> gnu.org>
To: 67132-done <at> debbugs.gnu.org
Subject: Re: bug#67132: [Shepherd] Bogus value from PID file can prevent
 service from being stopped
Date: Wed, 22 Nov 2023 11:16:37 +0100

Ludovic Courtès <ludo <at> gnu.org> skribis:

> ‘terminate-process’ in (shepherd service) assumes that, once it has sent
> SIGKILL to a given PID, it eventually gets a SIGCHLD notification for
> that PID.
>
> Unfortunately there are corner cases where it doesn’t hold.  Take PID 2:
>
> $ ps x 2
>   PID TTY      STAT   TIME COMMAND
>     2 ?        S      0:00 [kthreadd]
>
> That’s not a “real” process, so when you (kill 2 SIGKILL), nothing
> happens.  Thus, ‘terminate-process’ never returns; the corresponding
> service stays in “stopping” state and you cannot halt your machine.

Fixed in Shepherd commit cc9c5c029534458ae547d78200b6b51f729654e3.

Ludo’.

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