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From: Maxime Devos <maximedevos@telenet.be>
To: "Ludovic Courtès" <ludo@gnu.org>, 56674@debbugs.gnu.org
Subject: bug#56674: [Shepherd] Use of ‘waitpid’,  ‘system*’,  etc. in service code can cause deadlocks
Date: Thu, 21 Jul 2022 01:48:02 +0200	[thread overview]
Message-ID: <c4045c06-2024-b49e-cee9-88dafd3612e6@telenet.be> (raw)
In-Reply-To: <8735evpipv.fsf@inria.fr>


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On 20-07-2022 23:39, Ludovic Courtès wrote:
> Hi!
>
> We’ve just had a bad experience with the nginx service on berlin, where
> ‘herd restart nginx’ would cause shepherd to get stuck forever in
> ‘waitpid’ on the process that was supposed to start nginx.
>
> The details are unclear, but one thing is clear is that using ‘waitpid’
> (either directly or indirectly with ‘system*’, which is what
> ‘nginx-service-type’ does) is not great:
>
>    1. In the best case, shepherd (as of 0.9.1) is stuck while ‘system*’
>       is in ‘waitpid’ waiting for child process completion (“stuck” as
>       in: doesn’t do anything, not even answering ‘herd’ requests or
>       inetd connections.)
>
>    2. I don’t think that can happen with ‘system*’ (because it’s in C),
>       but generally speaking, there’s a possibility that shepherd’s event
>       loop will handle child process termination before some other
>       user-made ‘waitpid’ call does.
>
> Anyway, that’s a bad situation.
>
> So I can think of several ways to address it:
>
>    1. Change the nginx service ‘stop’ method to just
>       (make-kill-destructor), which should work just as well as invoking
>       “nginx -s stop”.
>
>    2. Have Shepherd provide a replacement for ‘system*’.
Why Shepherd and not guile fibers? Is this a Shepherd-specific problem?
>
> Thoughts?

3. Make waitpid (or a variant that does what we need) interact well with 
guile-fibers, like how 'accept' is doesn't inhibit switching to another 
fiber. There some Linux API with signal handlers or pid fds or such that 
might be useful here, though I don't recall the name. Presumably 
something similar can be done for the Hurd, though some C glue may be 
needed to access the right Hurd APIs if the signal handler API isn't 
portable.

Alternatively:

4. Do the waitpid in a separate thread (needs work-around for the 
multi-threaded fork problem, probably C things? Or modifying Guile and 
maybe glibc to avoid async-unsafe things or make more things async-safe 
or whatever the appropriate ...-safe is here.)

If not a Guile Fibers interaction problem, then the asynchronous signal 
handler API might still be useful.

Greetings,
Maxime


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  reply	other threads:[~2022-07-20 23:49 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-07-20 21:39 bug#56674: [Shepherd] Use of ‘waitpid’, ‘system*’, etc. in service code can cause deadlocks Ludovic Courtès
2022-07-20 23:48 ` Maxime Devos [this message]
2022-07-21 15:39   ` Ludovic Courtès
2022-08-13 14:59     ` Maxime Devos
2022-11-13 23:16 ` Ludovic Courtès
2022-11-14 16:32   ` bug#58926: Shepherd becomes unresponsive after an interrupt Ludovic Courtès

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