GNU bug report logs - #27553
[PATCH shepherd] Register SIGCHLD handler after primitive fork

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Message #20 received at 27553 <at> debbugs.gnu.org (full text, mbox):

From: Jelle Licht <jlicht <at> fsfe.org>
To: Ludovic Courtès <ludo <at> gnu.org>
Cc: 27553 <at> debbugs.gnu.org
Subject: Re: [bug#27553] [PATCH shepherd] Register SIGCHLD handler after
 primitive fork
Date: Thu, 7 Sep 2017 00:56:41 +0200

[Message part 1 (text/plain, inline)]

I just tested what Ludo' proposed, and it seems to work like a charm.
Seeing as we might be seeing more non-init shepherd instances w.r.t.
 user services and the possible service extension to `guix environment',
I think it would be a good call to fix this bug :-).

Regards,
Jelle



2017-07-27 16:32 GMT+02:00 Jelle Licht <jlicht <at> fsfe.org>:

> Hi Ludo,
>
> The documentation for the `daemonize' action specifies the following:
>
>>       "Go into the background.  Be careful, this means that a new
>> process will be created, so shepherd will not get SIGCHLD signals anymore
>> if previously spawned childs terminate.  Therefore, this action should
>> usually only be used (if at all) *before* childs get spawned for which
>> we want to receive these signals."
>>
>>
> In a sense, the problem that you describe can then be solved  by having
> the lazy SIGCHLD handler be registered in two places:
> - Immediately after a call to the `daemonize' action, as its documentation
> that if called, it should be done before starting any services
> - Before calling the lambda stored in the `start' slot of any
> non-root-service service
>
> I know how to do the first one (the newly forked process should lazily
> register the handler), but the second one seems a bit harder to do.
> I could add a special case to the `start' method so that it will lazily
> install the handler unless we are starting the root-service, but that seems
> inelegant somehow.
>
>
> 2017-07-17 10:33 GMT+02:00 Ludovic Courtès <ludo <at> gnu.org>:
>
>> Hi Jelle,
>>
>> Jelle Licht <jlicht <at> fsfe.org> skribis:
>>
>> > 2017-07-12 23:34 GMT+02:00 Ludovic Courtès <ludo <at> gnu.org>:
>> >
>> >> Hi Jelle,
>> >>
>> >> Jelle Licht <jlicht <at> fsfe.org> skribis:
>> >>
>> >> > I am not sure if this is also the proper ML for the GNU Shepherd, but
>> >> > looking in the archives lead me to believe it actually is. If not, I
>> >> > suggest the gnu.org page for shepherd be updated with the correct
>> info.
>> >>
>> >> It’s the right list.  :-)
>> >>
>> > I am glad it turned out to be :-). Perhaps [1] can be updated to the
>> same
>> > info as [2]?
>>
>> Done!
>>
>> >> > I recently starting playing around with user shepherd, and found out
>> that
>> >> > when running a shepherd 0.3.2 daemonized as non-init process (via
>> >> "(action
>> >> > 'shepherd 'daemonize)"), zombie processes are created whenever you
>> start
>> >> > and subsequently stop any service.
>> >> >
>> >> > Thinking I did something wrong, I asked lfam on #guix to share his
>> (very
>> >> > helpful) init.scm for user shepherd, yet I still noticed the same
>> >> behaviour.
>> >> >
>> >> > I believe commit `efa2f45c5f7dc735407381b7b8a83d6c37f828db'
>> >> inadvertently
>> >> > introduced an ordering issue, where the SIGCHLD handler is registered
>> >> > /before/ shepherd has the chance to daemonize. I believe the
>> following
>> >> > trivial patch addresses this snafu.
>> >>
>> >> The config file can start services, so the SIGCHLD handler must be
>> >> installed before we read the config file (otherwise we could be missing
>> >> some process termination notifications.)
>> >>
>> > What do you mean exactly? I think my config file does this, and I have
>> not
>> > yet noticed this issue,
>> > but I might just be confused about what you mean here.
>>
>> If the config file spawns a process and that process dies before we have
>> installed the SIGCHLD handler, then we’ll never know that it has
>> terminated.
>>
>> >> Perhaps a solution would be to install the SIGCHLD handler lazily upon
>> >> the first ‘fork+exec-command’ call?  That would ensure both that (1)
>> >> users have a chance to daemonize before the handler is installed, and
>> >> (2) that the handler is installed before services are started.
>> >>
>> >> Thoughts?
>> >>
>> > This seems like it would be for the best. I actually have no clue how to
>> > implement this though.
>>
>> I’d imagine something like a global variable (a Boolean) telling whether
>> the SIGCHLD handler is installed, and then:
>>
>>   (unless %sigchld-handler-installed?
>>     (sigaction …)
>>     (set! %sigchld-handler-installed? #t))
>>
>> Thoughts?
>>
>> Ludo’.
>>
>
>

[Message part 2 (text/html, inline)]

[0001-Lazily-register-SIGCHLD-hander-on-first-call-to-fork.patch (text/x-patch, attachment)]

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