bug#26931: GuixSD rebooting fails when tmux is running

[ludo@gnu.org (Ludovic Courtès)]

Hi,

Leo Famulari <leo@famulari.name> skribis:

> On Sun, May 14, 2017 at 11:36:17PM +0200, Ludovic Courtès wrote:
>> What does /var/log/shepherd.log show around the time where you hit
>> “halt”?
>> 
>> I get something like this:
>> 
>> --8<---------------cut here---------------start------------->8---
>> 18:06:26 Service mcron has been stopped.
>> 18:06:26 sending all processes the TERM signal
>
> For me, this is where it gets stuck:
>
> ------
> 2017-05-16 19:12:53 sending all processes the TERM signal
> 2017-05-16 19:12:58 waiting for process termination (processes left: (1 494)) 
> 2017-05-16 19:13:00 waiting for process termination (processes left: (1 494)) 
> 2017-05-16 19:13:02 waiting for process termination (processes left: (1 494)) 
> ------
>
> In my experience, it will wait here forever.
>
> And from `ps aux`:
>
> leo        494  0.0  0.1  27232  3676 ?        Ss   19:12   0:00 tmux

The bug was 100% reproducible in a VM, and AFAICS it is fixed by
7f090203d5fb033eb1b64778b03afad5bb35f5f2.

The problem was that the tmux server process would be left as a zombie,
and then the loop would always see it because the parent process of the
tmux server process is PID 1 and for some reason the PID 1 either didn’t
get SIGCHLD or the handler didn’t run.

The test that this commit adds does exactly the same thing: launch tmux
and then invoke “halt”.  I tried to create a synthetic test not
involving tmux, simply creating a process that gets PID 1 as its parent,
but it wouldn’t trigger the bug.  I’m unclear as to why tmux triggers it
and no that other simple test.

Thanks,
Ludo’.