bug#26931: GuixSD rebooting fails when tmux is running

["Clément Lassieur" <clement@lassieur.org>]

Ludovic Courtès <ludo@gnu.org> writes:

> 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 PID1 and for some reason the PID1 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 PID1 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.

FYI I have the exact same problem with guix-publish: I have to do 'herd
stop guix-publish', otherwise I can't reboot.  I'll report a bug soon.
I would be interested to know if someone else reproduces it.