bug#67839: shepherd: sometimes hangs on `guix system reconfigure`

[Ludovic Courtès]

Hi Attila,

Attila Lendvai <attila.lendvai@gmail.com> skribis:

> * modules/shepherd.scm (main): Switch with-service-registry and
> with-process-monitor.  This way the parameterize of the process monitor covers
> everything else.  This fixes the bug that caused `guix system reconfigure` to
> hang in certain situations.  Fix proposed by @emixa-d at:
> https://github.com/wingo/fibers/issues/29#issuecomment-1858922276.

[...]

> * modules/shepherd/service.scm (spawn-service-controller): Add two asserts.
> This is the bug that causes `guix system reconfigure ...` to sometimes hang,
> and subsequently all shepherd commands, because a match-error flies out from
> the service-controller of a replaced service, and thus its fiber dies.  These
> asserts get triggered without the previous commit that fixes the issue.

Good catch!!

I pushed these patches with small edits, in particular adding a test
that reproduces the bug without relying on assertion failures:

  5dbde1c support: ‘assert’ logs source location information.
  0bcf02a Update NEWS.
  c07f0a8 service: Add asserts to ensure a process monitor is running.
  9be0b7e shepherd: Make sure ‘with-process-monitor’ covers everything needed.

Thanks for the tedious but fruitful debugging work!

Ludo’.