[bug#43552] [PATCH] Add watchdog support.

[Mathieu Othacehe]

Hello,

> I've noticed that during several hours all Cuirass fibers seem to be
> stucked. I suspect that this is because a fiber is doing a blocking action,
> hence preventing its scheduler to run other fibers.

I think the issue is that we are communicating with guix-daemon over a
blocking socket. When Cuirass calls "build-derivations", and the "read"
over guix-daemon socket blocks, the whole Fiber scheduler is also
blocked.

As the GC is running for 6 - 7 hours a day[1], a Fiber scheduler is also
blocked for that duration. There's still some activity happening on
other schedulers, but that's still quite unfortunate. For instance,
nothing has been fetched since 4:00 this morning.

Thanks,

Mathieu

[1]: https://issues.guix.gnu.org/24937