shepherd, fibers, and signals (asyncs)

[Attila Lendvai]

dear Guix,

context:

Shepherd stops responding during "guix system reconfigure"
https://issues.guix.gnu.org/67538
https://issues.guix.gnu.org/65178
https://issues.guix.gnu.org/67230

i've added a ton of logging and asserts in my fork:

https://codeberg.org/attila-lendvai-patches/shepherd

which resulted in this report:

https://github.com/wingo/fibers/issues/29#issuecomment-1858319291

to which @emixa-d kindly responded:

https://github.com/wingo/fibers/issues/29#issuecomment-1858497720

which essentially identifies the following:

--------------

posix signal handlers are  async, and shepherd uses the fibers API from inside 
signal handlers, specifically in at least handle-SIGCHLD.

this violates the fibers API, and most probably leads to the root cause of the 
reconfigure hang: a match-error flying out from service-controller due to 
losing the value of the parameter called (current-process-monitor), which then 
makes that fiber exit.

i have little experience with posix signal handlers, so i probably won't come 
up with a fix for this, or at least not without someone's bird's eye view 
guidance.

maybe the solution could be something like packaging up posix signals and 
delivering them to the fibers universe by some form of polling of an atomic 
variable? or is there some signal-safe semaphore facility in guile that could 
be used in accordance with the fibers API?

-- 
• attila lendvai
• PGP: 963F 5D5F 45C7 DFCD 0A39
--
“Virtue is never left to stand alone. He who has it will have neighbors.”
        — Confucius (551–479 BC)