Re: shepherd, fibers, and signals (asyncs)

[Attila Lendvai]

@emixa-d kindly proposed something that turned out to be a fix:

https://github.com/wingo/fibers/issues/29#issuecomment-1858922276

i've sent it to:

shepherd: sometimes hangs on `guix system reconfigure`
https://issues.guix.gnu.org/67839#6

in essence:

shepherd violates the fibers API by calling it from an async signal handler, 
and this is an issue indeed, but the bugs caused by this should manifest rarely 
and randomly; i.e. the frozen recofigure behavior must be caused by something 
else.

the actual root cause here was that the with-process-monitor parameterize was 
not covering some code that it should have.

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• attila lendvai
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