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

[Attila Lendvai]

my fellow hackers,

i'm going to attach two patches that is essentially just adding a couple of 
asserts that trigger a test failure (tests/replacement.sh).

my current codebase 
(https://codeberg.org/attila-lendvai-patches/shepherd/commits/branch/attila) 
logs a whole lot more information, and has a more sophisticated error handling. 
triggering the same error on that codebase shows that the first assert is 
already failing (the one that is before spawning the new fiber for the 
controller of the replacement).

maybe the root cause is this: https://github.com/wingo/fibers/issues/29

HTH,

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• attila lendvai
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