bug#48521: opendht-service-type hangs Shepherd at boot

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello,

Maxim Cournoyer <maxim.cournoyer@gmail.com> writes:

> Hello,
>
> I just noticed about this problem following a reboot.  I can also
> reproduce it in 'guix system vm', simply adding the opendht-service-type
> to my operating-system declaration.
>
> The boot proceeds until 'error in finalization thread: Success' then
> hangs indefinitely.
>
> What is troubling for me is that the service is rather straightforwardly
> defined.  It uses the make-forkexec-constructor/container like so:
>
> (define (opendht-shepherd-service config)
>   "Return a <shepherd-service> running OpenDHT."
>   (shepherd-service
>    (documentation "Run an OpenDHT node.")
>    (provision '(opendht dhtnode dhtproxy))
>    (requirement '(user-processes syslogd))
>    (start #~(make-forkexec-constructor/container
>              (list #$@(opendht-configuration->command-line-arguments config))
>              #:mappings (list (file-system-mapping
>                                (source "/dev/log") ;for syslog
>                                (target source)))
>              #:user "opendht"))
>    (stop #~(make-kill-destructor))))
>
> I'm not sure how using such basic building blocks could lead to a hang
> in Shepherd ?

It seems Shepherd can't cope with a failing start procedure/script when
a variable was not bound.  To diagnose the problem, the best way ended
up being to extract the code of the constructor in a separate script to
run it separately.  This made the error quickly apparent: "Unbound
variable: file-system-mapping".

We should try to handle this class of errors in Shepherd and report a
useful message and *not* crash Shepherd or otherwise hang.

Pushed with commit a09cdf1f9d.

Closing.

Maxim