Re: shepherd service, unexplained permission errors

[Felix Lechner via "Development of GNU Guix and the GNU System distribution." <guix-devel@gnu.org>]

Hi Attila,

On Wed, Jun 28, 2023 at 1:27 PM Attila Lendvai <attila@lendvai.name> wrote:
>
> but through Shepherd the daemon fails while starting up due to a permission denied error:
>
> i created a user for that daemon, and added dialout as a supplementary group:
>
>  (supplementary-groups '("dialout"))
>
>            (fork+exec-command
>             cmd
>             #:log-file "/var/log/zigbee2mqtt.log"
>             #:user "zigbee2mqtt"
>             #:group "homeaut")))))))

The declaration of Shepherd's fork+exec-command has another optional
parameter for "supplementary-groups" [1] which is passed straight to
exec-command. [2]

With a double negation, the language in the manual is a bit hard to
parse, but the explicit argument may be required in order for the
supplementary group to work the way you expect:

"Likewise, command will be run under the current group, unless the
group keyword argument is present and not false, and
supplementary-groups is not '(). " [3]

Kind regards
Felix

[1] https://git.savannah.gnu.org/cgit/shepherd.git/tree/modules/shepherd/service.scm#n1569
[2] https://git.savannah.gnu.org/cgit/shepherd.git/tree/modules/shepherd/service.scm#n1610
[3] https://www.gnu.org/software/shepherd/manual/shepherd.html#index-exec_002dcommand