[bug#26830] Allow services to implement a 'reload' action

[ludo@gnu.org (Ludovic Courtès)]

Hello!

ludo@gnu.org (Ludovic Courtès) skribis:

> The logical next step of this series will be to have the service upgrade
> code in ‘guix system reconfigure’ invoke this action when it is defined.
> That will be awesome.
>
> Some comments:
>
> +                      #:actions (make-actions
> +                                 (reload
> +                                  "Reload the service's configuration files."
> +                                  #$(shepherd-service-reload service))))))))
>
> Here I think we should only define the action when it has a non-#f
> value.  That way we can distinguish between services that have a useful
> reload/reconfigure/upgrade action and those that don’t; in the latter
> case, we simply use ‘restart’ when upgrading.
>
> Regarding nginx:
>
> +              (stop (nginx-action "-s" "stop"))
> +              (reload (nginx-action "-s" "reload"))))))))
>
> Is this of any use in practice?  The nginx command line is something
> like:
>
>   /gnu/store/74kz9m850ycxpzkg6dvn9wbd3xjkwwrb-nginx-1.12.0/sbin/nginx -c /gnu/store/5w11ahw113fndvab3xmwcjzs2rw56sbh-nginx-config/bayfront.conf -p /var/run/nginx
>
> and the configuration file in /gnu/store is immutable, so “nginx -s
> reload” does nothing.  If the action took an argument, we could do:
>
>   herd reconfigure nginx /gnu/store/…-new-config.conf
>
> which would translate to:
>
>   nginx -s reload -c /gnu/store/…-new-config.conf

FWIW, with the patch at <https://bugs.gnu.org/32128>, adding such
actions becomes easy (it’s a generalization of what you did in this
patch series.)

Ludo’.