[bug#71324] [PATCH] news: Add entry for 'docker-service-type' changes.

[Carlo Zancanaro <carlo@zancanaro.id.au>]

On Mon, Jun 03 2024, Oleg Pykhalov wrote:
> Every docker-service-type user will need to add containerd-service-type
> to their system configurations, otherwise a message about not any
> service provides containerd will be displayed during reconfigure.

Is this necessary? If I understand correctly, the docker service has a
hard dependency on containerd, so having to specify it every time is
unnecessary boilerplate. Could we add an extension for containerd that
does nothing, but which the docker service can use just to request its
inclusion? Something like:

--8<---------------cut here---------------start------------->8---
(define containerd-service-type
  (service-type (name 'containerd)
                (description ...)
                (extensions ...)
                ;; Declare an extension point, so containerd can be requested
                ;; by other services without affecting its configuration.
                (compose (const #t))
                (extend (lambda (config _) config))
                (default-value (containerd-configuration))))

(define docker-service-type
  (service-type ...
                (extensions
                 ...
                 (service-extension containerd-service-type (const #t)))))
--8<---------------cut here---------------end--------------->8---

That way users could instantiate containerd themselves if they need some
particular configuration, but if not it would be pulled in automatically
by the docker service. Hopefully this would cause less of an issue for
existing configs that don't use a custom containerd (like mine).

We could even, for backwards compatibility, allow the docker service
type to pass through the containerd package from its old configuration
(while printing a warning). This could make it possible to separate
containerd from docker without breaking any existing configurations.

Carlo