bug#60657: Rethinking how service extensions work

[Bruno Victal <mirai@makinata.eu>]

Hi Ludo’,

On 2023-02-25 17:46, Ludovic Courtès wrote:
> Bruno Victal <mirai@makinata.eu> skribis:
>> In [1], the issue arises from using activation-service-type to create files/directories for services
>> when these should be either (1) shepherd one-shot services or moved into the 'start' procedure of the service.
>> 'activation-service-type' should only be used for doing things "listed on its label", that is, performing
>> actions at boot-time or after a system reconfigure.
> 
> Right.
> 
> As we once discussed on IRC, the conclusion to me is that some of the
> code currently implemented as activation snippets should rather be
> implemented either as part of the ‘start’ method of the corresponding
> Shepherd service, or as a one-shot Shepherd service that the main
> service would depend on.

I think moving them into the ‘start’ method is the best course of action.
I'm considering the following changes:
* Adding (gnu build activation) to %default-imported-modules + %default-modules in (gnu services shepherd).
  I expect that mkdir-p/perms is going to be used frequently enough, using the number of activation-service
  extensions in use as a rough estimate.
* Refactor the activation extensions into the ‘start’ method, where it makes sense to do so.


There's one issue I'm somewhat concerned about, consider the following snippet:

--8<---------------cut here---------------start------------->8---

(define log-directory "/var/log")
(define username "notroot")

(start
 #~(lambda _
    (mkdir-p/perms #$log-directory (getpw #$username) #o750)
    ...))

--8<---------------cut here---------------end--------------->8---

This is somewhat pitfall prone since you most likely don't want to chown /var/log to a non-root user.
I'm unsure what's the best course to take here, would a simple file-exist? check before mkdir-p/perms be sufficient?

In either case, with or without refactoring this issue is already present (but in activation-service extensions)
so it's no worse than the status quo.

>> (simple-service 'mount-overlayfs shepherd-root-service-type
>>                 (list (shepherd-service (requirement '(foo-mount))
>>                                         (provision '(overlayfs-foo))
>>                                         (documentation "Mount OverlayFS.")
>>                                         (one-shot? #t)
>>                                         (start (let ((util-linux (@ (gnu packages linux) util-linux)))
>>                                                  #~(lambda _
>>                                                      (system* #$(file-append util-linux "/bin/mount")
>>                                                               "-t" "overlay"
>>                                                               "-o" (string-append "noatime,nodev,noexec,ro,"
>>                                                                                   "lowerdir="
>>                                                                                   (string-join '("/srv/foo/overlays/top-layer"
>>                                                                                                  "/srv/foo/overlays/layer2"
>>                                                                                                  "/srv/foo/overlays/layer1"
>>                                                                                                  "/media/foo-base") ":"))
>>                                                               "none" "/media/foo" )))))))
> 
> Note that this should prolly be declared as a ‘file-system’ rather than
> as a custom service.  That way, it would get a “standard” Shepherd
> service.
> 
> There are cases where we add explicit dependencies on
> ‘file-system-/media/foo’ or similar.  <file-system> has a ‘dependencies’
> field specifically for this purpose (info "(guix) File Systems").
> 
> Would that work for you?

Unfortunately OverlayFS is filtered out from fstab by Guix (reported #60246) and the dependencies field IMO is too restrictive,
there should be a (sane) way to pass shepherd service symbols too. (for cases where a file system depends on 'networking or
depends on a particular interface e.g. NFS mount that uses a IPv6 link-local address)


Cheers,
Bruno