bug#27155: [PATCH 0/2] Support service extensions on the "final" service values

[Alex Kost <alezost@gmail.com>]

Ludovic Courtès (2017-05-30 23:58 +0200) wrote:

> Hello!
>
> This patch adds support for service extensions that modify the
> "final" values of a service.  This is meant to implement cross-cutting
> concerns as well as system-wide customization as discussed with Alex
> long ago:
>
>   https://lists.gnu.org/archive/html/guix-devel/2015-11/msg00623.html
>   https://lists.gnu.org/archive/html/guix-devel/2016-09/msg01505.html
>
> To summarize, a "finalization extension" (for lack of a better name)
> gets the final value of a service and returns a new value for that
> service.  This is in contrast with a "normal" extension which can only
> contribute to the value of a target service, and not inspect the value
> of that target service.
>
> For example, for the /etc service, a "normal" extension can only add
> entries for /etc.  A "finalization" extension can instead inspect and
> change all the /etc entries.  IOW, it is a sort of a "sudo" for service
> extensions; it's also quite inelegant compared to the "normal" extension
> mechanism, but it's certainly useful.

Definitely!

> A use case is given in the second patch: we change all the PAM services
> to use pam_elogind.so or pam_limits.so.  Likewise, the 'rename-etc-files'
> service below shows how to rename all the files in /etc (for illustration
> purposes only :-)):
>
>   (define rename-etc-files
>     (let ((rename (lambda (prefix entries)
> 		    (map (match-lambda
> 			   ((name . rest)
> 			    (cons (string-append prefix name)
> 				  rest)))
> 			 entries))))
>       (service-type
>        (name 'rename-etc-files)
>        (extensions (list (service-extension etc-service-type
> 					    (const '())
> 					    rename))))))
>
>
>   (operating-system
>     ;; ...
>     (services (cons* (service rename-etc-files "foo-")
>                      ...)))
>
> I think this should fulfill the need that Alex had expressed, which is
> to not only be able to add files to /etc, but also to have the ability
> to inspect and modify what goes to /etc.

This is great!  Just what I wanted, and thanks for this example!  Based
on it, I made the following service:

  (define replace-etc/profile-type
    (let ((replace
           (lambda (file entries)
             (cons `("profile" ,file)
                   (map (match-lambda
                          ((name . rest)
                           (cons (if (string= name "profile")
                                     (string-append "original-profile")
                                     name)
                                 rest)))
                        entries)))))
      (service-type
       (name 'replace-etc/profile)
       (extensions (list (service-extension etc-service-type
                                            (const '())
                                            replace))))))

  (service replace-etc/profile-type (local-file ".../my-system-profile"))

So now I can use my own "/etc/profile", moreover I can look at the
"/etc/original-profile" anytime.  I already use a system with this
service and I enjoy it, thanks a lot!

> The first patch currently lacks doc.  I'll work on it if there's consensus
> on the approach.

I agree with this approach!

-- 
Alex