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

From: Alex Kost
Subject: bug#27155: [PATCH 0/2] Support service extensions on the "final" service values
Date: Thu, 01 Jun 2017 12:57:09 +0300
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.2 (gnu/linux)

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:
> 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.


> A use case is given in the second patch: we change all the PAM services
> to use or  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 'replace-etc/profile)
       (extensions (list (service-extension etc-service-type
                                            (const '())

  (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!


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