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

[Ludovic Courtès]

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.

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.

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

Feedback welcome!

Ludo'.

Ludovic Courtès (2):
  DRAFT services: Extensions can specify a "finalization" procedure.
  system: pam: Remove custom API to transform PAM services.

 gnu/services.scm         | 52 ++++++++++++++++++++++++++++++++++++++----------
 gnu/services/base.scm    | 33 ++++++++++++++++--------------
 gnu/services/desktop.scm | 23 +++++++++++----------
 gnu/system/pam.scm       | 44 ++++++++--------------------------------
 tests/services.scm       | 34 +++++++++++++++++++++++++++++++
 5 files changed, 114 insertions(+), 72 deletions(-)

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2.13.0