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
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 :-)):
(let ((rename (lambda (prefix entries)
((name . rest)
(cons (string-append prefix name)
(extensions (list (service-extension etc-service-type
(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.
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(-)