services: guix: Generalize extensions.

* gnu/services/base.scm (<guix-extension>): New record type.
(guix-extension-merge): New procedure.
(guix-service-type): Honor extensions.
* doc/guix.texi (Base Services): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -103,6 +103,7 @@ Copyright @copyright{} 2022 Remco van 't Veer@*
 Copyright @copyright{} 2022 Aleksandr Vityazev@*
 Copyright @copyright{} 2022 Philip M@sup{c}Grath@*
 Copyright @copyright{} 2022 Karl Hallsby@*
+Copyright @copyright{} 2022 Justin Veilleux@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -17729,6 +17730,25 @@ A directory path where the @command{guix-daemon} will perform builds.
 @end table
 @end deftp
 
+@deftp {Data Type} guix-extension
+
+This data type represents the parameters of the Guix build daemon that
+are extendable. This is the type of the object that must be used within
+a guix service extension.
+@xref{Service Composition}, for more information.
+
+@table @asis
+@item @code{authorized-keys} (default: @code{'()})
+A list of file-like objects where each element contains a public key.
+
+@item @code{substitute-urls} (default: @code{'()})
+A list of strings where each element is a substitute URL.
+
+@item @code{chroot-directories} (default: @code{'()})
+A list of file-like objects or strings pointing to additional directories the build daemon can use.
+@end table
+@end deftp
+
 @deffn {Scheme Procedure} udev-service [#:udev @var{eudev} #:rules @code{'()}]
 Run @var{udev}, which populates the @file{/dev} directory dynamically.
 udev rules can be provided as a list of files through the @var{rules}

gnu/services/base.scm

@@ -17,6 +17,7 @@
 ;;; Copyright © 2021 Hui Lu <luhuins@163.com>
 ;;; Copyright © 2021, 2022 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2022 Guillaume Le Vaillant <glv@posteo.net>
+;;; Copyright © 2022 Justin Veilleux <terramorpha@cock.li>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -187,6 +188,12 @@
             guix-configuration-extra-options
             guix-configuration-log-file
 
+            guix-extension
+            guix-extension?
+            guix-extension-authorized-keys
+            guix-extension-substitute-urls
+            guix-extension-chroot-directories
+
             guix-service-type
             guix-publish-configuration
             guix-publish-configuration?
@@ -1768,6 +1775,25 @@ proxy of 'guix-daemon'...~%")
               (substitute-key-authorization authorized-keys guix)
               #~#f))))
 
+(define-record-type* <guix-extension>
+  guix-extension make-guix-extension
+  guix-extension?
+  (authorized-keys guix-extension-authorized-keys ;list of file-like
+                    (default '()))
+  (substitute-urls guix-extension-substitute-urls ;list of strings
+                    (default '()))
+  (chroot-directories guix-extension-chroot-directories ;list of file-like/strings
+                      (default '())))
+
+(define (guix-extension-merge a b)
+  (guix-extension
+   (authorized-keys (append (guix-extension-authorized-keys a)
+                            (guix-extension-authorized-keys b)))
+   (substitute-urls (append (guix-extension-substitute-urls a)
+                            (guix-extension-substitute-urls b)))
+   (chroot-directories (append (guix-extension-chroot-directories a)
+                               (guix-extension-chroot-directories b)))))
+
 (define guix-service-type
   (service-type
    (name 'guix)
@@ -1778,14 +1804,19 @@ proxy of 'guix-daemon'...~%")
           (service-extension profile-service-type
                              (compose list guix-configuration-guix))))
 
-   ;; Extensions can specify extra directories to add to the build chroot.
+   ;; Extensions can specify extra directories to add to the build chroot,
-   (compose concatenate)
+   ;; extra substitute urls and extra authorized keys
-   (extend (lambda (config directories)
+   (compose (lambda (args) (fold guix-extension-merge (guix-extension) args)))
+   (extend (lambda (config extension)
              (guix-configuration
               (inherit config)
+              (authorized-keys (append (guix-extension-authorized-keys extension)
+                                       (guix-configuration-authorized-keys config)))
+              (substitute-urls (append (guix-extension-substitute-urls extension)
+                                       (guix-configuration-substitute-urls config)))
               (chroot-directories
-               (append (guix-configuration-chroot-directories config)
+               (append (guix-extension-chroot-directories extension)
-                       directories)))))
+                       (guix-configuration-chroot-directories config))))))
 
    (default-value (guix-configuration))
    (description