gnu: services: Support channels when extending guix-service-type.

Most of the configuration for guix-service-type can be extended (build
machines, substitutes, etc), but channels currently cannot.  This commit adds
support for that.

* gnu/services/base.scm (guix-extension): Add `channels'.
(guix-extension-merge): Merge channels.
(guix-service-type): Add extension channels.
* doc/guix.texi (Getting Substitutes from Other Servers): Use a service
extension instead of modify-services.
(Base Services, guix-extension): Document channel field.

Change-Id: I26cd0556a536f49ecc61662fc10af080d6c6dc9f

doc/guix.texi

@@ -4018,12 +4018,9 @@ them in the specified order.  You also need to explicitly authorize the
 public keys of substitute servers to instruct Guix to accept the
 substitutes they sign.
 
-On Guix System, this is achieved by modifying the configuration of the
+On Guix System, this is achieved by extending the configuration of the
-@code{guix} service.  Since the @code{guix} service is part of the
+@code{guix} service.  @pxref{Service Composition, Service extensions}
-default lists of services, @code{%base-services} and
+allow users to easily add new configuration information.
-@code{%desktop-services}, you can use @code{modify-services} to change
-its configuration and add the URLs and substitute keys that you want
-(@pxref{Service Reference, @code{modify-services}}).
 
 As an example, suppose you want to fetch substitutes from
 @code{guix.example.org} and to authorize the signing key of that server,
@@ -4035,18 +4032,13 @@ configuration will look something like:
 (operating-system
   ;; @dots{}
   (services
-    ;; Assume we're starting from '%desktop-services'.  Replace it
+    (cons
-    ;; with the list of services you're actually using.
+     (simple-service
-    (modify-services %desktop-services
+      'my-guix-configuration guix-service-type
-      (guix-service-type config =>
+      (guix-extension
-                        (guix-configuration
+        (substitute-urls (list "https://guix.example.org"))
-                          (inherit config)
+        (authorized-keys (list (local-file "./key.pub")))))
-                          (substitute-urls
+     %desktop-services)))
-                            (append (list "https://guix.example.org")
-                                    %default-substitute-urls))
-                          (authorized-keys
-                            (append (list (local-file "./key.pub"))
-                                    %default-authorized-guix-keys)))))))
 @end lisp
 
 This assumes that the file @file{key.pub} contains the signing key of
@@ -20702,6 +20694,9 @@ a guix service extension.
 @xref{Service Composition}, for more information.
 
 @table @asis
+@item @code{channels} (default: @code{'()})
+A list of objects where each element is a channel record.
+
 @item @code{authorized-keys} (default: @code{'()})
 A list of file-like objects where each element contains a public key.
 

gnu/services/base.scm

@@ -241,6 +241,7 @@
             guix-extension
             guix-extension?
             guix-extension-authorized-keys
+            guix-extension-channels
             guix-extension-substitute-urls
             guix-extension-chroot-directories
 
@@ -2428,6 +2429,8 @@ guix-daemon have the right ownership."))
 (define-record-type* <guix-extension>
   guix-extension make-guix-extension
   guix-extension?
+  (channels guix-extension-channels     ;list of channel
+            (default '()))
   (authorized-keys guix-extension-authorized-keys ;list of file-like
                     (default '()))
   (substitute-urls guix-extension-substitute-urls ;list of strings
@@ -2439,6 +2442,8 @@ guix-daemon have the right ownership."))
 
 (define (guix-extension-merge a b)
   (guix-extension
+   (channels (append (guix-extension-channels a)
+                     (guix-extension-channels b)))
    (authorized-keys (append (guix-extension-authorized-keys a)
                             (guix-extension-authorized-keys b)))
    (substitute-urls (append (guix-extension-substitute-urls a)
@@ -2464,6 +2469,8 @@ guix-daemon have the right ownership."))
    (extend (lambda (config extension)
              (guix-configuration
               (inherit config)
+              (channels (append (guix-extension-channels extension)
+                                (guix-configuration-channels config)))
               (authorized-keys (append (guix-extension-authorized-keys extension)
                                        (guix-configuration-authorized-keys config)))
               (substitute-urls (append (guix-extension-substitute-urls extension)