services: guix: Add ‘channels’ field.

* doc/guix.texi (Base Services): Document
'guix-configuration-channels' field.
(Invoking guix pull): Add cross-reference.
* gnu/services/base.scm (install-channels-file): New procedure.
(guix-configuration): Add channels field.
(guix-activation): Use 'install-channels-file' procedure.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>
Change-Id: I4d89235bf0bc6dde69984138ccb894b48ace9d76

doc/guix.texi

@@ -4727,7 +4727,9 @@ the user's @file{~/.config/guix/channels.scm} file, unless @option{-q}
 is passed;
 @item
 the system-wide @file{/etc/guix/channels.scm} file, unless @option{-q}
-is passed;
+is passed (on Guix System, this file can be declared in the operating
+system configuration, @pxref{guix-configuration-channels,
+@code{channels} field of @code{guix-configuration}});
 @item
 the built-in default channels specified in the @code{%default-channels}
 variable.
@@ -19806,6 +19808,20 @@ few seconds when enough entropy is available and is only done once; you
 might want to turn it off for instance in a virtual machine that does
 not need it and where the extra boot time is a problem.
 
+@anchor{guix-configuration-channels}
+@item @code{channels} (default: @code{%default-channels})
+List of channels to be specified in @file{/etc/guix/channels.scm}, which
+is what @command{guix pull} uses by default (@pxref{Invoking guix
+pull}).
+
+@quotation Note
+When reconfiguring a system, the existing @file{/etc/guix/channels.scm}
+file is backed up as @file{/etc/guix/channels.scm.bak} if it was
+determined to be a manually modified file.  This is to facilitate
+migration from earlier versions, which allowed for in-place
+modifications to @file{/etc/guix/channels.scm}.
+@end quotation
+
 @item @code{max-silent-time} (default: @code{3600})
 @itemx @code{timeout} (default: @code{(* 3600 24)})
 The number of seconds of silence and the number of seconds of activity,

gnu/services/base.scm

@@ -83,6 +83,7 @@
   #:use-module ((gnu build file-systems)
                 #:select (mount-flags->bit-mask
                           swap-space->flags-bit-mask))
+  #:autoload   (guix channels) (%default-channels channel->code)
   #:use-module (guix gexp)
   #:use-module (guix records)
   #:use-module (guix modules)
@@ -211,6 +212,7 @@
             guix-configuration-use-substitutes?
             guix-configuration-substitute-urls
             guix-configuration-generate-substitute-key?
+            guix-configuration-channels
             guix-configuration-extra-options
             guix-configuration-log-file
             guix-configuration-environment
@@ -1740,6 +1742,31 @@ archive' public keys, with GUIX."
         ;; Installed the declared ACL.
         (symlink #+default-acl acl-file))))
 
+(define (install-channels-file channels)
+  "Return a gexp with code to install CHANNELS, a list of channels, in
+/etc/guix/channels.scm."
+  (define channels-file
+    (scheme-file "channels.scm"
+                 `(list ,@(map channel->code channels))))
+
+  (with-imported-modules '((guix build utils))
+    #~(begin
+        (use-modules (guix build utils))
+
+        ;; If channels.scm already exists, move it out of the way. Create a
+        ;; backup if it's a regular file: it's likely that the user
+        ;; manually defined it.
+        (if (file-exists? "/etc/guix/channels.scm")
+            (if (and (symbolic-link? "/etc/guix/channels.scm")
+                     (store-file-name? (readlink "/etc/guix/channels.scm")))
+                (delete-file "/etc/guix/channels.scm")
+                (rename-file "/etc/guix/channels.scm"
+                             "/etc/guix/channels.scm.bak"))
+            (mkdir-p "/etc/guix"))
+
+        ;; Installed the declared channels.
+        (symlink #+channels-file "/etc/guix/channels.scm"))))
+
 (define %default-authorized-guix-keys
   ;; List of authorized substitute keys.
   (list (file-append guix "/share/guix/berlin.guix.gnu.org.pub")
@@ -1795,6 +1822,8 @@ archive' public keys, with GUIX."
                     (default %default-substitute-urls))
   (generate-substitute-key? guix-configuration-generate-substitute-key?
                             (default #t))         ;Boolean
+  (channels         guix-configuration-channels ;file-like
+                    (default %default-channels))
   (chroot-directories guix-configuration-chroot-directories ;list of file-like/strings
                       (default '()))
   (max-silent-time  guix-configuration-max-silent-time ;integer
@@ -1988,7 +2017,7 @@ proxy of 'guix-daemon'...~%")
 (define (guix-activation config)
   "Return the activation gexp for CONFIG."
   (match-record config <guix-configuration>
-    (guix generate-substitute-key? authorize-key? authorized-keys)
+    (guix generate-substitute-key? authorize-key? authorized-keys channels)
     #~(begin
         ;; Assume that the store has BUILD-GROUP as its group.  We could
         ;; otherwise call 'chown' here, but the problem is that on a COW overlayfs,
@@ -2005,6 +2034,9 @@ proxy of 'guix-daemon'...~%")
               (substitute-key-authorization authorized-keys guix)
               #~#f)
 
+        ;; ... and /etc/guix/channels.scm...
+        #$(and channels (install-channels-file channels))
+
         ;; ... and /etc/guix/machines.scm.
         #$(if (guix-build-machines config)
               (guix-machines-files-installation