[bug#49610,v3] services: guix: Add channels field.

diff --git a/doc/guix.texi b/doc/guix.texi
index 31dc33fb97..d0367ab8c5 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -5397,8 +5397,8 @@  $ wget -O - \
 Guix and its package collection are updated by running @command{guix pull}
 (@pxref{Invoking guix pull}).  By default @command{guix pull} downloads and
 deploys Guix itself from the official GNU@tie{}Guix repository.  This can be
-customized by defining @dfn{channels} in the
-@file{~/.config/guix/channels.scm} file.  A channel specifies a URL and branch
+customized by defining @dfn{channels} in the @file{/etc/guix/channels.scm} and
+@file{~/.config/guix/channels.scm} files.  A channel specifies a URL and branch
 of a Git repository to be deployed, and @command{guix pull} can be instructed
 to pull from one or more channels.  In other words, channels can be used
 to @emph{customize} and to @emph{extend} Guix, as we will see below.
@@ -18858,6 +18858,19 @@  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.
 
+@item @code{channels} (default: @code{%default-channels-file})
+File-like object containing a list of channels to be used by
+@command{guix pull}, by default.  The file-like object is symlinked to
+@file{/etc/guix/channels.scm}.
+
+@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{0})
 @itemx @code{timeout} (default: @code{0})
 The number of seconds of silence and the number of seconds of activity,
diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index c5b06b57e8..e61bf6eac7 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -80,6 +80,7 @@  (define-module (gnu services base)
   #:use-module ((gnu build file-systems)
                 #:select (mount-flags->bit-mask
                           swap-space->flags-bit-mask))
+  #:use-module (guix channels)
   #:use-module (guix gexp)
   #:use-module (guix records)
   #:use-module (guix modules)
@@ -93,6 +94,7 @@  (define-module (gnu services base)
   #:use-module (srfi srfi-35)
   #:use-module (ice-9 match)
   #:use-module (ice-9 format)
+  #:use-module (ice-9 pretty-print)
   #:re-export (user-processes-service-type        ;backwards compatibility
                %default-substitute-urls)
   #:export (fstab-service-type
@@ -207,6 +209,7 @@  (define-module (gnu services base)
             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
@@ -1739,6 +1742,39 @@  (define acl-file #$%acl-file)
         ;; Installed the declared ACL.
         (symlink #+default-acl acl-file))))
 
+(define %default-channels-file
+  ;; File-like object containing the default list of channels.
+  (plain-file "channels.scm"
+              (with-output-to-string
+                (lambda _
+                  (pretty-print (map (lambda (channel)
+                                       (if (channel? channel)
+                                           (channel->code channel)
+                                           channel))
+                                     %default-channels))))))
+
+;; FIXME: Should this gexp be built before boot, like
+;; substitute-key-authorization does?
+(define (install-channels-file channels-file)
+  "Return a gexp with code to install CHANNELS-FILE, a file-like object."
+  (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")
@@ -1763,6 +1799,8 @@  (define-record-type* <guix-configuration>
                     (default %default-substitute-urls))
   (generate-substitute-key? guix-configuration-generate-substitute-key?
                             (default #t))         ;Boolean
+  (channels         guix-configuration-channels ;file-like
+                    (default %default-channels-file))
   (chroot-directories guix-configuration-chroot-directories ;list of file-like/strings
                       (default '()))
   (max-silent-time  guix-configuration-max-silent-time ;integer
@@ -1949,7 +1987,7 @@  (define (guix-accounts config)
 (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,
@@ -1963,7 +2001,9 @@  (define (guix-activation config)
 
         #$(if authorize-key?
               (substitute-key-authorization authorized-keys guix)
-              #~#f))))
+              #~#f)
+
+        #$(install-channels-file channels))))
 
 (define-record-type* <guix-extension>
   guix-extension make-guix-extension