gnu: guix: Define ‘guix-for-channels’ and document its use.

* gnu/packages/package-management.scm (guix-for-channels): New
procedure.
* doc/guix.texi (Customizing the System-Wide Guix): New section.
(Base Services): Add cross-reference.

Change-Id: Ied51c3bf9bf08dfc629bb3f0a152eb20b869a636

doc/guix.texi

@@ -5529,6 +5529,7 @@ account security concerns and deal with authenticated updates.
 * Specifying Additional Channels::  Extending the package collection.
 * Using a Custom Guix Channel::  Using a customized Guix.
 * Replicating Guix::            Running the @emph{exact same} Guix.
+* Customizing the System-Wide Guix:: Default channels on Guix System.
 * Channel Authentication::      How Guix verifies what it fetches.
 * Channels with Substitutes::   Using channels with available substitutes.
 * Creating a Channel::          How to write your custom channel.
@@ -5686,6 +5687,64 @@ artifacts with very fine grain, and to reproduce software environments at
 will---some sort of ``meta reproducibility'' capabilities, if you will.
 @xref{Inferiors}, for another way to take advantage of these super powers.
 
+@node Customizing the System-Wide Guix
+@section Customizing the System-Wide Guix
+
+@cindex system-wide Guix, customization
+@cindex channels, for the default Guix
+If you're running Guix System or building system images with it, maybe
+you will want to customize the system-wide @command{guix} it
+provides---specifically, @file{/run/current-system/profile/bin/guix}.
+For example, you might want to provide additional channels or to pin its
+revision.
+
+This can be done using the @code{guix-for-channels} procedure, which
+returns a package for the given channels, and using it as part of your
+operating system configuration, as in this example:
+
+@lisp
+(use-modules (guix channels))
+
+(define my-channels
+  ;; Channels that should be available to
+  ;; /run/current-system/profile/bin/guix.
+  (append
+   (list (channel
+          (name 'guix-science)
+          (url "https://github.com/guix-science/guix-science")
+          (branch "master")))
+   %default-channels))
+
+(operating-system
+  ;; @dots{}
+  (services
+    ;; Change the package used by 'guix-service-type'.
+    (modify-services %base-services
+      (guix-service-type
+       config => (guix-configuration
+                  (inherit config)
+                  (channels my-channels)
+                  (guix (guix-for-channels my-channels)))))))
+@end lisp
+
+The resulting operating system will have both the @code{guix} and the
+@code{guix-science} channels visible by default.  The @code{channels}
+field of @code{guix-configuration} above further ensures that
+@file{/etc/guix/channels.scm}, which is used by @command{guix pull},
+specifies the same set of channels (@pxref{guix-configuration-channels,
+@code{channels} field of @code{guix-configuration}}).
+
+The @code{(gnu packages package-management)} module exports the
+@code{guix-for-channels} procedure, described below.
+
+@deffn {Procedure} guix-for-channels @var{channels}
+Return a package corresponding to @var{channels}.
+
+The result is a ``regular'' package, which can be used in
+@code{guix-configuration} as shown above or in any other place that
+expects a package.
+@end deffn
+
 @node Channel Authentication
 @section Channel Authentication
 
@@ -19749,7 +19808,8 @@ This data type represents the configuration of the Guix build daemon.
 
 @table @asis
 @item @code{guix} (default: @var{guix})
-The Guix package to use.
+The Guix package to use.  @xref{Customizing the System-Wide Guix} to
+learn how to provide a package with a pre-configured set of channels.
 
 @item @code{build-group} (default: @code{"guixbuild"})
 Name of the group for build user accounts.

gnu/packages/package-management.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013-2023 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013-2024 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015, 2017, 2020, 2021, 2022, 2023 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2017 Muriithi Frederick Muriuki <fredmanglis@gmail.com>
 ;;; Copyright © 2017, 2018 Oleg Pykhalov <go.wigust@gmail.com>
@@ -649,6 +649,18 @@ the Nix package manager.")
       (modify-inputs (package-propagated-inputs guix)
         (delete "guile-ssh"))))))
 
+(define-public (guix-for-channels channels)
+  "Return a package corresponding to CHANNELS."
+  (package
+    (inherit guix)
+    (source (find guix-channel? channels))
+    (build-system channel-build-system)
+    (arguments
+     `(#:channels ,(remove guix-channel? channels)))
+    (inputs '())
+    (native-inputs '())
+    (propagated-inputs '())))
+
 (define-public current-guix-package
   ;; This parameter allows callers to override the package that 'current-guix'
   ;; returns.  This is useful when 'current-guix' cannot compute it by itself,