services: shepherd: Allow custom 'shepherd' package.

* gnu/services/shepherd.scm (<shepherd-configuration>): New record.
(shepherd-boot-gexp, shepherd-root-service-type): Use it.
(scm->go, shepherd-configuration-file): Allow passing custom
shepherd package.
* gnu/system.scm (operating-system-shepherd-service-names): Use the new
record.
* guix/scripts/system.scm (export-shepherd-graph): Adjust accordingly.
* doc/guix.texi (Shepherd Services). Document it.

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

doc/guix.texi

@@ -32782,9 +32782,24 @@ The service type for the Shepherd ``root service''---i.e., PID@tie{}1.
 
 This is the service type that extensions target when they want to create
 shepherd services (@pxref{Service Types and Services}, for an example).
-Each extension must pass a list of @code{<shepherd-service>}.
+Each extension must pass a list of @code{<shepherd-service>}.  Its
+value must be a @code{shepherd-configuration}, as described below.
 @end defvr
 
+@deftp {Data Type} shepherd-configuration
+This data type represents the Shepherd's configuration.
+
+@table @code
+@item shepherd (default: @code{shepherd})
+The Shepherd package to use.
+
+@item services (default: @code{'()})
+A list of @code{<shepherd-service>} to start.
+You should probably use the service extension
+mechanism instead (@pxref{Shepherd Services}).
+@end table
+@end deftp
+
 @defvr {Scheme Variable} %shepherd-root-service
 This service represents PID@tie{}1.
 @end defvr

gnu/services/shepherd.scm

@@ -3,6 +3,7 @@
 ;;; Copyright © 2017 Clément Lassieur <clement@lassieur.org>
 ;;; Copyright © 2018 Carlo Zancanaro <carlo@zancanaro.id.au>
 ;;; Copyright © 2020 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -36,7 +37,12 @@ (define-module (gnu services shepherd)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
-  #:export (shepherd-root-service-type
+  #:export (shepherd-configuration
+            shepherd-configuration?
+            shepherd-configuration-shepherd
+            shepherd-configuration-services
+
+            shepherd-root-service-type
             %shepherd-root-service
             shepherd-service-type
 
@@ -76,7 +82,18 @@ (define-module (gnu services shepherd)
 ;;; Code:
 
 
-(define (shepherd-boot-gexp services)
+(define-record-type* <shepherd-configuration>
+  shepherd-configuration make-shepherd-configuration
+  shepherd-configuration?
+  (shepherd shepherd-configuration-shepherd
+            (default shepherd)) ; package
+  (services shepherd-configuration-services
+            (default '()))) ; list of <shepherd-service>
+
+(define (shepherd-boot-gexp config)
+  "Return a gexp starting the shepherd service."
+  (let ((shepherd (shepherd-configuration-shepherd config))
+        (services (shepherd-configuration-services config)))
   #~(begin
       ;; Keep track of the booted system.
       (false-if-exception (delete-file "/run/booted-system"))
@@ -95,7 +112,10 @@ (define (shepherd-boot-gexp services)
       ;; Start shepherd.
       (execl #$(file-append shepherd "/bin/shepherd")
              "shepherd" "--config"
-             #$(shepherd-configuration-file services))))
+             #$(shepherd-configuration-file services shepherd)))))
+
+(define shepherd-packages
+  (compose list shepherd-configuration-shepherd))
 
 (define shepherd-root-service-type
   (service-type
@@ -103,20 +123,25 @@ (define shepherd-root-service-type
    ;; Extending the root shepherd service (aka. PID 1) happens by
    ;; concatenating the list of services provided by the extensions.
    (compose concatenate)
-   (extend append)
+   (extend (lambda (config extra-services)
+             (shepherd-configuration
+               (inherit config)
+               (services (append (shepherd-configuration-services config)
+                                 extra-services)))))
    (extensions (list (service-extension boot-service-type
                                         shepherd-boot-gexp)
                      (service-extension profile-service-type
-                                        (const (list shepherd)))))
+                                        shepherd-packages)))
+   (default-value (shepherd-configuration))
    (description
     "Run the GNU Shepherd as PID 1---i.e., the operating system's first
 process.  The Shepherd takes care of managing services such as daemons by
 ensuring they are started and stopped in the right order.")))
 
 (define %shepherd-root-service
-  ;; The root shepherd service, aka. PID 1.  Its parameter is a list of
-  ;; <shepherd-service> objects.
-  (service shepherd-root-service-type '()))
+  ;; The root shepherd service, aka. PID 1.  Its parameter is a
+  ;; <shepherd-configuration>.
+  (service shepherd-root-service-type))
 
 (define-syntax shepherd-service-type
   (syntax-rules (description)
@@ -270,9 +295,9 @@ (define (shepherd-service-file service)
                                    #~(#$name #$doc #$proc)))
                                 (shepherd-service-actions service))))))))
 
-(define (scm->go file)
+(define (scm->go file shepherd)
   "Compile FILE, which contains code to be loaded by shepherd's config file,
-and return the resulting '.go' file."
+and return the resulting '.go' file. SHEPHERD is used as shepherd package."
   (let-system (system target)
     (with-extensions (list shepherd)
       (computed-file (string-append (basename (scheme-file-name file) ".scm")
@@ -294,11 +319,13 @@ (define (scm->go file)
                      #:options '(#:local-build? #t
                                  #:substitutable? #f)))))
 
-(define (shepherd-configuration-file services)
-  "Return the shepherd configuration file for SERVICES."
+(define (shepherd-configuration-file services shepherd)
+  "Return the shepherd configuration file for SERVICES.  SHEPHERD is used
+as shepherd package."
   (assert-valid-graph services)
 
-  (let ((files (map shepherd-service-file services)))
+  (let ((files (map shepherd-service-file services))
+        (scm->go (cute scm->go <> shepherd)))
     (define config
       #~(begin
           (use-modules (srfi srfi-34)

gnu/system.scm

@@ -12,6 +12,7 @@
 ;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2020 Jan (janneke) Nieuwenhuizen <jannek@gnu.org>
 ;;; Copyright © 2020 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -1134,10 +1135,11 @@ (define (operating-system-user-accounts os)
 (define (operating-system-shepherd-service-names os)
   "Return the list of Shepherd service names for OS."
   (append-map shepherd-service-provision
-              (service-value
-               (fold-services (operating-system-services os)
-                              #:target-type
-                              shepherd-root-service-type))))
+              (shepherd-configuration-services
+               (service-value
+                (fold-services (operating-system-services os)
+                               #:target-type
+                               shepherd-root-service-type)))))
 
 (define* (operating-system-derivation os)
   "Return a derivation that builds OS."

guix/scripts/system.scm

@@ -915,7 +915,8 @@ (define* (export-shepherd-graph os port
   (let* ((services  (operating-system-services os))
          (pid1      (fold-services services
                                    #:target-type shepherd-root-service-type))
-         (shepherds (service-value pid1))         ;list of <shepherd-service>
+         ;; Get the list of <shepherd-service>.
+         (shepherds (shepherd-configuration-services (service-value pid1)))
          (sinks     (filter (lambda (service)
                               (null? (shepherd-service-requirement service)))
                             shepherds)))