services: mpd: Always create the "mpd" user account.

* gnu/services/audio.scm (%mpd-accounts): New variable.
(mpd-service-type)[extensions]: Add ACCOUNT-SERVICE-TYPE extension.

gnu/services/audio.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017 Peter Mikkelsen <petermikkelsen10@gmail.com>
 ;;; Copyright © 2019 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2020 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -21,6 +22,8 @@
   #:use-module (guix gexp)
   #:use-module (gnu services)
   #:use-module (gnu services shepherd)
+  #:use-module (gnu system shadow)
+  #:use-module (gnu packages admin)
   #:use-module (gnu packages mpd)
   #:use-module (guix records)
   #:use-module (ice-9 match)
@@ -162,6 +165,22 @@ audio_output {
           (mkdir-p directory)
           (chown directory (passwd:uid %user) (passwd:gid %user))))))
 
+
+(define %mpd-accounts
+  ;; Default account and group for MPD.
+  (list (user-group (name "mpd") (system? #t))
+        (user-account
+         (name "mpd")
+         (group "mpd")
+         (system? #t)
+         (comment "Music Player Daemon (MPD) user")
+
+         ;; Note: /var/run/mpd hosts one sub-directory per user, of which
+         ;; /var/run/mpd/mpd corresponds to the "mpd" user.
+         (home-directory "/var/run/mpd/mpd")
+
+         (shell (file-append shadow "/sbin/nologin")))))
+
 (define mpd-service-type
   (service-type
    (name 'mpd)
@@ -169,6 +188,8 @@ audio_output {
    (extensions
     (list (service-extension shepherd-root-service-type
                              (compose list mpd-shepherd-service))
+          (service-extension account-service-type
+                             (const %mpd-accounts))
           (service-extension activation-service-type
                              mpd-service-activation)))
    (default-value (mpd-configuration))))