services: mpd: Use proper records for user and group fields.

Deprecate using strings for these fields and prefer user-account
(resp. user-group) instead to avoid duplication within account-service-type.

Fixes #61570 <https://issues.guix.gnu.org/61570>.

* gnu/services/audio.scm (%mpd-user, %mpd-group)
(mpd-serialize-user-account, mpd-serialize-user-group)
(mpd-user-sanitizer, mpd-group-sanitizer): New variables.
(mpd-configuration)[user]: Use user-account as value type.
Sanitize via mpd-user-sanitizer.
[group]: Use user-group as value type.
Sanitize via mpd-group-sanitizer.
(mpd-shepherd-service): Adjust accordingly.
(mpd-accounts): Likewise.
* doc/guix.texi (Audio Services)[Music Player Daemon]: Likewise.

Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>

doc/guix.texi

@@ -6211,7 +6211,7 @@ Transformation Options}) so it should be lossless.
 
 @item --profile=@var{profile}
 @itemx -p @var{profile}
-Create an environment containing the packages installed in @var{profile}. 
+Create an environment containing the packages installed in @var{profile}.
 Use @command{guix package} (@pxref{Invoking guix package}) to create
 and manage profiles.
 
@@ -6657,7 +6657,7 @@ interpreted as packages that will be added to the environment directly.
 
 @item --profile=@var{profile}
 @itemx -p @var{profile}
-Create an environment containing the packages installed in @var{profile}. 
+Create an environment containing the packages installed in @var{profile}.
 Use @command{guix package} (@pxref{Invoking guix package}) to create
 and manage profiles.
 
@@ -12667,7 +12667,7 @@ candidates, and even to test their impact on packages that depend on
 them:
 
 @example
-guix build elogind --with-source=@dots{}/shepherd-0.9.0rc1.tar.gz 
+guix build elogind --with-source=@dots{}/shepherd-0.9.0rc1.tar.gz
 @end example
 
 @dots{} or to build from a checkout in a pristine environment:
@@ -23783,7 +23783,7 @@ created for.
 Restricts all controllers to the specified transport. @code{'dual} means both
 BR/EDR and LE are enabled (if supported by the hardware).
 
-Possible values are: 
+Possible values are:
 
 @itemize @bullet
 @item
@@ -33494,14 +33494,17 @@ Data type representing the configuration of @command{mpd}.
 @item @code{package} (default: @code{mpd}) (type: file-like)
 The MPD package.
 
-@item @code{user} (default: @code{"mpd"}) (type: string)
+@item @code{user} (default: @code{%mpd-user}) (type: user-account)
 The user to run mpd as.
 
-@item @code{group} (default: @code{"mpd"}) (type: string)
+The default @code{%mpd-user} is a system user with the name ``mpd'',
+who is a part of the group @var{group} (see below).
+@item @code{group} (default: @code{%mpd-group}) (type: user-group)
 The group to run mpd as.
 
+The default @code{%mpd-group} is a system group with name ``mpd''.
 @item @code{shepherd-requirement} (default: @code{()}) (type: list-of-symbol)
-This is a list of symbols naming Shepherd services that this service
+A list of symbols naming Shepherd services that this service
 will depend on.
 
 @item @code{environment-variables} (default: @code{("PULSE_CLIENTCONFIG=/etc/pulse/client.conf" "PULSE_CONFIG=/etc/pulse/daemon.conf")}) (type: list-of-strings)
@@ -41215,7 +41218,7 @@ A clause can have one of the following forms:
 (@var{field-name}
  (@var{type} @var{default-value})
  @var{documentation})
- 
+
 (@var{field-name}
  (@var{type} @var{default-value})
  @var{documentation}
@@ -41289,7 +41292,7 @@ A simple serializer procedure could look like this:
 (define (serialize-boolean field-name value)
   (let ((value (if value "true" "false")))
     #~(string-append #$field-name #$value)))
-@end lisp  
+@end lisp
 
 In some cases multiple different configuration records might be defined
 in the same file, but their serializers for the same type might have to
@@ -41307,7 +41310,7 @@ manually specify a custom @var{serializer} for every field.
 
 (define (bar-serialize-string field-name value)
   @dots{})
-  
+
 (define-configuration foo-configuration
   (label
    (string)
@@ -41339,7 +41342,7 @@ macro which is a shorthand of this.
   (field
    (string "test")
    "Some documentation."))
-@end lisp   
+@end lisp
 @end defmac
 
 @defmac define-maybe type
@@ -44145,7 +44148,7 @@ down in its dependency graph.  As it turns out, GLib does not have a
    from /gnu/store/@dots{}-glib-2.62.6/lib/libglib-2.0.so.0
 #1  0x00007ffff608a7d6 in gobject_init_ctor ()
    from /gnu/store/@dots{}-glib-2.62.6/lib/libgobject-2.0.so.0
-#2  0x00007ffff7fe275a in call_init (l=<optimized out>, argc=argc@@entry=1, argv=argv@@entry=0x7fffffffcfd8, 
+#2  0x00007ffff7fe275a in call_init (l=<optimized out>, argc=argc@@entry=1, argv=argv@@entry=0x7fffffffcfd8,
     env=env@@entry=0x7fffffffcfe8) at dl-init.c:72
 #3  0x00007ffff7fe2866 in call_init (env=0x7fffffffcfe8, argv=0x7fffffffcfd8, argc=1, l=<optimized out>)
     at dl-init.c:118
@@ -44174,7 +44177,7 @@ Starting program: /gnu/store/@dots{}-profile/bin/sh -c exec\ inkscape
 #0  g_getenv (variable=variable@@entry=0x7ffff60c7a2e "GOBJECT_DEBUG") at ../glib-2.62.6/glib/genviron.c:252
 #1  0x00007ffff608a7d6 in gobject_init () at ../glib-2.62.6/gobject/gtype.c:4380
 #2  gobject_init_ctor () at ../glib-2.62.6/gobject/gtype.c:4493
-#3  0x00007ffff7fe275a in call_init (l=<optimized out>, argc=argc@@entry=3, argv=argv@@entry=0x7fffffffd088, 
+#3  0x00007ffff7fe275a in call_init (l=<optimized out>, argc=argc@@entry=3, argv=argv@@entry=0x7fffffffd088,
     env=env@@entry=0x7fffffffd0a8) at dl-init.c:72
 @dots{}
 @end example

gnu/services/audio.scm

@@ -140,6 +140,14 @@
 (define list-of-symbol?
   (list-of symbol?))
 
+;; Helpers for deprecated field types, to be removed later.
+(define %lazy-group (make-symbol "%lazy-group"))
+
+(define (%set-user-group user group)
+  (user-account
+   (inherit user)
+   (group (user-group-name group))))
+
 
 ;;;
 ;;; MPD
@@ -164,10 +172,31 @@
 (define (mpd-serialize-list-of-strings field-name value)
   #~(string-append #$@(map (cut mpd-serialize-string field-name <>) value)))
 
+(define (mpd-serialize-user-account field-name value)
+  (mpd-serialize-string field-name (user-account-name value)))
+
+(define (mpd-serialize-user-group field-name value)
+  (mpd-serialize-string field-name (user-group-name value)))
+
 (define-maybe string (prefix mpd-))
 (define-maybe list-of-strings (prefix mpd-))
 (define-maybe boolean (prefix mpd-))
 
+(define %mpd-user
+  (user-account
+      (name "mpd")
+      (group %lazy-group)
+      (system? #t)
+      (comment "Music Player Daemon (MPD) user")
+      ;; MPD can use $HOME (or $XDG_CONFIG_HOME) to place its data
+      (home-directory "/var/lib/mpd")
+      (shell (file-append shadow "/sbin/nologin"))))
+
+(define %mpd-group
+  (user-group
+   (name "mpd")
+   (system? #t)))
+
 ;;; TODO: Procedures for deprecated fields, to be removed.
 
 (define mpd-deprecated-fields '((music-dir . music-directory)
@@ -197,6 +226,33 @@
 
 (define-maybe port (prefix mpd-))
 
+;;; Procedures for unsupported value types, to be removed.
+
+(define (mpd-user-sanitizer value)
+  (cond ((user-account? value) value)
+        ((string? value)
+         (warning (G_ "string value for 'user' is deprecated, use \
+user-account instead~%"))
+         (user-account
+          (inherit %mpd-user)
+          (name value)
+          ;; XXX: This is to be lazily substituted in (…-accounts)
+          ;; with the value from 'group'.
+          (group %lazy-group)))
+        (else
+         (configuration-field-error #f 'user value))))
+
+(define (mpd-group-sanitizer value)
+  (cond ((user-group? value) value)
+        ((string? value)
+         (warning (G_ "string value for 'group' is deprecated, use \
+user-group instead~%"))
+         (user-group
+          (inherit %mpd-group)
+          (name value)))
+        (else
+         (configuration-field-error #f 'group value))))
+
 ;;;
 
 ;; Generic MPD plugin record, lists only the most prevalent fields.
@@ -347,12 +403,14 @@ to be appended to the audio output configuration.")
    empty-serializer)
 
   (user
-   (string "mpd")
+   (user-account %mpd-user)
-   "The user to run mpd as.")
+   "The user to run mpd as."
+   (sanitizer mpd-user-sanitizer))
 
   (group
-   (string "mpd")
+   (user-group %mpd-group)
-   "The group to run mpd as.")
+   "The group to run mpd as."
+   (sanitizer mpd-group-sanitizer))
 
   (shepherd-requirement
    (list-of-symbol '())
@@ -517,7 +575,8 @@ appended to the configuration.")
                                             log-file playlist-directory
                                             db-file state-file sticker-file
                                             environment-variables)
-    (let* ((config-file (mpd-serialize-configuration config)))
+    (let ((config-file (mpd-serialize-configuration config))
+          (username (user-account-name user)))
       (shepherd-service
        (documentation "Run the MPD (Music Player Daemon)")
        (requirement `(user-processes loopback ,@shepherd-requirement))
@@ -526,7 +585,7 @@ appended to the configuration.")
                   (and=> #$(maybe-value log-file)
                          (compose mkdir-p dirname))
 
-                  (let ((user (getpw #$user)))
+                  (let ((user (getpw #$username)))
                     (for-each
                      (lambda (x)
                        (when (and x (not (file-exists? x)))
@@ -560,17 +619,11 @@ appended to the configuration.")
 
 (define (mpd-accounts config)
   (match-record config <mpd-configuration> (user group)
-    (list (user-group
+    ;; TODO: Deprecation code, to be removed.
-           (name group)
+    (let ((user (if (eq? (user-account-group user) %lazy-group)
-           (system? #t))
+                    (%set-user-group user group)
-          (user-account
+                    user)))
-           (name user)
+      (list user group))))
-           (group group)
-           (system? #t)
-           (comment "Music Player Daemon (MPD) user")
-           ;; MPD can use $HOME (or $XDG_CONFIG_HOME) to place its data
-           (home-directory "/var/lib/mpd")
-           (shell (file-append shadow "/sbin/nologin"))))))
 
 (define mpd-service-type
   (service-type