bug#63082: [PATCH 09/17] services: mpd: Let Shepherd effect the user/group change.

[Maxim Cournoyer]

Relates to <https://issues.guix.gnu.org/63082>.

Quoting a MPD developer, regarding MPD's feature to switch user itself:
"that's legacy for the dark ages when proper service managers did not exist"
:-).

* gnu/services/audio.scm (mpd-serialize-user-account)
(mpd-serialize-user-group): Delete procedures.
* gnu/services/audio.scm (mpd-configuration) [user]: Do not serialize.
[group]: Likewise.
(mpd-shepherd-service): Provide the #:user, #:group and #:supplementary-groups
arguments.
(mympd-shepherd-service): Likewise, and remove the '--user' argument.
* doc/guix.texi (Audio Services): Decorate mpd with @command.
---
 doc/guix.texi          |  4 ++--
 gnu/services/audio.scm | 31 ++++++++++++++++++++++---------
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index a71a05bcf3..19320c2185 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -33569,7 +33569,7 @@ Audio Services
 The MPD package.
 
 @item @code{user} (type: user-account)
-The user to run mpd as.
+The user to run @command{mpd} as.
 
 @item @code{group} (default: @code{#f}) (type: boolean)
 Obsolete.  Do not use.
@@ -33612,7 +33612,7 @@ Audio Services
 The location of the sticker database.
 
 @item @code{default-port} (default: @code{6600}) (type: maybe-port)
-The default port to run mpd on.
+The default port to run @command{mpd} on.
 
 @item @code{endpoints} (type: maybe-list-of-strings)
 The addresses that mpd will bind to.  A port different from
diff --git a/gnu/services/audio.scm b/gnu/services/audio.scm
index cccf5c2693..550ccc542c 100644
--- a/gnu/services/audio.scm
+++ b/gnu/services/audio.scm
@@ -3,6 +3,7 @@
 ;;; Copyright © 2019 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2022⁠–⁠2023 Bruno Victal <mirai@makinata.eu>
+;;; Copyright © 2023 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -164,9 +165,6 @@ (define mpd-serialize-boolean mpd-serialize-field)
 (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-maybe string (prefix mpd-))
 (define-maybe list-of-strings (prefix mpd-))
 (define-maybe boolean (prefix mpd-))
@@ -387,10 +385,14 @@ (define-configuration mpd-configuration
    "The MPD package."
    empty-serializer)
 
+  ;; Note: The user and its group are not serialized, otherwise MPD would
+  ;; attempt to switch the user/group itself.  The task of switching the
+  ;; user/group is left to Shepherd instead.
   (user
    (user-account %mpd-user)
-   "The user to run mpd as."
-   (sanitizer mpd-user-sanitizer))
+   "The user to run @command{mpd} as."
+   (sanitizer mpd-user-sanitizer)
+   (serializer empty-serializer))
 
   (group
    (boolean #f)
@@ -454,7 +456,7 @@ (define-configuration mpd-configuration
 
   (default-port
    (maybe-port 6600)
-   "The default port to run mpd on.")
+   "The default port to run @command{mpd} on.")
 
   (endpoints
    maybe-list-of-strings
@@ -595,7 +597,11 @@ (define (mpd-shepherd-service config)
                    (list #$(file-append package "/bin/mpd")
                          "--no-daemon"
                          #$config-file)
-                   #:environment-variables '#$environment-variables)))
+                   #:environment-variables '#$environment-variables
+                   #:user #$username
+                   #:group #$(user-account-group user)
+                   #:supplementary-groups
+                   '#$(user-account-supplementary-groups user))))
        (stop  #~(make-kill-destructor))
        (actions
         (list (shepherd-configuration-action config-file)
@@ -876,12 +882,19 @@ (define (mympd-shepherd-service config)
 
                   (make-forkexec-constructor
                    `(#$(file-append package "/bin/mympd")
-                     "--user" #$username
                      #$@(if (eq? log-to 'syslog) '("--syslog") '())
                      "--workdir" #$work-directory
                      "--cachedir" #$cache-directory)
                    #:environment-variables (list #$log-level*)
-                   #:log-file #$(if (string? log-to) log-to #f))))
+                   #:log-file #$(if (string? log-to) log-to #f)
+                   #:user #$username
+                   ;; Note: the group of the <user-account> record or that of
+                   ;; the <user-group> record can be used interchangeably
+                   ;; here, since they've been synced in the 'mympd-accounts'
+                   ;; procedure.
+                   #:group #$(user-account-group user)
+                   #:supplementary-groups
+                   '#$(user-account-supplementary-groups user))))
        (stop #~(make-kill-destructor))))))
 
 (define (mympd-accounts config)
-- 
2.39.2