[bug#54986] [PATCH v2 3/3 WIP] services: mpd: Support socket activation.

[Liliana Marie Prikler <liliana.prikler@gmail.com>]

* gnu/services/audio.scm (<mpd-configuration>)[shepherd-endpoints]: New field.
(mpd-shepherd-service): Use it.
* doc/guix.texi (Music Player Daemon): Document it.
---
 doc/guix.texi          |  7 +++++++
 gnu/services/audio.scm | 45 ++++++++++++++++++++++++++++++------------
 2 files changed, 39 insertions(+), 13 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 38aeda1d2d..6bfa854b1d 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -30883,6 +30883,13 @@ The port to run mpd on.
 The address that mpd will bind to.  To use a Unix domain socket,
 an absolute path can be specified here.
 
+@item @code{shepherd-endpoints} (default: @code{'()})
+The endpoints shepherd shall bind and spawn MPD from.  If this field is
+not empty (checked via @code{null?}), a systemd-style service is used
+rather than a forkexec-service.  This delays the start of MPD until the
+first client connects.  As a side effect @code{port} and @code{address}
+may be ignored.
+
 @item @code{outputs} (default: @code{"(list (mpd-output))"})
 The audio outputs that MPD can use.  By default this is a single output using pulseaudio.
 
diff --git a/gnu/services/audio.scm b/gnu/services/audio.scm
index c60053f33c..b184ac596a 100644
--- a/gnu/services/audio.scm
+++ b/gnu/services/audio.scm
@@ -78,6 +78,8 @@ (define-record-type* <mpd-configuration>
                 (default "6600"))
   (address      mpd-configuration-address
                 (default "any"))
+  (shepherd-endpoints mpd-configuration-shepherd-endpoints
+                      (default '())) ; list of <shepherd-endpoint>
   (outputs      mpd-configuration-outputs
                 (default (list (mpd-output)))))
 
@@ -140,19 +142,36 @@ (define (mpd-shepherd-service config)
    (documentation "Run the MPD (Music Player Daemon)")
    (requirement '(user-processes))
    (provision '(mpd))
-   (start #~(make-forkexec-constructor
-             (list #$(file-append mpd "/bin/mpd")
-                   "--no-daemon"
-                   #$(mpd-config->file config))
-             #:environment-variables
-             ;; Required to detect PulseAudio when run under a user account.
-             (list (string-append
-                    "XDG_RUNTIME_DIR=/run/user/"
-                    (number->string
-                     (passwd:uid
-                      (getpwnam #$(mpd-configuration-user config))))))
-             #:log-file #$(mpd-file-name config "log")))
-   (stop  #~(make-kill-destructor))))
+   (start (if (null? (mpd-configuration-shepherd-endpoints config))
+              #~(make-forkexec-constructor
+                 (list #$(file-append mpd "/bin/mpd")
+                       "--no-daemon"
+                       #$(mpd-config->file config))
+                 #:environment-variables
+                 ;; Required to detect PulseAudio when run under a user account.
+                 (list (string-append
+                        "XDG_RUNTIME_DIR=/run/user/"
+                        (number->string
+                         (passwd:uid
+                          (getpwnam #$(mpd-configuration-user config))))))
+                 #:log-file #$(mpd-file-name config "log"))
+              #~(make-systemd-constructor
+                 (list #$(file-append mpd "/bin/mpd")
+                       "--systemd"
+                       #$(mpd-config->file config))
+                 (list #$@(map shepherd-endpoint->sexp
+                               (mpd-configuration-shepherd-endpoints config)))
+                 #:environment-variables
+                 ;; Required to detect PulseAudio when run under a user account.
+                 (list (string-append
+                        "XDG_RUNTIME_DIR=/run/user/"
+                        (number->string
+                         (passwd:uid
+                          (getpwnam #$(mpd-configuration-user config))))))
+                 #:log-file #$(mpd-file-name config "log"))))
+   (stop  (if (null? (mpd-configuration-shepherd-endpoints config))
+              #~(make-kill-destructor)
+              #~(make-systemd-destructor)))))
 
 (define (mpd-service-activation config)
   (with-imported-modules '((guix build utils))
-- 
2.35.1