services: mpd: Do not hardcode environment variables.

Services should not expect for XDG_RUNTIME_DIR to be set.
Inferring from the past comment, this seemed to resolve an issue when
the service was launched with an _interactive_ user-account,
which tends to have this variable set by the login-manager.

This is not the case for system accounts and setting this variable
results in PulseAudio (launched by the same system user) failing to start
since it attempts to use a nonexistent directory.

Ideally, this service should have a home-service counterpart but the old
behavior can be emulated by setting the environment-variables field to:

(environment-variables
  (list
    (string-append "XDG_RUNTIME_DIR=/run/user/"
                   (number->string (passwd:uid (getpwnam "myuser"))))))

* gnu/services/audio.scm
(mpd-configuration)[environment-variables]: New field.
(mpd-shepherd-service)[start]: Use new field.
* doc/guix.texi (Audio Services)[Music Player Daemon]: Document it.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index cd18959db0..e7d56f2504 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -33199,6 +33199,9 @@ The group to run mpd as.
 This is a list of symbols naming Shepherd services that this service
 will depend on.
 
+@item @code{environment-variables} (default: @code{()}) (type: list-of-string)
+A list of strings specifying environment variables.
+
 @item @code{log-file} (default: @code{"/var/log/mpd/log"}) (type: maybe-string)
 The location of the log file.  Set to @code{syslog} to use the local
 syslog daemon or @code{%unset-value} to omit this directive from the