bug#61570: Backward incompatible changes in mpd-service-type

[Maxim Cournoyer]

Hi!

I wanted to note some findings regarding the improved
mpd-configuration/mdp-service-type (thanks!)
(5c5f0fc1135ff15f9c4adfc5f27eadd9a592b5d1, "services: mpd: Refactor MPD
service.") after having reconfigured my machine yesterday.  I've noted
these two backward incompatible changes:

1. the mixer type must now be a string instead of a symbol, and takes
"none" instead of 'null for the null mixer:

--8<---------------cut here---------------start------------->8---
@@ -239,7 +239,7 @@
                        (mpd-output
                         (name "streaming")
                         (type "httpd")
-                        (mixer-type 'null)
+                        (mixer-type "none")
                         (extra-options
                          `((encoder . "lame")
                            (port    . "6601")
--8<---------------cut here---------------end--------------->8---

It'd be nice if there was some deprecation warning and automatic healing
code for now.  The doc should also be updated, because it still mention
the @code{null} value:

--8<---------------cut here---------------start------------->8---
     ‘mixer-type’ (default: ‘"none"’) (type: string)
          This field accepts a string that specifies which mixer should
          be used for this audio output: the ‘hardware’ mixer, the
          ‘software’ mixer, the ‘null’ mixer (allows setting the volume,
          but with no effect; this can be used as a trick to implement
          an external mixer External Mixer) or no mixer (‘none’).
--8<---------------cut here---------------end--------------->8---

2.  The MPD user appears to be created instead of using an existing
one.  I was using my own account, like this:

--8<---------------cut here---------------start------------->8---
      (service mpd-service-type
               (mpd-configuration
                (user "maxim")
                (address "0.0.0.0")
                (outputs
                 (list (mpd-output)
                       (mpd-output
                        (name "streaming")
                        (type "httpd")
                        (mixer-type "none")
                        (extra-options
                         `((encoder . "lame")
                           (port    . "6601")
                           (bitrate . "320"))))))))
--8<---------------cut here---------------end--------------->8---

and 'guix system reconfigure' is now warning that a user is defined
twice, and my user description following reconfiguration is: "Music
Player Daemon (MPD) user" :-).

Perhaps it should check if a user already exists and not add it if it
does?  Else an error rather than a warning when multiple same-name users
are defined would be more appropriate, I think.

-- 
Thanks,
Maxim