mpd: Make 'decoders' return an empty response to please ncmpcpp

docs/changes.rst

@@ -88,6 +88,10 @@ long time been our most requested feature. Finally, it's here!
 - The MPD command ``list`` will no longer return artist names, album names, or
   dates that are blank.
 
+- The MPD command ``decoders`` will now return an empty response instead of a
+  "not implemented" error to make the ncmpcpp browse view work the first time
+  it is opened.
+
 **MPRIS frontend**
 
 - The MPRIS playlists interface is now supported by our MPRIS frontend. This

mopidy/frontends/mpd/protocol/reflection.py

@@ -1,7 +1,6 @@
 from __future__ import unicode_literals
 
 from mopidy.frontends.mpd.protocol import handle_request, mpd_commands
-from mopidy.frontends.mpd.exceptions import MpdNotImplemented
 
 
 @handle_request(r'^commands$', auth_required=False)
@@ -47,8 +46,15 @@ def decoders(context):
             mime_type: audio/mpeg
             plugin: mpcdec
             suffix: mpc
+
+    *Clarifications:*
+
+    - ncmpcpp asks for decoders the first time you open the browse view. By
+      returning nothing and OK instead of an not implemented error, we avoid
+      "Not implemented" showing up in the ncmpcpp interface, and we get the
+      list of playlists without having to enter the browse interface twice.
     """
-    raise MpdNotImplemented  # TODO
+    return  # TODO
 
 
 @handle_request(r'^notcommands$', auth_required=False)

tests/frontends/mpd/protocol/reflection_test.py

@@ -38,7 +38,7 @@ class ReflectionHandlerTest(protocol.BaseTestCase):
 
     def test_decoders(self):
         self.sendRequest('decoders')
-        self.assertInResponse('ACK [0@0] {} Not implemented')
+        self.assertInResponse('OK')
 
     def test_notcommands_returns_only_kill_and_ok(self):
         response = self.sendRequest('notcommands')