bpd: support decoders command

This uses GStreamer APIs to extract a list of audio decoders and the
relevant MIME types and file extensions. Some clients like ncmpcpp use
this command to fetch a list of supported file extensions.

beetsplug/bpd/__init__.py

@@ -1321,6 +1321,16 @@ class Server(BaseServer):
             u'db_update: ' + six.text_type(int(self.updated_time)),
         )
 
+    def cmd_decoders(self, conn):
+        """Send list of supported decoders and formats."""
+        decoders = self.player.get_decoders()
+        for name, (mimes, exts) in decoders.items():
+            yield u'plugin: {}'.format(name)
+            for ext in exts:
+                yield u'suffix: {}'.format(ext)
+            for mime in mimes:
+                yield u'mime_type: {}'.format(mime)
+
     # Searching.
 
     tagtype_map = {

beetsplug/bpd/gstplayer.py

@@ -215,6 +215,59 @@ class GstPlayer(object):
         while self.playing:
             time.sleep(1)
 
+    def get_decoders(self):
+        return get_decoders()
+
+
+def get_decoders():
+    """Get supported audio decoders from GStreamer.
+    Returns a dict mapping decoder element names to the associated media types
+    and file extensions.
+    """
+    # We only care about audio decoder elements.
+    filt = (Gst.ELEMENT_FACTORY_TYPE_DEPAYLOADER |
+            Gst.ELEMENT_FACTORY_TYPE_DEMUXER |
+            Gst.ELEMENT_FACTORY_TYPE_PARSER |
+            Gst.ELEMENT_FACTORY_TYPE_DECODER |
+            Gst.ELEMENT_FACTORY_TYPE_MEDIA_AUDIO)
+
+    decoders = {}
+    mime_types = set()
+    for f in Gst.ElementFactory.list_get_elements(filt, Gst.Rank.NONE):
+        for pad in f.get_static_pad_templates():
+            if pad.direction == Gst.PadDirection.SINK:
+                caps = pad.static_caps.get()
+                mimes = set()
+                for i in range(caps.get_size()):
+                    struct = caps.get_structure(i)
+                    mime = struct.get_name()
+                    if mime == 'unknown/unknown':
+                        continue
+                    mimes.add(mime)
+                    mime_types.add(mime)
+                if mimes:
+                    decoders[f.get_name()] = (mimes, set())
+
+    # Check all the TypeFindFactory plugin features form the registry. If they
+    # are associated with an audio media type that we found above, get the list
+    # of corresponding file extensions.
+    mime_extensions = {mime: set() for mime in mime_types}
+    for feat in Gst.Registry.get().get_feature_list(Gst.TypeFindFactory):
+        caps = feat.get_caps()
+        if caps:
+            for i in range(caps.get_size()):
+                struct = caps.get_structure(i)
+                mime = struct.get_name()
+                if mime in mime_types:
+                    mime_extensions[mime].update(feat.get_extensions())
+
+    # Fill in the slot we left for file extensions.
+    for name, (mimes, exts) in decoders.items():
+        for mime in mimes:
+            exts.update(mime_extensions[mime])
+
+    return decoders
+
 
 def play_simple(paths):
     """Play the files in paths in a straightforward way, without

test/test_player.py

@@ -44,13 +44,14 @@ def _gstplayer_play(*_):  # noqa: 42
 gstplayer._GstPlayer = mock.MagicMock(
     spec_set=[
         "time", "volume", "playing", "run", "play_file", "pause", "stop",
-        "seek", "play"
+        "seek", "play", "get_decoders",
     ], **{
         'playing': False,
         'volume': 0,
         'time.return_value': (0, 0),
         'play_file.side_effect': _gstplayer_play,
         'play.side_effect': _gstplayer_play,
+        'get_decoders.return_value': {'default': ({'audio/mpeg'}, {'mp3'})},
     })
 gstplayer.GstPlayer = lambda _: gstplayer._GstPlayer
 sys.modules["beetsplug.bpd.gstplayer"] = gstplayer
@@ -879,9 +880,16 @@ class BPDDeviceTest(BPDTestHelper):
 
 class BPDReflectionTest(BPDTestHelper):
     test_implements_reflection = implements({
-            'config', 'commands', 'notcommands', 'urlhandlers',
-            'decoders',
-            }, expectedFailure=True)
+        'config', 'commands', 'notcommands', 'urlhandlers',
+    }, expectedFailure=True)
+
+    def test_cmd_decoders(self):
+        with self.run_bpd() as client:
+            response = client.send_command('decoders')
+        self._assert_ok(response)
+        self.assertEqual('default', response.data['plugin'])
+        self.assertEqual('mp3', response.data['suffix'])
+        self.assertEqual('audio/mpeg', response.data['mime_type'])
 
 
 class BPDPeersTest(BPDTestHelper):