[mpd] implement plchanges to report all songs in the queue as changed

(ignores the playlist version parameter).

This is necessary in order to get mpd clients (e. g. sonata) relying on
this command to display the queue.

src/mpd.c

@@ -1797,10 +1797,49 @@ mpd_command_playlistinfo(struct evbuffer *evbuf, int argc, char **argv, char **e
   return 0;
 }
 
+/*
+ * Command handler function for 'plchanges'
+ * Lists all changed songs in the queue since the given playlist version in argv[1].
+ */
 static int
 mpd_command_plchanges(struct evbuffer *evbuf, int argc, char **argv, char **errmsg)
 {
-  DPRINTF(E_WARN, L_MPD, "Ignore command %s\n", argv[0]);
+  struct player_queue *queue;
+  int pos_pl;
+  int i;
+  int ret;
+
+  /*
+   * forked-daapd does not keep track of changes in the queue based on the playlist version,
+   * therefor plchanges returns all songs in the queue as changed ignoring the given version.
+   */
+  queue = player_queue_get(0, -1, 0);
+
+  if (!queue)
+    {
+      // Queue is emtpy
+      return 0;
+    }
+
+  pos_pl = queue->start_pos;
+  for (i = 0; i < queue->count; i++)
+    {
+      ret = mpd_add_mediainfo_byid(evbuf, queue->queue[i], pos_pl);
+      if (ret < 0)
+	{
+	  ret = asprintf(errmsg, "Error adding media info for file with id: %d", queue->queue[i]);
+
+	  queue_free(queue);
+
+	  if (ret < 0)
+	    DPRINTF(E_LOG, L_MPD, "Out of memory\n");
+	  return ACK_ERROR_UNKNOWN;
+	}
+
+      pos_pl++;
+    }
+
+  queue_free(queue);
   return 0;
 }