bpd: skipping backwards through zero keeps playing

Previously issuing the 'previous' command when at position 0 on the
playlist would cause bpd to stop playing. MPD instead just restarts the
currently playing song instead, so we now match this behaviour.

beetsplug/bpd/__init__.py

@@ -554,9 +554,8 @@ class BaseServer(object):
         """Step back to the last song."""
         self.current_index = self._prev_idx()
         if self.current_index < 0:
-            return self.cmd_stop(conn)
-        else:
-            return self.cmd_play(conn)
+            self.current_index = 0
+        return self.cmd_play(conn)
 
     def cmd_pause(self, conn, state=None):
         """Set the pause state playback."""

test/test_player.py

@@ -519,7 +519,7 @@ class BPDPlaybackTest(BPDTestHelper):
 
 class BPDControlTest(BPDTestHelper):
     test_implements_control = implements({
-            'pause', 'playid', 'previous', 'seek',
+            'pause', 'playid', 'seek',
             'seekid', 'seekcur', 'stop',
             }, expectedFailure=True)
 
@@ -552,6 +552,23 @@ class BPDControlTest(BPDTestHelper):
         self.assertEqual('2', responses[3].data['Id'])
         self.assertEqual('stop', responses[5].data['state'])
 
+    def test_cmd_previous(self):
+        with self.run_bpd() as client:
+            self._bpd_add(client, self.item1, self.item2)
+            responses = client.send_commands(
+                    ('play', '1'),
+                    ('currentsong',),
+                    ('previous',),
+                    ('currentsong',),
+                    ('previous',),
+                    ('status',),
+                    ('currentsong',))
+        self._assert_ok(*responses)
+        self.assertEqual('2', responses[1].data['Id'])
+        self.assertEqual('1', responses[3].data['Id'])
+        self.assertEqual('play', responses[5].data['state'])
+        self.assertEqual('1', responses[6].data['Id'])
+
 
 class BPDQueueTest(BPDTestHelper):
     test_implements_queue = implements({