fix occasional gstreamer hang

The backend would hang unrecoverably in the buffer dequeue; this change seems to
have fixed it. I believe that the problem was caused by the fact that buffers
could be decoded before the "playing" flag was set on the object, leading to
missed buffers when Gstreamer ran fast.
- The main thread started the pipeline and acquired the semaphore.
- The file was decoded very quickly, and _new_buffer was called repeatedly but
  did nothing because self.playing was False.
- Finally, when self.playing became True, the file had already been decoded, so
  the queue was empty. Thus the self.queue.get() blocked forever.
It was a dumb idea to make the modification of the "playing" flag unsynchronized
with respect to its accesses in _pad_added.

README.rst

@@ -64,7 +64,9 @@ Version History
 ---------------
 
 0.6
-  Make FFmpeg timeout more robust. Dump FFmpeg output on timeout.
+  Make FFmpeg timeout more robust.
+  Dump FFmpeg output on timeout.
+  Fix a nondeterministic hang in the Gstreamer backend.
 
 0.5
   Fix crash when FFmpeg fails to report a duration.

audioread/gstdec.py

@@ -106,11 +106,9 @@ class MainLoopThread(threading.Thread):
     def __init__(self):   
         super(MainLoopThread, self).__init__()             
         self.loop = gobject.MainLoop()
-        self.running = False
         self.daemon = True
 
     def run(self):    
-        self.running = True
         self.loop.run()
 
 
@@ -202,13 +200,13 @@ def __init__(self, path):
         self.read_exc = None
 
         # Return as soon as the stream is ready!
+        self.running = True
         self.pipeline.set_state(gst.STATE_PLAYING)
         self.ready_sem.acquire()
         if self.read_exc:
             # An error occurred before the stream became ready.
             self.close(True)
             raise self.read_exc
-        self.running = True
 
 
     # Gstreamer callbacks.
@@ -254,9 +252,9 @@ def _no_more_pads(self, element):
         # Sent when the pads are done adding (i.e., there are no more
         # streams in the file). If we haven't gotten at least one
         # decodable stream, raise an exception.
-        if not self.running and not self._got_a_pad:
+        if not self._got_a_pad:
             self.read_exc = NoStreamError()
-            self.ready_sem.release()
+            self.ready_sem.release()  # No effect if we've already started.
 
     def _new_buffer(self, sink):
         if self.running: