Don't send WINDOWUPDATEs for empty DATA frames.

The spec mandates that the minimum size of the window increment in a
WINDOWUPDATE frame be 1. Stop sending the WINDOWUPDATE frame when we
receive an empty DATA frame.

Also, do increment the window size for the connection when data is
received, or we'll stop recieving data at all.

Author: Lukasa

hyper/http20/connection.py

@@ -399,11 +399,14 @@ def _recv_cb(self):
         )
 
         # Maintain our flow control window. We don't care about flow control
-        # really, so increment the window by however much data was sent.
+        # really, so increment the window by however much data was sent unless
+        # no data was sent at all, in which case we don't need to do anything.
         if (isinstance(frame, DataFrame) and
-            not isinstance(frame, HeadersFrame)):
+            not isinstance(frame, HeadersFrame) and
+            len(frame.data)):
             outframe = WindowUpdateFrame(0)
             outframe.window_increment = len(frame.data)
+            self._send_cb(outframe)
 
         # Work out to whom this frame should go.
         if frame.stream_id != 0:

hyper/http20/stream.py

@@ -142,10 +142,12 @@ def listlen(list):
                 )
                 break
 
-            # Increase the window size.
-            w = WindowUpdateFrame(self.stream_id)
-            w.window_increment = len(frame.data)
-            self._data_cb(w)
+            # Increase the window size. Only do this if the data frame contains
+            # actual data.
+            if len(frame.data):
+                w = WindowUpdateFrame(self.stream_id)
+                w.window_increment = len(frame.data)
+                self._data_cb(w)
 
         return b''.join(data)
 

test/test_hyper.py

@@ -842,6 +842,24 @@ def test_closed_connections_are_reset(self):
         assert c._out_flow_control_window == 65535
         assert c._in_flow_control_window == 65535
 
+    def test_connection_doesnt_send_window_update_on_zero_length_data_frame(self):
+        # Prepare a socket with a data frame in it that has no length.
+        sock = DummySocket()
+        sock.buffer = BytesIO(DataFrame(1).serialize())
+        c = HTTP20Connection('www.google.com')
+        c._sock = sock
+
+        # We open a request here just to allocate a stream, but we throw away
+        # the frames it sends.
+        c.request('GET', '/')
+        sock.queue = []
+
+        # Read the frame.
+        c._recv_cb()
+
+        # No frame should have been sent on the connection.
+        assert len(sock.queue) == 0
+
 
 class TestHyperStream(object):
     def test_streams_have_ids(self):