Fix another race.

on_transmit() buffer mutation must be linearized with enqueue() buffer
mutation.

econtext/core.py

@@ -347,7 +347,11 @@ class Stream(BasicStream):
         """Transmit buffered messages."""
         IOLOG.debug('%r.on_transmit()', self)
         written = os.write(self.write_side.fd, self._output_buf[:4096])
-        self._output_buf = self._output_buf[written:]
+        self._lock.acquire()
+        try:
+            self._output_buf = self._output_buf[written:]
+        finally:
+            self._lock.release()
         if (not self._output_buf) and not self._context.broker.graceful_count:
             self.on_disconnect()
 
@@ -357,10 +361,10 @@ class Stream(BasicStream):
     def enqueue(self, handle, obj):
         """Enqueue `obj` to `handle`, and tell the broker we have output."""
         IOLOG.debug('%r.enqueue(%r, %r)', self, handle, obj)
+        encoded = cPickle.dumps((handle, obj), protocol=2)
+        msg = struct.pack('>L', len(encoded)) + encoded
         self._lock.acquire()
         try:
-            encoded = cPickle.dumps((handle, obj), protocol=2)
-            msg = struct.pack('>L', len(encoded)) + encoded
             self._whmac.update(msg)
             self._output_buf += self._whmac.digest() + msg
         finally: