Minimal change to push back to dofn generating output if the sdk is producing faster than runner is consuming.

Author: scwhittle

sdks/python/apache_beam/runners/worker/data_plane.py

@@ -456,7 +456,8 @@ class _GrpcDataChannel(DataChannel):
   def __init__(self, data_buffer_time_limit_ms=0):
     # type: (int) -> None
     self._data_buffer_time_limit_ms = data_buffer_time_limit_ms
-    self._to_send = queue.Queue()  # type: queue.Queue[DataOrTimers]
+    self._to_send = queue.Queue(maxsize=10)  # type: queue.Queue[DataOrTimers]
+    _LOGGER.info('DO_NOT_SUBMIT Using to_send maxsize=10')
     self._received = collections.defaultdict(
         lambda: queue.Queue(maxsize=5)
     )  # type: DefaultDict[str, queue.Queue[DataOrTimers]]