feat: allow disabling response stream pre-fetch

google/api_core/grpc_helpers.py

@@ -62,14 +62,15 @@ def error_remapped_callable(*args, **kwargs):
 
 
 class _StreamingResponseIterator(grpc.Call):
-    def __init__(self, wrapped):
+    def __init__(self, wrapped, prefetch_first_result=True):
         self._wrapped = wrapped
 
         # This iterator is used in a retry context, and returned outside after init.
         # gRPC will not throw an exception until the stream is consumed, so we need
         # to retrieve the first result, in order to fail, in order to trigger a retry.
         try:
-            self._stored_first_result = six.next(self._wrapped)
+            if prefetch_first_result:
+                self._stored_first_result = six.next(self._wrapped)
         except TypeError:
             # It is possible the wrapped method isn't an iterable (a grpc.Call
             # for instance). If this happens don't store the first result.
@@ -141,7 +142,12 @@ def _wrap_stream_errors(callable_):
     def error_remapped_callable(*args, **kwargs):
         try:
             result = callable_(*args, **kwargs)
-            return _StreamingResponseIterator(result)
+            # Auto-fetching the first result causes PubSub client's streaming pull
+            # to hang when re-opening the stream, thus we need examine the hacky
+            # hidden flag to see if pre-fetching is disabled.
+            # https://github.com/googleapis/python-pubsub/issues/93#issuecomment-630762257
+            prefetch_first = getattr(callable_, "_prefetch_first_result_", True)
+            return _StreamingResponseIterator(result, prefetch_first_result=prefetch_first)
         except grpc.RpcError as exc:
             six.raise_from(exceptions.from_grpc_error(exc), exc)
 

tests/unit/test_grpc_helpers.py

@@ -80,6 +80,19 @@ def test_wrap_stream_okay():
     assert responses == expected_responses
 
 
+def test_wrap_stream_prefetch_disabled():
+    responses = [1, 2, 3]
+    iter_responses = iter(responses)
+    callable_ = mock.Mock(spec=["__call__"], return_value=iter_responses)
+    callable_._prefetch_first_result_ = False
+
+    wrapped_callable = grpc_helpers._wrap_stream_errors(callable_)
+    wrapped_callable(1, 2, three="four")
+
+    assert list(iter_responses) == responses  # no items should have been pre-fetched
+    callable_.assert_called_once_with(1, 2, three="four")
+
+
 def test_wrap_stream_iterable_iterface():
     response_iter = mock.create_autospec(grpc.Call, instance=True)
     callable_ = mock.Mock(spec=["__call__"], return_value=response_iter)