fix fragmentation edge case on empty payload

Author: jell-o-fishi

rsocket/frame_fragmenter.py

@@ -38,6 +38,10 @@ def _get_next_fragment_body_size(self) -> int:
 
     def __iter__(self):
 
+        if self._data_length == 0 and self._metadata_length == 0:
+            yield Fragment(None, None, is_last=True, is_first=True)
+            return
+
         data_reader = BytesIO(self.data)
         metadata_reader = BytesIO(self.metadata)
 

tests/rsocket/test_fragments.py

@@ -15,6 +15,7 @@ def test_create_data():
 
 
 @pytest.mark.parametrize('data, metadata, fragment_size_bytes, expected_frame_count', (
+        (b'', b'', 64, 1),  # empty payload
         (b'', create_data(b'123abc456def', 20), 64, 5),  # only data
         (create_data(b'123abc456def', 20), b'', 64, 5),  # only metadata
         (create_data(b'123abc456def', 20), create_data(b'123abc456def', 20, 55), 64, 6),  # metadata fits in first frame

tests/rsocket/test_request_response.py

@@ -112,15 +112,33 @@ async def request_response(self, payload: Payload):
     assert response.metadata == b'(server (client metadata))'
 
 
-async def test_request_response_fragmented(lazy_pipe):
+@pytest.mark.parametrize('data_size_multiplier', (
+        1,
+        10
+))
+async def test_request_response_fragmented(lazy_pipe, data_size_multiplier):
     class Handler(BaseRequestHandler):
         async def request_response(self, request: Payload):
             return future_from_payload(request)
 
     async with lazy_pipe(
             server_arguments={'handler_factory': Handler, 'fragment_size_bytes': 64},
             client_arguments={'fragment_size_bytes': 64}) as (server, client):
-        data = b'dog-dog-dog-dog-dog-dog-dog-dog-dog' * 10
+        data = b'dog-dog-dog-dog-dog-dog-dog-dog-dog' * data_size_multiplier
         response = await client.request_response(Payload(data, b'cat'))
 
         assert response.data == b'data: ' + data
+
+
+async def test_request_response_fragmented_empty_payload(lazy_pipe):
+    class Handler(BaseRequestHandler):
+        async def request_response(self, request: Payload):
+            return create_future(Payload())
+
+    async with lazy_pipe(
+            server_arguments={'handler_factory': Handler, 'fragment_size_bytes': 64},
+            client_arguments={'fragment_size_bytes': 64}) as (server, client):
+        response = await client.request_response(Payload())
+
+        assert response.data == b''
+        assert response.metadata == b''