Remove consumed frames in place

Deleting the consumed prefix preserves the bytearray's amortized left-delete behavior instead of copying the entire remaining buffer after every frame.

Closes #474

Author: utkarshalpha

src/h2/frame_buffer.py

@@ -155,7 +155,7 @@ def __next__(self) -> Frame:
 
         # At this point, as we know we'll use or discard the entire frame, we
         # can update the data.
-        self._data = self._data[9+length:]
+        del self._data[:9+length]
 
         # Pass the frame through the header buffer.
         new_frame = self._update_header_buffer(f)

tests/test_basic_logic.py

@@ -23,6 +23,20 @@
 from . import helpers
 
 
+class TestFrameBuffer:
+    def test_consumed_frames_are_removed_in_place(self) -> None:
+        frame = hyperframe.frame.SettingsFrame(0).serialize()
+        buffer = h2.frame_buffer.FrameBuffer()
+        buffer.max_frame_size = 65535
+        buffer.add_data(frame * 2)
+        data = buffer._data
+
+        next(buffer)
+
+        assert buffer._data is data
+        assert buffer._data == frame
+
+
 class TestBasicClient:
     """
     Basic client-side tests.