performance: remove consumed frames in place from data buffer (#1321)

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 +155,14 @@ 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:]
+        # Deleting the consumed prefix mutates the bytearray in place instead
+        # of copying the remaining bytes into a new object, as slicing would.
+        # ``del s[i:j]`` is documented for mutable sequences in
+        # https://docs.python.org/3/library/stdtypes.html#mutable-sequence-types
+        # and CPython's bytearray tracks an internal offset (``ob_start`` in
+        # Objects/bytearrayobject.c) that makes repeated deletes from the
+        # front amortized O(1) per byte rather than O(len) per frame.
+        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 +23,24 @@
 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)
+
+        # ``is`` checks object identity (CPython compares ``id(...)`` of both
+        # operands): the buffer must still be the very same bytearray object,
+        # proving the consumed frame was deleted in place rather than the
+        # buffer being replaced by a sliced copy.
+        assert buffer._data is data
+        assert buffer._data == frame
+
+
 class TestBasicClient:
     """
     Basic client-side tests.