Preserve flow-control windows when rejecting local increments

Signed-off-by: Xucheng Zhou <aden1350@outlook.com>

Author: adenzhou1350

src/h2/windows.py

@@ -64,12 +64,13 @@ def window_opened(self, size: int) -> None:
         :returns: Nothing
         :rtype: ``None``
         """
-        self.current_window_size += size
+        new_window_size = self.current_window_size + size
 
-        if self.current_window_size > LARGEST_FLOW_CONTROL_WINDOW:
+        if new_window_size > LARGEST_FLOW_CONTROL_WINDOW:
             msg = f"Flow control window mustn't exceed {LARGEST_FLOW_CONTROL_WINDOW}"
             raise FlowControlError(msg)
 
+        self.current_window_size = new_window_size
         self.max_window_size = max(self.current_window_size, self.max_window_size)
 
     def process_bytes(self, size: int) -> int | None:

tests/test_flow_control_window.py

@@ -650,6 +650,29 @@ def test_reject_local_overlarge_increase_stream_window(self) -> None:
         with pytest.raises(h2.exceptions.FlowControlError):
             c.increment_flow_control_window(increment=increment, stream_id=1)
 
+    @pytest.mark.parametrize("stream_id", [None, 1])
+    def test_rejected_local_window_increase_preserves_window(self, frame_factory, stream_id: int | None) -> None:
+        """
+        Rejecting a local increase must not change the advertised window or
+        prevent a subsequent valid increase.
+        """
+        c = h2.connection.H2Connection()
+        c.initiate_connection()
+        c.send_headers(1, self.example_request_headers)
+        c.clear_outbound_data_buffer()
+        before = c.inbound_flow_control_window
+
+        with pytest.raises(h2.exceptions.FlowControlError):
+            c.increment_flow_control_window(2**31 - before, stream_id=stream_id)
+
+        assert c.inbound_flow_control_window == before
+        assert c.data_to_send() == b""
+        c.increment_flow_control_window(1, stream_id=stream_id)
+        expected = frame_factory.build_window_update_frame(
+            stream_id=stream_id or 0, increment=1,
+        )
+        assert c.data_to_send() == expected.serialize()
+
     def test_send_update_on_closed_streams(self, frame_factory) -> None:
         c = h2.connection.H2Connection()
         c.initiate_connection()