h2: Guard against KeyError for data received on a closed stream

[gonzageraci]

What

The HTTP/2 protocol crashes the connection with a KeyError when a DataReceived event arrives for a stream that is no longer tracked in self.streams.

Why

When an application sends its complete response and the stream is closed (removed from self.streams via _close_stream) before the client has finished sending the request body, the h2 state machine can still emit DataReceived events for that stream. The DataReceived branch in _handle_events indexes self.streams[event.stream_id] without guarding for a missing key, so the lookup raises KeyError and the connection task dies with Unhandled exception in client_connected_cb.

This is observed in production behind an HTTP/2 load balancer:

  File ".../hypercorn/protocol/h2.py", line 258, in _handle_events
    await self.streams[event.stream_id].handle(
KeyError: 1

The adjacent StreamEnded branch already guards the same lookup with a try/except KeyError for exactly this "response sent before full request received" case; the DataReceived branch was missing the equivalent guard.

Change

Wrap the DataReceived stream dispatch in try/except KeyError, mirroring the existing StreamEnded handling. Connection-level flow control is still acknowledged via acknowledge_received_data, since the stream remains valid at the h2 layer.

Added a regression test that reproduces the race (stream closed, then body data received) and asserts the connection no longer raises. Without the fix the test fails with KeyError: 1; with it the full tests/protocol/test_h2.py suite passes, and black, isort, flake8 and mypy are clean.