Ignore leftover frames received after the connection is closed

CHANGELOG.rst

@@ -1,6 +1,14 @@
 Release History
 ===============
 
+4.4.2 (unreleased)
+------------------
+
+**Bugfixes**
+
+- Ignore leftover frames received after the connection has closed, instead of
+  raising ``ProtocolError``. Further GOAWAY frames are still processed.
+
 4.4.1 (2026-08-03)
 ------------------
 

src/h2/connection.py

@@ -1541,6 +1541,13 @@ def _receive_frame(self, frame: Frame) -> list[Event]:
         """
         events: list[Event]
         self.config.logger.trace("Received frame: %s", repr(frame))
+        # Leftover frames can still be in the TCP buffer after we closed.
+        # Extra GOAWAYs are already valid; anything else is not a protocol error.
+        if (
+            self.state_machine.state == ConnectionState.CLOSED
+            and not isinstance(frame, GoAwayFrame)
+        ):
+            return []
         try:
             # I don't love using __class__ here, maybe reconsider it.
             frames, events = self._frame_dispatch_table[frame.__class__](frame)

tests/test_basic_logic.py

@@ -1788,6 +1788,31 @@ def test_receiving_goaway_frame(self, frame_factory) -> None:
 
         assert not c.data_to_send()
 
+    def test_frames_after_goaway_are_ignored(self, frame_factory) -> None:
+        """
+        Frames already in the receive buffer after GOAWAY must not raise
+        ProtocolError. Further GOAWAY frames are still accepted.
+        """
+        c = h2.connection.H2Connection(config=self.server_config)
+        c.initiate_connection()
+        c.receive_data(frame_factory.preamble())
+        c.clear_outbound_data_buffer()
+
+        goaway = frame_factory.build_goaway_frame(last_stream_id=0)
+        events = c.receive_data(goaway.serialize())
+        assert len(events) == 1
+        assert isinstance(events[0], h2.events.ConnectionTerminated)
+
+        ping = frame_factory.build_ping_frame(b"\x01\x02\x03\x04\x05\x06\x07\x08")
+        events = c.receive_data(ping.serialize())
+        assert events == []
+        assert not c.data_to_send()
+        assert c.state_machine.state == h2.connection.ConnectionState.CLOSED
+
+        events = c.receive_data(goaway.serialize())
+        assert len(events) == 1
+        assert isinstance(events[0], h2.events.ConnectionTerminated)
+
     def test_receiving_multiple_goaway_frames(self, frame_factory) -> None:
         """
         Multiple GOAWAY frames can be received at once, and are allowed. Each