Resolve "Default max_queue blocks websocket cancellation with high traffic"

[btschwertfeger]

Problem statement

If max_queue is not set to None and the connection closes while receiving a high volume of messages, it can trigger websockets.asyncio.client.messages.py:Assembler.paused == True. This results in self.pause() blocking termination, as frames are still pending processing. This issue arises only when the connection handles a significant influx of messages between the initiation of cancellation and its final acknowledgment.

Solution proposal

In order to avoid this behavior, we introduce Assembler.prepare_close() to continue any paused processing while setting Assembler.closing=True allows skipping further calls of Assembler.maybe_pause().

  # src.websockets.asyncio.connection.py:915... (see MR diff)
  async with asyncio_timeout_at(self.close_deadline):
+     self.recv_messages.prepare_close()
      await asyncio.shield(self.connection_lost_waiter)

Tasks:

• add proper MR description and reasoning behind the change
• add/update documentation (not needed I guess)
• test the change properly
• remove repro file (after first review)

Closes #1555