async def close_connection(self) -> None:
        """
        7.1.1. Close the WebSocket Connection

        When the opening handshake succeeds, :meth:`connection_open` starts
        this coroutine in a task. It waits for the data transfer phase to
        complete then it closes the TCP connection cleanly.

        When the opening handshake fails, :meth:`fail_connection` does the
        same. There's no data transfer phase in that case.

        """
        try:
            # Wait for the data transfer phase to complete.
            if hasattr(self, "transfer_data_task"):
                try:
                    await self.transfer_data_task
                except asyncio.CancelledError:
                    pass

            # Cancel the keepalive ping task.
            if hasattr(self, "keepalive_ping_task"):
                self.keepalive_ping_task.cancel()

            # A client should wait for a TCP close from the server.
            if self.is_client and hasattr(self, "transfer_data_task"):
                if await self.wait_for_connection_lost():
                    return
                logger.debug("%s ! timed out waiting for TCP close", self.side)

            # Half-close the TCP connection if possible (when there's no TLS).
            if self.writer.can_write_eof():
                logger.debug("%s x half-closing TCP connection", self.side)
                self.writer.write_eof()

                if await self.wait_for_connection_lost():
                    return
                logger.debug("%s ! timed out waiting for TCP close", self.side)

        finally:
            # The try/finally ensures that the transport never remains open,
            # even if this coroutine is canceled (for example).

            # If connection_lost() was called, the TCP connection is closed.
            # However, if TLS is enabled, the transport still needs closing.
            # Else asyncio complains: ResourceWarning: unclosed transport.
            if self.connection_lost_waiter.done() and not self.secure:
                return

            # Close the TCP connection. Buffers are flushed asynchronously.
            logger.debug("%s x closing TCP connection", self.side)
            self.writer.close()

            if await self.wait_for_connection_lost():
                return
            logger.debug("%s ! timed out waiting for TCP close", self.side)

            # Abort the TCP connection. Buffers are discarded.
            logger.debug("%s x aborting TCP connection", self.side)
            # mypy thinks self.writer.transport is a BaseTransport, not a Transport.
            self.writer.transport.abort()  # type: ignore

            # connection_lost() is called quickly after aborting.
            await self.wait_for_connection_lost()
7.1.1. Close the WebSocket Connection

        When the opening handshake succeeds, :meth:`connection_open` starts
        this coroutine in a task. It waits for the data transfer phase to
        complete then it closes the TCP connection cleanly.

        When the opening handshake fails, :meth:`fail_connection` does the
        same. There's no data transfer phase in that case.