Wait for the initial SSL handshake to complete.

        If a ``callback`` is given, it will be called with no
        arguments once the handshake is complete; otherwise this
        method returns a `.Future` which will resolve to the
        stream itself after the handshake is complete.

        Once the handshake is complete, information such as
        the peer's certificate and NPN/ALPN selections may be
        accessed on ``self.socket``.

        This method is intended for use on server-side streams
        or after using `IOStream.start_tls`; it should not be used
        with `IOStream.connect` (which already waits for the
        handshake to complete). It may only be called once per stream.

        .. versionadded:: 4.2

        .. versionchanged:: 6.0

           The ``callback`` argument was removed. Use the returned
           `.Future` instead.

def wait_for_handshake(self) -> "Future[SSLIOStream]":
        """Wait for the initial SSL handshake to complete.

        If a ``callback`` is given, it will be called with no
        arguments once the handshake is complete; otherwise this
        method returns a `.Future` which will resolve to the
        stream itself after the handshake is complete.

        Once the handshake is complete, information such as
        the peer's certificate and NPN/ALPN selections may be
        accessed on ``self.socket``.

        This method is intended for use on server-side streams
        or after using `IOStream.start_tls`; it should not be used
        with `IOStream.connect` (which already waits for the
        handshake to complete). It may only be called once per stream.

        .. versionadded:: 4.2

        .. versionchanged:: 6.0

           The ``callback`` argument was removed. Use the returned
           `.Future` instead.

        """
        if self._ssl_connect_future is not None:
            raise RuntimeError("Already waiting")
        future = self._ssl_connect_future = Future()
        if not self._ssl_accepting:
            self._finish_ssl_connect()
        return future