# Source code for tornado.websocket

    """Implementation of the WebSocket protocol.

    `WebSockets <http://dev.w3.org/html5/websockets/>`_ allow for bidirectional
    communication between the browser and server. WebSockets are supported in the
    current versions of all major browsers.

    This module implements the final version of the WebSocket protocol as
    defined in `RFC 6455 <http://tools.ietf.org/html/rfc6455>`_.

    .. versionchanged:: 4.0
       Removed support for the draft 76 protocol version.
    """

    import abc
    import asyncio
    import base64
    import hashlib
    import os
    import sys
    import struct
    import tornado
    from urllib.parse import urlparse
    import warnings
    import zlib

    from tornado.concurrent import Future, future_set_result_unless_cancelled
    from tornado.escape import utf8, native_str, to_unicode
    from tornado import gen, httpclient, httputil
    from tornado.ioloop import IOLoop, PeriodicCallback
    from tornado.iostream import StreamClosedError, IOStream
    from tornado.log import gen_log, app_log
    from tornado.netutil import Resolver
    from tornado import simple_httpclient
    from tornado.queues import Queue
    from tornado.tcpclient import TCPClient
    from tornado.util import _websocket_mask

    from typing import (
        TYPE_CHECKING,
        cast,
        Any,
        Optional,
        Dict,
        Union,
        List,
        Awaitable,
        Callable,
        Tuple,
        Type,
    )
    from types import TracebackType

    if TYPE_CHECKING:
        from typing_extensions import Protocol

        # The zlib compressor types aren't actually exposed anywhere
        # publicly, so declare protocols for the portions we use.
        class _Compressor(Protocol):
            def compress(self, data: bytes) -> bytes:
                pass

            def flush(self, mode: int) -> bytes:
                pass

        class _Decompressor(Protocol):
            unconsumed_tail = b""  # type: bytes

            def decompress(self, data: bytes, max_length: int) -> bytes:
                pass

        class _WebSocketDelegate(Protocol):
            # The common base interface implemented by WebSocketHandler on
            # the server side and WebSocketClientConnection on the client
            # side.
            def on_ws_connection_close(
                self, close_code: Optional[int] = None, close_reason: Optional[str] = None
            ) -> None:
                pass

            def on_message(self, message: Union[str, bytes]) -> Optional["Awaitable[None]"]:
                pass

            def on_ping(self, data: bytes) -> None:
                pass

            def on_pong(self, data: bytes) -> None:
                pass

            def log_exception(
                self,
                typ: Optional[Type[BaseException]],
                value: Optional[BaseException],
                tb: Optional[TracebackType],
            ) -> None:
                pass

    _default_max_message_size = 10 * 1024 * 1024

    class WebSocketError(Exception):
        pass

    class WebSocketClosedError(WebSocketError):
        """Raised by operations on a closed connection.

        .. versionadded:: 3.2
        """

        pass

    class _DecompressTooLargeError(Exception):
        pass

    class _WebSocketParams(object):
        def __init__(
            self,
            ping_interval: Optional[float] = None,
            ping_timeout: Optional[float] = None,
            max_message_size: int = _default_max_message_size,
            compression_options: Optional[Dict[str, Any]] = None,
        ) -> None:
            self.ping_interval = ping_interval
            self.ping_timeout = ping_timeout
            self.max_message_size = max_message_size
            self.compression_options = compression_options

    class WebSocketHandler(tornado.web.RequestHandler):
        """Subclass this class to create a basic WebSocket handler.

        Override `on_message` to handle incoming messages, and use
        `write_message` to send messages to the client. You can also
        override `open` and `on_close` to handle opened and closed
        connections.

        Custom upgrade response headers can be sent by overriding
        `~tornado.web.RequestHandler.set_default_headers` or
        `~tornado.web.RequestHandler.prepare`.

        See http://dev.w3.org/html5/websockets/ for details on the
        JavaScript interface.  The protocol is specified at
        http://tools.ietf.org/html/rfc6455.

        Here is an example WebSocket handler that echos back all received messages
        back to the client:

        .. testcode::

          class EchoWebSocket(tornado.websocket.WebSocketHandler):
              def open(self):
                  print("WebSocket opened")

              def on_message(self, message):
                  self.write_message(u"You said: " + message)

              def on_close(self):
                  print("WebSocket closed")

        .. testoutput::
           :hide:

        WebSockets are not standard HTTP connections. The "handshake" is
        HTTP, but after the handshake, the protocol is
        message-based. Consequently, most of the Tornado HTTP facilities
        are not available in handlers of this type. The only communication
        methods available to you are `write_message()`, `ping()`, and
        `close()`. Likewise, your request handler class should implement
        `open()` method rather than ``get()`` or ``post()``.

        If you map the handler above to ``/websocket`` in your application, you can
        invoke it in JavaScript with::

          var ws = new WebSocket("ws://localhost:8888/websocket");
          ws.onopen = function() {
             ws.send("Hello, world");
          };
          ws.onmessage = function (evt) {
             alert(evt.data);
          };

        This script pops up an alert box that says "You said: Hello, world".

        Web browsers allow any site to open a websocket connection to any other,
        instead of using the same-origin policy that governs other network
        access from JavaScript.  This can be surprising and is a potential
        security hole, so since Tornado 4.0 `WebSocketHandler` requires
        applications that wish to receive cross-origin websockets to opt in
        by overriding the `~WebSocketHandler.check_origin` method (see that
        method's docs for details).  Failure to do so is the most likely
        cause of 403 errors when making a websocket connection.

        When using a secure websocket connection (``wss://``) with a self-signed
        certificate, the connection from a browser may fail because it wants
        to show the "accept this certificate" dialog but has nowhere to show it.
        You must first visit a regular HTML page using the same certificate
        to accept it before the websocket connection will succeed.

        If the application setting ``websocket_ping_interval`` has a non-zero
        value, a ping will be sent periodically, and the connection will be
        closed if a response is not received before the ``websocket_ping_timeout``.

        Messages larger than the ``websocket_max_message_size`` application setting
        (default 10MiB) will not be accepted.

        .. versionchanged:: 4.5
           Added ``websocket_ping_interval``, ``websocket_ping_timeout``, and
           ``websocket_max_message_size``.
        """

        def __init__(
            self,
            application: tornado.web.Application,
            request: httputil.HTTPServerRequest,
            **kwargs: Any
        ) -> None:
            super().__init__(application, request, **kwargs)
            self.ws_connection = None  # type: Optional[WebSocketProtocol]
            self.close_code = None  # type: Optional[int]
            self.close_reason = None  # type: Optional[str]
            self._on_close_called = False

        async def get(self, *args: Any, **kwargs: Any) -> None:
            self.open_args = args
            self.open_kwargs = kwargs

            # Upgrade header should be present and should be equal to WebSocket
            if self.request.headers.get("Upgrade", "").lower() != "websocket":
                self.set_status(400)
                log_msg = 'Can "Upgrade" only to "WebSocket".'
                self.finish(log_msg)
                gen_log.debug(log_msg)
                return

            # Connection header should be upgrade.
            # Some proxy servers/load balancers
            # might mess with it.
            headers = self.request.headers
            connection = map(
                lambda s: s.strip().lower(), headers.get("Connection", "").split(",")
            )
            if "upgrade" not in connection:
                self.set_status(400)
                log_msg = '"Connection" must be "Upgrade".'
                self.finish(log_msg)
                gen_log.debug(log_msg)
                return

            # Handle WebSocket Origin naming convention differences
            # The difference between version 8 and 13 is that in 8 the
            # client sends a "Sec-Websocket-Origin" header and in 13 it's
            # simply "Origin".
            if "Origin" in self.request.headers:
                origin = self.request.headers.get("Origin")
            else:
                origin = self.request.headers.get("Sec-Websocket-Origin", None)

            # If there was an origin header, check to make sure it matches
            # according to check_origin. When the origin is None, we assume it
            # did not come from a browser and that it can be passed on.
            if origin is not None and not self.check_origin(origin):
                self.set_status(403)
                log_msg = "Cross origin websockets not allowed"
                self.finish(log_msg)
                gen_log.debug(log_msg)
                return

            self.ws_connection = self.get_websocket_protocol()
            if self.ws_connection:
                await self.ws_connection.accept_connection(self)
            else:
                self.set_status(426, "Upgrade Required")
                self.set_header("Sec-WebSocket-Version", "7, 8, 13")

        @property
        def ping_interval(self) -> Optional[float]:
            """The interval for websocket keep-alive pings.

            Set websocket_ping_interval = 0 to disable pings.
            """
            return self.settings.get("websocket_ping_interval", None)

        @property
        def ping_timeout(self) -> Optional[float]:
            """If no ping is received in this many seconds,
            close the websocket connection (VPNs, etc. can fail to cleanly close ws connections).
            Default is max of 3 pings or 30 seconds.
            """
            return self.settings.get("websocket_ping_timeout", None)

        @property
        def max_message_size(self) -> int:
            """Maximum allowed message size.

            If the remote peer sends a message larger than this, the connection
            will be closed.

            Default is 10MiB.
            """
            return self.settings.get(
                "websocket_max_message_size", _default_max_message_size
            )

        def write_message(
            self, message: Union[bytes, str, Dict[str, Any]], binary: bool = False
        ) -> "Future[None]":
            """Sends the given message to the client of this Web Socket.

            The message may be either a string or a dict (which will be
            encoded as json).  If the ``binary`` argument is false, the
            message will be sent as utf8; in binary mode any byte string
            is allowed.

            If the connection is already closed, raises `WebSocketClosedError`.
            Returns a `.Future` which can be used for flow control.

            .. versionchanged:: 3.2
               `WebSocketClosedError` was added (previously a closed connection
               would raise an `AttributeError`)

            .. versionchanged:: 4.3
               Returns a `.Future` which can be used for flow control.

            .. versionchanged:: 5.0
               Consistently raises `WebSocketClosedError`. Previously could
               sometimes raise `.StreamClosedError`.
            """
            if self.ws_connection is None or self.ws_connection.is_closing():
                raise WebSocketClosedError()
            if isinstance(message, dict):
                message = tornado.escape.json_encode(message)
            return self.ws_connection.write_message(message, binary=binary)

        def select_subprotocol(self, subprotocols: List[str]) -> Optional[str]:
            """Override to implement subprotocol negotiation.

            ``subprotocols`` is a list of strings identifying the
            subprotocols proposed by the client.  This method may be
            overridden to return one of those strings to select it, or
            ``None`` to not select a subprotocol.

            Failure to select a subprotocol does not automatically abort
            the connection, although clients may close the connection if
            none of their proposed subprotocols was selected.

            The list may be empty, in which case this method must return
            None. This method is always called exactly once even if no
            subprotocols were proposed so that the handler can be advised
            of this fact.

            .. versionchanged:: 5.1

               Previously, this method was called with a list containing
               an empty string instead of an empty list if no subprotocols
               were proposed by the client.
            """
            return None

        @property
        def selected_subprotocol(self) -> Optional[str]:
            """The subprotocol returned by `select_subprotocol`.

            .. versionadded:: 5.1
            """
            assert self.ws_connection is not None
            return self.ws_connection.selected_subprotocol

        def get_compression_options(self) -> Optional[Dict[str, Any]]:
            """Override to return compression options for the connection.

            If this method returns None (the default), compression will
            be disabled.  If it returns a dict (even an empty one), it
            will be enabled.  The contents of the dict may be used to
            control the following compression options:

            ``compression_level`` specifies the compression level.

            ``mem_level`` specifies the amount of memory used for the internal compression state.

             These parameters are documented in details here:
             https://docs.python.org/3.6/library/zlib.html#zlib.compressobj

            .. versionadded:: 4.1

            .. versionchanged:: 4.5

               Added ``compression_level`` and ``mem_level``.
            """
            # TODO: Add wbits option.
            return None

        def open(self, *args: str, **kwargs: str) -> Optional[Awaitable[None]]:
            """Invoked when a new WebSocket is opened.

            The arguments to `open` are extracted from the `tornado.web.URLSpec`
            regular expression, just like the arguments to
            `tornado.web.RequestHandler.get`.

            `open` may be a coroutine. `on_message` will not be called until
            `open` has returned.

            .. versionchanged:: 5.1

               ``open`` may be a coroutine.
            """
            pass

        def on_message(self, message: Union[str, bytes]) -> Optional[Awaitable[None]]:
            """Handle incoming messages on the WebSocket

            This method must be overridden.

            .. versionchanged:: 4.5

               ``on_message`` can be a coroutine.
            """
            raise NotImplementedError

        def ping(self, data: Union[str, bytes] = b"") -> None:
            """Send ping frame to the remote end.

            The data argument allows a small amount of data (up to 125
            bytes) to be sent as a part of the ping message. Note that not
            all websocket implementations expose this data to
            applications.

            Consider using the ``websocket_ping_interval`` application
            setting instead of sending pings manually.

            .. versionchanged:: 5.1

               The data argument is now optional.

            """
            data = utf8(data)
            if self.ws_connection is None or self.ws_connection.is_closing():
                raise WebSocketClosedError()
            self.ws_connection.write_ping(data)

        def on_pong(self, data: bytes) -> None:
            """Invoked when the response to a ping frame is received."""
            pass

        def on_ping(self, data: bytes) -> None:
            """Invoked when the a ping frame is received."""
            pass

        def on_close(self) -> None:
            """Invoked when the WebSocket is closed.

            If the connection was closed cleanly and a status code or reason
            phrase was supplied, these values will be available as the attributes
            ``self.close_code`` and ``self.close_reason``.

            .. versionchanged:: 4.0

               Added ``close_code`` and ``close_reason`` attributes.
            """
            pass

        def close(self, code: Optional[int] = None, reason: Optional[str] = None) -> None:
            """Closes this Web Socket.

            Once the close handshake is successful the socket will be closed.

            ``code`` may be a numeric status code, taken from the values
            defined in `RFC 6455 section 7.4.1
            <https://tools.ietf.org/html/rfc6455#section-7.4.1>`_.
            ``reason`` may be a textual message about why the connection is
            closing.  These values are made available to the client, but are
            not otherwise interpreted by the websocket protocol.

            .. versionchanged:: 4.0

               Added the ``code`` and ``reason`` arguments.
            """
            if self.ws_connection:
                self.ws_connection.close(code, reason)
                self.ws_connection = None

        def check_origin(self, origin: str) -> bool:
            """Override to enable support for allowing alternate origins.

            The ``origin`` argument is the value of the ``Origin`` HTTP
            header, the url responsible for initiating this request.  This
            method is not called for clients that do not send this header;
            such requests are always allowed (because all browsers that
            implement WebSockets support this header, and non-browser
            clients do not have the same cross-site security concerns).

            Should return ``True`` to accept the request or ``False`` to
            reject it. By default, rejects all requests with an origin on
            a host other than this one.

            This is a security protection against cross site scripting attacks on
            browsers, since WebSockets are allowed to bypass the usual same-origin
            policies and don't use CORS headers.

            .. warning::

               This is an important security measure; don't disable it
               without understanding the security implications. In
               particular, if your authentication is cookie-based, you
               must either restrict the origins allowed by
               ``check_origin()`` or implement your own XSRF-like
               protection for websocket connections. See `these
               <https://www.christian-schneider.net/CrossSiteWebSocketHijacking.html>`_
               `articles
               <https://devcenter.heroku.com/articles/websocket-security>`_
               for more.

            To accept all cross-origin traffic (which was the default prior to
            Tornado 4.0), simply override this method to always return ``True``::

                def check_origin(self, origin):
                    return True

            To allow connections from any subdomain of your site, you might
            do something like::

                def check_origin(self, origin):
                    parsed_origin = urllib.parse.urlparse(origin)
                    return parsed_origin.netloc.endswith(".mydomain.com")

            .. versionadded:: 4.0

            """
            parsed_origin = urlparse(origin)
            origin = parsed_origin.netloc
            origin = origin.lower()

            host = self.request.headers.get("Host")

            # Check to see that origin matches host directly, including ports
            return origin == host

        def set_nodelay(self, value: bool) -> None:
            """Set the no-delay flag for this stream.

            By default, small messages may be delayed and/or combined to minimize
            the number of packets sent.  This can sometimes cause 200-500ms delays
            due to the interaction between Nagle's algorithm and TCP delayed
            ACKs.  To reduce this delay (at the expense of possibly increasing
            bandwidth usage), call ``self.set_nodelay(True)`` once the websocket
            connection is established.

            See `.BaseIOStream.set_nodelay` for additional details.

            .. versionadded:: 3.1
            """
            assert self.ws_connection is not None
            self.ws_connection.set_nodelay(value)

        def on_connection_close(self) -> None:
            if self.ws_connection:
                self.ws_connection.on_connection_close()
                self.ws_connection = None
            if not self._on_close_called:
                self._on_close_called = True
                self.on_close()
                self._break_cycles()

        def on_ws_connection_close(
            self, close_code: Optional[int] = None, close_reason: Optional[str] = None
        ) -> None:
            self.close_code = close_code
            self.close_reason = close_reason
            self.on_connection_close()

        def _break_cycles(self) -> None:
            # WebSocketHandlers call finish() early, but we don't want to
            # break up reference cycles (which makes it impossible to call
            # self.render_string) until after we've really closed the
            # connection (if it was established in the first place,
            # indicated by status code 101).
            if self.get_status() != 101 or self._on_close_called:
                super()._break_cycles()

        def get_websocket_protocol(self) -> Optional["WebSocketProtocol"]:
            websocket_version = self.request.headers.get("Sec-WebSocket-Version")
            if websocket_version in ("7", "8", "13"):
                params = _WebSocketParams(
                    ping_interval=self.ping_interval,
                    ping_timeout=self.ping_timeout,
                    max_message_size=self.max_message_size,
                    compression_options=self.get_compression_options(),
                )
                return WebSocketProtocol13(self, False, params)
            return None

        def _detach_stream(self) -> IOStream:
            # disable non-WS methods
            for method in [
                "write",
                "redirect",
                "set_header",
                "set_cookie",
                "set_status",
                "flush",
                "finish",
            ]:
                setattr(self, method, _raise_not_supported_for_websockets)
            return self.detach()

    def _raise_not_supported_for_websockets(*args: Any, **kwargs: Any) -> None:
        raise RuntimeError("Method not supported for Web Sockets")

    class WebSocketProtocol(abc.ABC):
        """Base class for WebSocket protocol versions."""

        def __init__(self, handler: "_WebSocketDelegate") -> None:
            self.handler = handler
            self.stream = None  # type: Optional[IOStream]
            self.client_terminated = False
            self.server_terminated = False

        def _run_callback(
            self, callback: Callable, *args: Any, **kwargs: Any
        ) -> "Optional[Future[Any]]":
            """Runs the given callback with exception handling.

            If the callback is a coroutine, returns its Future. On error, aborts the
            websocket connection and returns None.
            """
            try:
                result = callback(*args, **kwargs)
            except Exception:
                self.handler.log_exception(*sys.exc_info())
                self._abort()
                return None
            else:
                if result is not None:
                    result = gen.convert_yielded(result)
                    assert self.stream is not None
                    self.stream.io_loop.add_future(result, lambda f: f.result())
                return result

        def on_connection_close(self) -> None:
            self._abort()

        def _abort(self) -> None:
            """Instantly aborts the WebSocket connection by closing the socket"""
            self.client_terminated = True
            self.server_terminated = True
            if self.stream is not None:
                self.stream.close()  # forcibly tear down the connection
            self.close()  # let the subclass cleanup

        @abc.abstractmethod
        def close(self, code: Optional[int] = None, reason: Optional[str] = None) -> None:
            raise NotImplementedError()

        @abc.abstractmethod
        def is_closing(self) -> bool:
            raise NotImplementedError()

        @abc.abstractmethod
        async def accept_connection(self, handler: WebSocketHandler) -> None:
            raise NotImplementedError()

        @abc.abstractmethod
        def write_message(
            self, message: Union[str, bytes, Dict[str, Any]], binary: bool = False
        ) -> "Future[None]":
            raise NotImplementedError()

        @property
        @abc.abstractmethod
        def selected_subprotocol(self) -> Optional[str]:
            raise NotImplementedError()

        @abc.abstractmethod
        def write_ping(self, data: bytes) -> None:
            raise NotImplementedError()

        # The entry points below are used by WebSocketClientConnection,
        # which was introduced after we only supported a single version of
        # WebSocketProtocol. The WebSocketProtocol/WebSocketProtocol13
        # boundary is currently pretty ad-hoc.
        @abc.abstractmethod
        def _process_server_headers(
            self, key: Union[str, bytes], headers: httputil.HTTPHeaders
        ) -> None:
            raise NotImplementedError()

        @abc.abstractmethod
        def start_pinging(self) -> None:
            raise NotImplementedError()

        @abc.abstractmethod
        async def _receive_frame_loop(self) -> None:
            raise NotImplementedError()

        @abc.abstractmethod
        def set_nodelay(self, x: bool) -> None:
            raise NotImplementedError()

    class _PerMessageDeflateCompressor(object):
        def __init__(
            self,
            persistent: bool,
            max_wbits: Optional[int],
            compression_options: Optional[Dict[str, Any]] = None,
        ) -> None:
            if max_wbits is None:
                max_wbits = zlib.MAX_WBITS
            # There is no symbolic constant for the minimum wbits value.
            if not (8 <= max_wbits <= zlib.MAX_WBITS):
                raise ValueError(
                    "Invalid max_wbits value %r; allowed range 8-%d",
                    max_wbits,
                    zlib.MAX_WBITS,
                )
            self._max_wbits = max_wbits

            if (
                compression_options is None
                or "compression_level" not in compression_options
            ):
                self._compression_level = tornado.web.GZipContentEncoding.GZIP_LEVEL
            else:
                self._compression_level = compression_options["compression_level"]

            if compression_options is None or "mem_level" not in compression_options:
                self._mem_level = 8
            else:
                self._mem_level = compression_options["mem_level"]

            if persistent:
                self._compressor = self._create_compressor()  # type: Optional[_Compressor]
            else:
                self._compressor = None

        def _create_compressor(self) -> "_Compressor":
            return zlib.compressobj(
                self._compression_level, zlib.DEFLATED, -self._max_wbits, self._mem_level
            )

        def compress(self, data: bytes) -> bytes:
            compressor = self._compressor or self._create_compressor()
            data = compressor.compress(data) + compressor.flush(zlib.Z_SYNC_FLUSH)
            assert data.endswith(b"\x00\x00\xff\xff")
            return data[:-4]

    class _PerMessageDeflateDecompressor(object):
        def __init__(
            self,
            persistent: bool,
            max_wbits: Optional[int],
            max_message_size: int,
            compression_options: Optional[Dict[str, Any]] = None,
        ) -> None:
            self._max_message_size = max_message_size
            if max_wbits is None:
                max_wbits = zlib.MAX_WBITS
            if not (8 <= max_wbits <= zlib.MAX_WBITS):
                raise ValueError(
                    "Invalid max_wbits value %r; allowed range 8-%d",
                    max_wbits,
                    zlib.MAX_WBITS,
                )
            self._max_wbits = max_wbits
            if persistent:
                self._decompressor = (
                    self._create_decompressor()
                )  # type: Optional[_Decompressor]
            else:
                self._decompressor = None

        def _create_decompressor(self) -> "_Decompressor":
            return zlib.decompressobj(-self._max_wbits)

        def decompress(self, data: bytes) -> bytes:
            decompressor = self._decompressor or self._create_decompressor()
            result = decompressor.decompress(
                data + b"\x00\x00\xff\xff", self._max_message_size
            )
            if decompressor.unconsumed_tail:
                raise _DecompressTooLargeError()
            return result

    class WebSocketProtocol13(WebSocketProtocol):
        """Implementation of the WebSocket protocol from RFC 6455.

        This class supports versions 7 and 8 of the protocol in addition to the
        final version 13.
        """

        # Bit masks for the first byte of a frame.
        FIN = 0x80
        RSV1 = 0x40
        RSV2 = 0x20
        RSV3 = 0x10
        RSV_MASK = RSV1 | RSV2 | RSV3
        OPCODE_MASK = 0x0F

        stream = None  # type: IOStream

        def __init__(
            self,
            handler: "_WebSocketDelegate",
            mask_outgoing: bool,
            params: _WebSocketParams,
        ) -> None:
            WebSocketProtocol.__init__(self, handler)
            self.mask_outgoing = mask_outgoing
            self.params = params
            self._final_frame = False
            self._frame_opcode = None
            self._masked_frame = None
            self._frame_mask = None  # type: Optional[bytes]
            self._frame_length = None
            self._fragmented_message_buffer = None  # type: Optional[bytearray]
            self._fragmented_message_opcode = None
            self._waiting = None  # type: object
            self._compression_options = params.compression_options
            self._decompressor = None  # type: Optional[_PerMessageDeflateDecompressor]
            self._compressor = None  # type: Optional[_PerMessageDeflateCompressor]
            self._frame_compressed = None  # type: Optional[bool]
            # The total uncompressed size of all messages received or sent.
            # Unicode messages are encoded to utf8.
            # Only for testing; subject to change.
            self._message_bytes_in = 0
            self._message_bytes_out = 0
            # The total size of all packets received or sent.  Includes
            # the effect of compression, frame overhead, and control frames.
            self._wire_bytes_in = 0
            self._wire_bytes_out = 0
            self.ping_callback = None  # type: Optional[PeriodicCallback]
            self.last_ping = 0.0
            self.last_pong = 0.0
            self.close_code = None  # type: Optional[int]
            self.close_reason = None  # type: Optional[str]

        # Use a property for this to satisfy the abc.
        @property
        def selected_subprotocol(self) -> Optional[str]:
            return self._selected_subprotocol

        @selected_subprotocol.setter
        def selected_subprotocol(self, value: Optional[str]) -> None:
            self._selected_subprotocol = value

        async def accept_connection(self, handler: WebSocketHandler) -> None:
            try:
                self._handle_websocket_headers(handler)
            except ValueError:
                handler.set_status(400)
                log_msg = "Missing/Invalid WebSocket headers"
                handler.finish(log_msg)
                gen_log.debug(log_msg)
                return

            try:
                await self._accept_connection(handler)
            except asyncio.CancelledError:
                self._abort()
                return
            except ValueError:
                gen_log.debug("Malformed WebSocket request received", exc_info=True)
                self._abort()
                return

        def _handle_websocket_headers(self, handler: WebSocketHandler) -> None:
            """Verifies all invariant- and required headers

            If a header is missing or have an incorrect value ValueError will be
            raised
            """
            fields = ("Host", "Sec-Websocket-Key", "Sec-Websocket-Version")
            if not all(map(lambda f: handler.request.headers.get(f), fields)):
                raise ValueError("Missing/Invalid WebSocket headers")

        @staticmethod
        def compute_accept_value(key: Union[str, bytes]) -> str:
            """Computes the value for the Sec-WebSocket-Accept header,
            given the value for Sec-WebSocket-Key.
            """
            sha1 = hashlib.sha1()
            sha1.update(utf8(key))
            sha1.update(b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11")  # Magic value
            return native_str(base64.b64encode(sha1.digest()))

        def _challenge_response(self, handler: WebSocketHandler) -> str:
            return WebSocketProtocol13.compute_accept_value(
                cast(str, handler.request.headers.get("Sec-Websocket-Key"))
            )

        async def _accept_connection(self, handler: WebSocketHandler) -> None:
            subprotocol_header = handler.request.headers.get("Sec-WebSocket-Protocol")
            if subprotocol_header:
                subprotocols = [s.strip() for s in subprotocol_header.split(",")]
            else:
                subprotocols = []
            self.selected_subprotocol = handler.select_subprotocol(subprotocols)
            if self.selected_subprotocol:
                assert self.selected_subprotocol in subprotocols
                handler.set_header("Sec-WebSocket-Protocol", self.selected_subprotocol)

            extensions = self._parse_extensions_header(handler.request.headers)
            for ext in extensions:
                if ext[0] == "permessage-deflate" and self._compression_options is not None:
                    # TODO: negotiate parameters if compression_options
                    # specifies limits.
                    self._create_compressors("server", ext[1], self._compression_options)
                    if (
                        "client_max_window_bits" in ext[1]
                        and ext[1]["client_max_window_bits"] is None
                    ):
                        # Don't echo an offered client_max_window_bits
                        # parameter with no value.
                        del ext[1]["client_max_window_bits"]
                    handler.set_header(
                        "Sec-WebSocket-Extensions",
                        httputil._encode_header("permessage-deflate", ext[1]),
                    )
                    break

            handler.clear_header("Content-Type")
            handler.set_status(101)
            handler.set_header("Upgrade", "websocket")
            handler.set_header("Connection", "Upgrade")
            handler.set_header("Sec-WebSocket-Accept", self._challenge_response(handler))
            handler.finish()

            self.stream = handler._detach_stream()

            self.start_pinging()
            try:
                open_result = handler.open(*handler.open_args, **handler.open_kwargs)
                if open_result is not None:
                    await open_result
            except Exception:
                handler.log_exception(*sys.exc_info())
                self._abort()
                return

            await self._receive_frame_loop()

        def _parse_extensions_header(
            self, headers: httputil.HTTPHeaders
        ) -> List[Tuple[str, Dict[str, str]]]:
            extensions = headers.get("Sec-WebSocket-Extensions", "")
            if extensions:
                return [httputil._parse_header(e.strip()) for e in extensions.split(",")]
            return []

        def _process_server_headers(
            self, key: Union[str, bytes], headers: httputil.HTTPHeaders
        ) -> None:
            """Process the headers sent by the server to this client connection.

            'key' is the websocket handshake challenge/response key.
            """
            assert headers["Upgrade"].lower() == "websocket"
            assert headers["Connection"].lower() == "upgrade"
            accept = self.compute_accept_value(key)
            assert headers["Sec-Websocket-Accept"] == accept

            extensions = self._parse_extensions_header(headers)
            for ext in extensions:
                if ext[0] == "permessage-deflate" and self._compression_options is not None:
                    self._create_compressors("client", ext[1])
                else:
                    raise ValueError("unsupported extension %r", ext)

            self.selected_subprotocol = headers.get("Sec-WebSocket-Protocol", None)

        def _get_compressor_options(
            self,
            side: str,
            agreed_parameters: Dict[str, Any],
            compression_options: Optional[Dict[str, Any]] = None,
        ) -> Dict[str, Any]:
            """Converts a websocket agreed_parameters set to keyword arguments
            for our compressor objects.
            """
            options = dict(
                persistent=(side + "_no_context_takeover") not in agreed_parameters
            )  # type: Dict[str, Any]
            wbits_header = agreed_parameters.get(side + "_max_window_bits", None)
            if wbits_header is None:
                options["max_wbits"] = zlib.MAX_WBITS
            else:
                options["max_wbits"] = int(wbits_header)
            options["compression_options"] = compression_options
            return options

        def _create_compressors(
            self,
            side: str,
            agreed_parameters: Dict[str, Any],
            compression_options: Optional[Dict[str, Any]] = None,
        ) -> None:
            # TODO: handle invalid parameters gracefully
            allowed_keys = set(
                [
                    "server_no_context_takeover",
                    "client_no_context_takeover",
                    "server_max_window_bits",
                    "client_max_window_bits",
                ]
            )
            for key in agreed_parameters:
                if key not in allowed_keys:
                    raise ValueError("unsupported compression parameter %r" % key)
            other_side = "client" if (side == "server") else "server"
            self._compressor = _PerMessageDeflateCompressor(
                **self._get_compressor_options(side, agreed_parameters, compression_options)
            )
            self._decompressor = _PerMessageDeflateDecompressor(
                max_message_size=self.params.max_message_size,
                **self._get_compressor_options(
                    other_side, agreed_parameters, compression_options
                )
            )

        def _write_frame(
            self, fin: bool, opcode: int, data: bytes, flags: int = 0
        ) -> "Future[None]":
            data_len = len(data)
            if opcode & 0x8:
                # All control frames MUST have a payload length of 125
                # bytes or less and MUST NOT be fragmented.
                if not fin:
                    raise ValueError("control frames may not be fragmented")
                if data_len > 125:
                    raise ValueError("control frame payloads may not exceed 125 bytes")
            if fin:
                finbit = self.FIN
            else:
                finbit = 0
            frame = struct.pack("B", finbit | opcode | flags)
            if self.mask_outgoing:
                mask_bit = 0x80
            else:
                mask_bit = 0
            if data_len < 126:
                frame += struct.pack("B", data_len | mask_bit)
            elif data_len <= 0xFFFF:
                frame += struct.pack("!BH", 126 | mask_bit, data_len)
            else:
                frame += struct.pack("!BQ", 127 | mask_bit, data_len)
            if self.mask_outgoing:
                mask = os.urandom(4)
                data = mask + _websocket_mask(mask, data)
            frame += data
            self._wire_bytes_out += len(frame)
            return self.stream.write(frame)

        def write_message(
            self, message: Union[str, bytes, Dict[str, Any]], binary: bool = False
        ) -> "Future[None]":
            """Sends the given message to the client of this Web Socket."""
            if binary:
                opcode = 0x2
            else:
                opcode = 0x1
            if isinstance(message, dict):
                message = tornado.escape.json_encode(message)
            message = tornado.escape.utf8(message)
            assert isinstance(message, bytes)
            self._message_bytes_out += len(message)
            flags = 0
            if self._compressor:
                message = self._compressor.compress(message)
                flags |= self.RSV1
            # For historical reasons, write methods in Tornado operate in a semi-synchronous
            # mode in which awaiting the Future they return is optional (But errors can
            # still be raised). This requires us to go through an awkward dance here
            # to transform the errors that may be returned while presenting the same
            # semi-synchronous interface.
            try:
                fut = self._write_frame(True, opcode, message, flags=flags)
            except StreamClosedError:
                raise WebSocketClosedError()

            async def wrapper() -> None:
                try:
                    await fut
                except StreamClosedError:
                    raise WebSocketClosedError()

            return asyncio.ensure_future(wrapper())

        def write_ping(self, data: bytes) -> None:
            """Send ping frame."""
            assert isinstance(data, bytes)
            self._write_frame(True, 0x9, data)

        async def _receive_frame_loop(self) -> None:
            try:
                while not self.client_terminated:
                    await self._receive_frame()
            except StreamClosedError:
                self._abort()
            self.handler.on_ws_connection_close(self.close_code, self.close_reason)

        async def _read_bytes(self, n: int) -> bytes:
            data = await self.stream.read_bytes(n)
            self._wire_bytes_in += n
            return data

        async def _receive_frame(self) -> None:
            # Read the frame header.
            data = await self._read_bytes(2)
            header, mask_payloadlen = struct.unpack("BB", data)
            is_final_frame = header & self.FIN
            reserved_bits = header & self.RSV_MASK
            opcode = header & self.OPCODE_MASK
            opcode_is_control = opcode & 0x8
            if self._decompressor is not None and opcode != 0:
                # Compression flag is present in the first frame's header,
                # but we can't decompress until we have all the frames of
                # the message.
                self._frame_compressed = bool(reserved_bits & self.RSV1)
                reserved_bits &= ~self.RSV1
            if reserved_bits:
                # client is using as-yet-undefined extensions; abort
                self._abort()
                return
            is_masked = bool(mask_payloadlen & 0x80)
            payloadlen = mask_payloadlen & 0x7F

            # Parse and validate the length.
            if opcode_is_control and payloadlen >= 126:
                # control frames must have payload < 126
                self._abort()
                return
            if payloadlen < 126:
                self._frame_length = payloadlen
            elif payloadlen == 126:
                data = await self._read_bytes(2)
                payloadlen = struct.unpack("!H", data)[0]
            elif payloadlen == 127:
                data = await self._read_bytes(8)
                payloadlen = struct.unpack("!Q", data)[0]
            new_len = payloadlen
            if self._fragmented_message_buffer is not None:
                new_len += len(self._fragmented_message_buffer)
            if new_len > self.params.max_message_size:
                self.close(1009, "message too big")
                self._abort()
                return

            # Read the payload, unmasking if necessary.
            if is_masked:
                self._frame_mask = await self._read_bytes(4)
            data = await self._read_bytes(payloadlen)
            if is_masked:
                assert self._frame_mask is not None
                data = _websocket_mask(self._frame_mask, data)

            # Decide what to do with this frame.
            if opcode_is_control:
                # control frames may be interleaved with a series of fragmented
                # data frames, so control frames must not interact with
                # self._fragmented_*
                if not is_final_frame:
                    # control frames must not be fragmented
                    self._abort()
                    return
            elif opcode == 0:  # continuation frame
                if self._fragmented_message_buffer is None:
                    # nothing to continue
                    self._abort()
                    return
                self._fragmented_message_buffer.extend(data)
                if is_final_frame:
                    opcode = self._fragmented_message_opcode
                    data = bytes(self._fragmented_message_buffer)
                    self._fragmented_message_buffer = None
            else:  # start of new data message
                if self._fragmented_message_buffer is not None:
                    # can't start new message until the old one is finished
                    self._abort()
                    return
                if not is_final_frame:
                    self._fragmented_message_opcode = opcode
                    self._fragmented_message_buffer = bytearray(data)

            if is_final_frame:
                handled_future = self._handle_message(opcode, data)
                if handled_future is not None:
                    await handled_future

        def _handle_message(self, opcode: int, data: bytes) -> "Optional[Future[None]]":
            """Execute on_message, returning its Future if it is a coroutine."""
            if self.client_terminated:
                return None

            if self._frame_compressed:
                assert self._decompressor is not None
                try:
                    data = self._decompressor.decompress(data)
                except _DecompressTooLargeError:
                    self.close(1009, "message too big after decompression")
                    self._abort()
                    return None

            if opcode == 0x1:
                # UTF-8 data
                self._message_bytes_in += len(data)
                try:
                    decoded = data.decode("utf-8")
                except UnicodeDecodeError:
                    self._abort()
                    return None
                return self._run_callback(self.handler.on_message, decoded)
            elif opcode == 0x2:
                # Binary data
                self._message_bytes_in += len(data)
                return self._run_callback(self.handler.on_message, data)
            elif opcode == 0x8:
                # Close
                self.client_terminated = True
                if len(data) >= 2:
                    self.close_code = struct.unpack(">H", data[:2])[0]
                if len(data) > 2:
                    self.close_reason = to_unicode(data[2:])
                # Echo the received close code, if any (RFC 6455 section 5.5.1).
                self.close(self.close_code)
            elif opcode == 0x9:
                # Ping
                try:
                    self._write_frame(True, 0xA, data)
                except StreamClosedError:
                    self._abort()
                self._run_callback(self.handler.on_ping, data)
            elif opcode == 0xA:
                # Pong
                self.last_pong = IOLoop.current().time()
                return self._run_callback(self.handler.on_pong, data)
            else:
                self._abort()
            return None

        def close(self, code: Optional[int] = None, reason: Optional[str] = None) -> None:
            """Closes the WebSocket connection."""
            if not self.server_terminated:
                if not self.stream.closed():
                    if code is None and reason is not None:
                        code = 1000  # "normal closure" status code
                    if code is None:
                        close_data = b""
                    else:
                        close_data = struct.pack(">H", code)
                    if reason is not None:
                        close_data += utf8(reason)
                    try:
                        self._write_frame(True, 0x8, close_data)
                    except StreamClosedError:
                        self._abort()
                self.server_terminated = True
            if self.client_terminated:
                if self._waiting is not None:
                    self.stream.io_loop.remove_timeout(self._waiting)
                    self._waiting = None
                self.stream.close()
            elif self._waiting is None:
                # Give the client a few seconds to complete a clean shutdown,
                # otherwise just close the connection.
                self._waiting = self.stream.io_loop.add_timeout(
                    self.stream.io_loop.time() + 5, self._abort
                )
            if self.ping_callback:
                self.ping_callback.stop()
                self.ping_callback = None

        def is_closing(self) -> bool:
            """Return ``True`` if this connection is closing.

            The connection is considered closing if either side has
            initiated its closing handshake or if the stream has been
            shut down uncleanly.
            """
            return self.stream.closed() or self.client_terminated or self.server_terminated

        @property
        def ping_interval(self) -> Optional[float]:
            interval = self.params.ping_interval
            if interval is not None:
                return interval
            return 0

        @property
        def ping_timeout(self) -> Optional[float]:
            timeout = self.params.ping_timeout
            if timeout is not None:
                return timeout
            assert self.ping_interval is not None
            return max(3 * self.ping_interval, 30)

        def start_pinging(self) -> None:
            """Start sending periodic pings to keep the connection alive"""
            assert self.ping_interval is not None
            if self.ping_interval > 0:
                self.last_ping = self.last_pong = IOLoop.current().time()
                self.ping_callback = PeriodicCallback(
                    self.periodic_ping, self.ping_interval * 1000
                )
                self.ping_callback.start()

        def periodic_ping(self) -> None:
            """Send a ping to keep the websocket alive

            Called periodically if the websocket_ping_interval is set and non-zero.
            """
            if self.is_closing() and self.ping_callback is not None:
                self.ping_callback.stop()
                return

            # Check for timeout on pong. Make sure that we really have
            # sent a recent ping in case the machine with both server and
            # client has been suspended since the last ping.
            now = IOLoop.current().time()
            since_last_pong = now - self.last_pong
            since_last_ping = now - self.last_ping
            assert self.ping_interval is not None
            assert self.ping_timeout is not None
            if (
                since_last_ping < 2 * self.ping_interval
                and since_last_pong > self.ping_timeout
            ):
                self.close()
                return

            self.write_ping(b"")
            self.last_ping = now

        def set_nodelay(self, x: bool) -> None:
            self.stream.set_nodelay(x)

    class WebSocketClientConnection(simple_httpclient._HTTPConnection):
        """WebSocket client connection.

        This class should not be instantiated directly; use the
        `websocket_connect` function instead.
        """

        protocol = None  # type: WebSocketProtocol

        def __init__(
            self,
            request: httpclient.HTTPRequest,
            on_message_callback: Optional[Callable[[Union[None, str, bytes]], None]] = None,
            compression_options: Optional[Dict[str, Any]] = None,
            ping_interval: Optional[float] = None,
            ping_timeout: Optional[float] = None,
            max_message_size: int = _default_max_message_size,
            subprotocols: Optional[List[str]] = None,
            resolver: Optional[Resolver] = None,
        ) -> None:
            self.connect_future = Future()  # type: Future[WebSocketClientConnection]
            self.read_queue = Queue(1)  # type: Queue[Union[None, str, bytes]]
            self.key = base64.b64encode(os.urandom(16))
            self._on_message_callback = on_message_callback
            self.close_code = None  # type: Optional[int]
            self.close_reason = None  # type: Optional[str]
            self.params = _WebSocketParams(
                ping_interval=ping_interval,
                ping_timeout=ping_timeout,
                max_message_size=max_message_size,
                compression_options=compression_options,
            )

            scheme, sep, rest = request.url.partition(":")
            scheme = {"ws": "http", "wss": "https"}[scheme]
            request.url = scheme + sep + rest
            request.headers.update(
                {
                    "Upgrade": "websocket",
                    "Connection": "Upgrade",
                    "Sec-WebSocket-Key": self.key,
                    "Sec-WebSocket-Version": "13",
                }
            )
            if subprotocols is not None:
                request.headers["Sec-WebSocket-Protocol"] = ",".join(subprotocols)
            if compression_options is not None:
                # Always offer to let the server set our max_wbits (and even though
                # we don't offer it, we will accept a client_no_context_takeover
                # from the server).
                # TODO: set server parameters for deflate extension
                # if requested in self.compression_options.
                request.headers["Sec-WebSocket-Extensions"] = (
                    "permessage-deflate; client_max_window_bits"
                )

            # Websocket connection is currently unable to follow redirects
            request.follow_redirects = False

            self.tcp_client = TCPClient(resolver=resolver)
            super().__init__(
                None,
                request,
                lambda: None,
                self._on_http_response,
                104857600,
                self.tcp_client,
                65536,
                104857600,
            )

        def __del__(self) -> None:
            if self.protocol is not None:
                # Unclosed client connections can sometimes log "task was destroyed but
                # was pending" warnings if shutdown strikes at the wrong time (such as
                # while a ping is being processed due to ping_interval). Log our own
                # warning to make it a little more deterministic (although it's still
                # dependent on GC timing).
                warnings.warn("Unclosed WebSocketClientConnection", ResourceWarning)

        def close(self, code: Optional[int] = None, reason: Optional[str] = None) -> None:
            """Closes the websocket connection.

            ``code`` and ``reason`` are documented under
            `WebSocketHandler.close`.

            .. versionadded:: 3.2

            .. versionchanged:: 4.0

               Added the ``code`` and ``reason`` arguments.
            """
            if self.protocol is not None:
                self.protocol.close(code, reason)
                self.protocol = None  # type: ignore

        def on_connection_close(self) -> None:
            if not self.connect_future.done():
                self.connect_future.set_exception(StreamClosedError())
            self._on_message(None)
            self.tcp_client.close()
            super().on_connection_close()

        def on_ws_connection_close(
            self, close_code: Optional[int] = None, close_reason: Optional[str] = None
        ) -> None:
            self.close_code = close_code
            self.close_reason = close_reason
            self.on_connection_close()

        def _on_http_response(self, response: httpclient.HTTPResponse) -> None:
            if not self.connect_future.done():
                if response.error:
                    self.connect_future.set_exception(response.error)
                else:
                    self.connect_future.set_exception(
                        WebSocketError("Non-websocket response")
                    )

        async def headers_received(
            self,
            start_line: Union[httputil.RequestStartLine, httputil.ResponseStartLine],
            headers: httputil.HTTPHeaders,
        ) -> None:
            assert isinstance(start_line, httputil.ResponseStartLine)
            if start_line.code != 101:
                await super().headers_received(start_line, headers)
                return

            if self._timeout is not None:
                self.io_loop.remove_timeout(self._timeout)
                self._timeout = None

            self.headers = headers
            self.protocol = self.get_websocket_protocol()
            self.protocol._process_server_headers(self.key, self.headers)
            self.protocol.stream = self.connection.detach()

            IOLoop.current().add_callback(self.protocol._receive_frame_loop)
            self.protocol.start_pinging()

            # Once we've taken over the connection, clear the final callback
            # we set on the http request.  This deactivates the error handling
            # in simple_httpclient that would otherwise interfere with our
            # ability to see exceptions.
            self.final_callback = None  # type: ignore

            future_set_result_unless_cancelled(self.connect_future, self)

        def write_message(
            self, message: Union[str, bytes, Dict[str, Any]], binary: bool = False
        ) -> "Future[None]":
            """Sends a message to the WebSocket server.

            If the stream is closed, raises `WebSocketClosedError`.
            Returns a `.Future` which can be used for flow control.

            .. versionchanged:: 5.0
               Exception raised on a closed stream changed from `.StreamClosedError`
               to `WebSocketClosedError`.
            """
            if self.protocol is None:
                raise WebSocketClosedError("Client connection has been closed")
            return self.protocol.write_message(message, binary=binary)

        def read_message(
            self,
            callback: Optional[Callable[["Future[Union[None, str, bytes]]"], None]] = None,
        ) -> Awaitable[Union[None, str, bytes]]:
            """Reads a message from the WebSocket server.

            If on_message_callback was specified at WebSocket
            initialization, this function will never return messages

            Returns a future whose result is the message, or None
            if the connection is closed.  If a callback argument
            is given it will be called with the future when it is
            ready.
            """

            awaitable = self.read_queue.get()
            if callback is not None:
                self.io_loop.add_future(asyncio.ensure_future(awaitable), callback)
            return awaitable

        def on_message(self, message: Union[str, bytes]) -> Optional[Awaitable[None]]:
            return self._on_message(message)

        def _on_message(
            self, message: Union[None, str, bytes]
        ) -> Optional[Awaitable[None]]:
            if self._on_message_callback:
                self._on_message_callback(message)
                return None
            else:
                return self.read_queue.put(message)

        def ping(self, data: bytes = b"") -> None:
            """Send ping frame to the remote end.

            The data argument allows a small amount of data (up to 125
            bytes) to be sent as a part of the ping message. Note that not
            all websocket implementations expose this data to
            applications.

            Consider using the ``ping_interval`` argument to
            `websocket_connect` instead of sending pings manually.

            .. versionadded:: 5.1

            """
            data = utf8(data)
            if self.protocol is None:
                raise WebSocketClosedError()
            self.protocol.write_ping(data)

        def on_pong(self, data: bytes) -> None:
            pass

        def on_ping(self, data: bytes) -> None:
            pass

        def get_websocket_protocol(self) -> WebSocketProtocol:
            return WebSocketProtocol13(self, mask_outgoing=True, params=self.params)

        @property
        def selected_subprotocol(self) -> Optional[str]:
            """The subprotocol selected by the server.

            .. versionadded:: 5.1
            """
            return self.protocol.selected_subprotocol

        def log_exception(
            self,
            typ: "Optional[Type[BaseException]]",
            value: Optional[BaseException],
            tb: Optional[TracebackType],
        ) -> None:
            assert typ is not None
            assert value is not None
            app_log.error("Uncaught exception %s", value, exc_info=(typ, value, tb))

    def websocket_connect(
        url: Union[str, httpclient.HTTPRequest],
        callback: Optional[Callable[["Future[WebSocketClientConnection]"], None]] = None,
        connect_timeout: Optional[float] = None,
        on_message_callback: Optional[Callable[[Union[None, str, bytes]], None]] = None,
        compression_options: Optional[Dict[str, Any]] = None,
        ping_interval: Optional[float] = None,
        ping_timeout: Optional[float] = None,
        max_message_size: int = _default_max_message_size,
        subprotocols: Optional[List[str]] = None,
        resolver: Optional[Resolver] = None,
    ) -> "Awaitable[WebSocketClientConnection]":
        """Client-side websocket support.

        Takes a url and returns a Future whose result is a
        `WebSocketClientConnection`.

        ``compression_options`` is interpreted in the same way as the
        return value of `.WebSocketHandler.get_compression_options`.

        The connection supports two styles of operation. In the coroutine
        style, the application typically calls
        `~.WebSocketClientConnection.read_message` in a loop::

            conn = yield websocket_connect(url)
            while True:
                msg = yield conn.read_message()
                if msg is None: break
                # Do something with msg

        In the callback style, pass an ``on_message_callback`` to
        ``websocket_connect``. In both styles, a message of ``None``
        indicates that the connection has been closed.

        ``subprotocols`` may be a list of strings specifying proposed
        subprotocols. The selected protocol may be found on the
        ``selected_subprotocol`` attribute of the connection object
        when the connection is complete.

        .. versionchanged:: 3.2
           Also accepts ``HTTPRequest`` objects in place of urls.

        .. versionchanged:: 4.1
           Added ``compression_options`` and ``on_message_callback``.

        .. versionchanged:: 4.5
           Added the ``ping_interval``, ``ping_timeout``, and ``max_message_size``
           arguments, which have the same meaning as in `WebSocketHandler`.

        .. versionchanged:: 5.0
           The ``io_loop`` argument (deprecated since version 4.1) has been removed.

        .. versionchanged:: 5.1
           Added the ``subprotocols`` argument.

        .. versionchanged:: 6.3
           Added the ``resolver`` argument.
        """
        if isinstance(url, httpclient.HTTPRequest):
            assert connect_timeout is None
            request = url
            # Copy and convert the headers dict/object (see comments in
            # AsyncHTTPClient.fetch)
            request.headers = httputil.HTTPHeaders(request.headers)
        else:
            request = httpclient.HTTPRequest(url, connect_timeout=connect_timeout)
        request = cast(
            httpclient.HTTPRequest,
            httpclient._RequestProxy(request, httpclient.HTTPRequest._DEFAULTS),
        )
        conn = WebSocketClientConnection(
            request,
            on_message_callback=on_message_callback,
            compression_options=compression_options,
            ping_interval=ping_interval,
            ping_timeout=ping_timeout,
            max_message_size=max_message_size,
            subprotocols=subprotocols,
            resolver=resolver,
        )
        if callback is not None:
            IOLoop.current().add_future(conn.connect_future, callback)
        return conn.connect_future