websocket.py

"""Server-side implementation of the WebSocket protocol.

`WebSockets <http://dev.w3.org/html5/websockets/>`_ allow for bidirectional
communication between the browser and server.

.. warning::

   The WebSocket protocol was recently finalized as `RFC 6455
   <http://tools.ietf.org/html/rfc6455>`_ and is not yet supported in
   all browsers.  Refer to http://caniuse.com/websockets for details
   on compatibility.  In addition, during development the protocol
   went through several incompatible versions, and some browsers only
   support older versions.  By default this module only supports the
   latest version of the protocol, but optional support for an older
   version (known as "draft 76" or "hixie-76") can be enabled by
   overriding `WebSocketHandler.allow_draft76` (see that method's
   documentation for caveats).
"""
# Author: Jacob Kristhammar, 2010

import array
import functools
import hashlib
import logging
import struct
import time
import base64
import tornado.escape
import tornado.web

from tornado.util import bytes_type, b

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

    Override on_message to handle incoming messages. You can also override
    open and on_close to handle opened and closed connections.

    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 Web Socket handler that echos back all received messages
    back to the client::

      class EchoWebSocket(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"

    Web Sockets 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()
    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".
    """
    def __init__(self, application, request, **kwargs):
        tornado.web.RequestHandler.__init__(self, application, request,
                                            **kwargs)
        self.stream = request.connection.stream
        self.ws_connection = None

    def _execute(self, transforms, *args, **kwargs):
        self.open_args = args
        self.open_kwargs = kwargs

        # Websocket only supports GET method
        if self.request.method != 'GET':
            self.stream.write(tornado.escape.utf8(
                "HTTP/1.1 405 Method Not Allowed\r\n\r\n"
            ))
            self.stream.close()
            return

        # Upgrade header should be present and should be equal to WebSocket
        if self.request.headers.get("Upgrade", "").lower() != 'websocket':
            self.stream.write(tornado.escape.utf8(
                "HTTP/1.1 400 Bad Request\r\n\r\n"
                "Can \"Upgrade\" only to \"WebSocket\"."
            ))
            self.stream.close()
            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.stream.write(tornado.escape.utf8(
                "HTTP/1.1 400 Bad Request\r\n\r\n"
                "\"Connection\" must be \"Upgrade\"."
            ))
            self.stream.close()
            return

        # 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 self.request.headers.get("Sec-WebSocket-Version") in ("7", "8", "13"):
            self.ws_connection = WebSocketProtocol13(self)
            self.ws_connection.accept_connection()
        elif (self.allow_draft76() and
              "Sec-WebSocket-Version" not in self.request.headers):
            self.ws_connection = WebSocketProtocol76(self)
            self.ws_connection.accept_connection()
        else:
            self.stream.write(tornado.escape.utf8(
                "HTTP/1.1 426 Upgrade Required\r\n"
                "Sec-WebSocket-Version: 8\r\n\r\n"))
            self.stream.close()

    def write_message(self, message, binary=False):
        """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 isinstance(message, dict):
            message = tornado.escape.json_encode(message)
        self.ws_connection.write_message(message, binary=binary)

    def select_subprotocol(self, subprotocols):
        """Invoked when a new WebSocket requests specific subprotocols.

        ``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.
        """
        return None

    def open(self):
        """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`.
        """
        pass

    def on_message(self, message):
        """Handle incoming messages on the WebSocket

        This method must be overridden.
        """
        raise NotImplementedError

    def on_close(self):
        """Invoked when the WebSocket is closed."""
        pass

    def close(self):
        """Closes this Web Socket.

        Once the close handshake is successful the socket will be closed.
        """
        self.ws_connection.close()

    def allow_draft76(self):
        """Override to enable support for the older "draft76" protocol.

        The draft76 version of the websocket protocol is disabled by
        default due to security concerns, but it can be enabled by
        overriding this method to return True.

        Connections using the draft76 protocol do not support the
        ``binary=True`` flag to `write_message`.

        Support for the draft76 protocol is deprecated and will be
        removed in a future version of Tornado.
        """
        return False

    def get_websocket_scheme(self):
        """Return the url scheme used for this request, either "ws" or "wss".

        This is normally decided by HTTPServer, but applications
        may wish to override this if they are using an SSL proxy
        that does not provide the X-Scheme header as understood
        by HTTPServer.
        
        Note that this is only used by the draft76 protocol.
        """
        return "wss" if self.request.protocol == "https" else "ws"

    def async_callback(self, callback, *args, **kwargs):
        """Wrap callbacks with this if they are used on asynchronous requests.

        Catches exceptions properly and closes this WebSocket if an exception
        is uncaught.  (Note that this is usually unnecessary thanks to
        `tornado.stack_context`)
        """
        return self.ws_connection.async_callback(callback, *args, **kwargs)

    def _not_supported(self, *args, **kwargs):
        raise Exception("Method not supported for Web Sockets")

    def on_connection_close(self):
        if self.ws_connection:
            self.ws_connection.on_connection_close()
            self.ws_connection = None
            self.on_close()


for method in ["write", "redirect", "set_header", "send_error", "set_cookie",
               "set_status", "flush", "finish"]:
    setattr(WebSocketHandler, method, WebSocketHandler._not_supported)


class WebSocketProtocol(object):
    """Base class for WebSocket protocol versions.
    """
    def __init__(self, handler):
        self.handler = handler
        self.request = handler.request
        self.stream = handler.stream
        self.client_terminated = False
        self.server_terminated = False

    def async_callback(self, callback, *args, **kwargs):
        """Wrap callbacks with this if they are used on asynchronous requests.

        Catches exceptions properly and closes this WebSocket if an exception
        is uncaught.
        """
        if args or kwargs:
            callback = functools.partial(callback, *args, **kwargs)
        def wrapper(*args, **kwargs):
            try:
                return callback(*args, **kwargs)
            except Exception:
                logging.error("Uncaught exception in %s",
                              self.request.path, exc_info=True)
                self._abort()
        return wrapper

    def on_connection_close(self):
        self._abort()

    def _abort(self):
        """Instantly aborts the WebSocket connection by closing the socket"""
        self.client_terminated = True
        self.server_terminated = True
        self.stream.close()  # forcibly tear down the connection
        self.close()  # let the subclass cleanup


class WebSocketProtocol76(WebSocketProtocol):
    """Implementation of the WebSockets protocol, version hixie-76.

    This class provides basic functionality to process WebSockets requests as
    specified in
    http://tools.ietf.org/html/draft-hixie-thewebsocketprotocol-76
    """
    def __init__(self, handler):
        WebSocketProtocol.__init__(self, handler)
        self.challenge = None
        self._waiting = None

    def accept_connection(self):
        try:
            self._handle_websocket_headers()
        except ValueError:
            logging.debug("Malformed WebSocket request received")
            self._abort()
            return

        scheme = self.handler.get_websocket_scheme()

        # draft76 only allows a single subprotocol
        subprotocol_header = ''
        subprotocol = self.request.headers.get("Sec-WebSocket-Protocol", None)
        if subprotocol:
            selected = self.handler.select_subprotocol([subprotocol])
            if selected:
                assert selected == subprotocol
                subprotocol_header = "Sec-WebSocket-Protocol: %s\r\n" % selected

        # Write the initial headers before attempting to read the challenge.
        # This is necessary when using proxies (such as HAProxy), which
        # need to see the Upgrade headers before passing through the
        # non-HTTP traffic that follows.
        self.stream.write(tornado.escape.utf8(
            "HTTP/1.1 101 WebSocket Protocol Handshake\r\n"
            "Upgrade: WebSocket\r\n"
            "Connection: Upgrade\r\n"
            "Server: TornadoServer/%(version)s\r\n"
            "Sec-WebSocket-Origin: %(origin)s\r\n"
            "Sec-WebSocket-Location: %(scheme)s://%(host)s%(uri)s\r\n"
            "%(subprotocol)s"
            "\r\n" % (dict(
                    version=tornado.version,
                    origin=self.request.headers["Origin"],
                    scheme=scheme,
                    host=self.request.host,
                    uri=self.request.uri,
                    subprotocol=subprotocol_header))))
        self.stream.read_bytes(8, self._handle_challenge)

    def challenge_response(self, challenge):
        """Generates the challenge response that's needed in the handshake

        The challenge parameter should be the raw bytes as sent from the
        client.
        """
        key_1 = self.request.headers.get("Sec-Websocket-Key1")
        key_2 = self.request.headers.get("Sec-Websocket-Key2")
        try:
            part_1 = self._calculate_part(key_1)
            part_2 = self._calculate_part(key_2)
        except ValueError:
            raise ValueError("Invalid Keys/Challenge")
        return self._generate_challenge_response(part_1, part_2, challenge)

    def _handle_challenge(self, challenge):
        try:
            challenge_response = self.challenge_response(challenge)
        except ValueError:
            logging.debug("Malformed key data in WebSocket request")
            self._abort()
            return
        self._write_response(challenge_response)

    def _write_response(self, challenge):
        self.stream.write(challenge)
        self.async_callback(self.handler.open)(*self.handler.open_args, **self.handler.open_kwargs)
        self._receive_message()

    def _handle_websocket_headers(self):
        """Verifies all invariant- and required headers

        If a header is missing or have an incorrect value ValueError will be
        raised
        """
        fields = ("Origin", "Host", "Sec-Websocket-Key1",
                  "Sec-Websocket-Key2")
        if not all(map(lambda f: self.request.headers.get(f), fields)):
            raise ValueError("Missing/Invalid WebSocket headers")

    def _calculate_part(self, key):
        """Processes the key headers and calculates their key value.

        Raises ValueError when feed invalid key."""
        number = int(''.join(c for c in key if c.isdigit()))
        spaces = len([c for c in key if c.isspace()])
        try:
            key_number = number // spaces
        except (ValueError, ZeroDivisionError):
            raise ValueError
        return struct.pack(">I", key_number)

    def _generate_challenge_response(self, part_1, part_2, part_3):
        m = hashlib.md5()
        m.update(part_1)
        m.update(part_2)
        m.update(part_3)
        return m.digest()

    def _receive_message(self):
        self.stream.read_bytes(1, self._on_frame_type)

    def _on_frame_type(self, byte):
        frame_type = ord(byte)
        if frame_type == 0x00:
            self.stream.read_until(b("\xff"), self._on_end_delimiter)
        elif frame_type == 0xff:
            self.stream.read_bytes(1, self._on_length_indicator)
        else:
            self._abort()

    def _on_end_delimiter(self, frame):
        if not self.client_terminated:
            self.async_callback(self.handler.on_message)(
                    frame[:-1].decode("utf-8", "replace"))
        if not self.client_terminated:
            self._receive_message()

    def _on_length_indicator(self, byte):
        if ord(byte) != 0x00:
            self._abort()
            return
        self.client_terminated = True
        self.close()

    def write_message(self, message, binary=False):
        """Sends the given message to the client of this Web Socket."""
        if binary:
            raise ValueError(
                "Binary messages not supported by this version of websockets")
        if isinstance(message, unicode):
            message = message.encode("utf-8")
        assert isinstance(message, bytes_type)
        self.stream.write(b("\x00") + message + b("\xff"))

    def close(self):
        """Closes the WebSocket connection."""
        if not self.server_terminated:
            if not self.stream.closed():
                self.stream.write("\xff\x00")
            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:
            self._waiting = self.stream.io_loop.add_timeout(
                time.time() + 5, self._abort)


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.
    """
    def __init__(self, handler):
        WebSocketProtocol.__init__(self, handler)
        self._final_frame = False
        self._frame_opcode = None
        self._frame_mask = None
        self._frame_length = None
        self._fragmented_message_buffer = None
        self._fragmented_message_opcode = None
        self._waiting = None

    def accept_connection(self):
        try:
            self._handle_websocket_headers()
            self._accept_connection()
        except ValueError:
            logging.debug("Malformed WebSocket request received")
            self._abort()
            return

    def _handle_websocket_headers(self):
        """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: self.request.headers.get(f), fields)):
            raise ValueError("Missing/Invalid WebSocket headers")

    def _challenge_response(self):
        sha1 = hashlib.sha1()
        sha1.update(tornado.escape.utf8(
                self.request.headers.get("Sec-Websocket-Key")))
        sha1.update(b("258EAFA5-E914-47DA-95CA-C5AB0DC85B11")) # Magic value
        return tornado.escape.native_str(base64.b64encode(sha1.digest()))

    def _accept_connection(self):
        subprotocol_header = ''
        subprotocols = self.request.headers.get("Sec-WebSocket-Protocol", '')
        subprotocols = [s.strip() for s in subprotocols.split(',')]
        if subprotocols:
            selected = self.handler.select_subprotocol(subprotocols)
            if selected:
                assert selected in subprotocols
                subprotocol_header = "Sec-WebSocket-Protocol: %s\r\n" % selected

        self.stream.write(tornado.escape.utf8(
            "HTTP/1.1 101 Switching Protocols\r\n"
            "Upgrade: websocket\r\n"
            "Connection: Upgrade\r\n"
            "Sec-WebSocket-Accept: %s\r\n"
            "%s"
            "\r\n" % (self._challenge_response(), subprotocol_header)))

        self.async_callback(self.handler.open)(*self.handler.open_args, **self.handler.open_kwargs)
        self._receive_frame()

    def _write_frame(self, fin, opcode, data):
        if fin:
            finbit = 0x80
        else:
            finbit = 0
        frame = struct.pack("B", finbit | opcode)
        l = len(data)
        if l < 126:
            frame += struct.pack("B", l)
        elif l <= 0xFFFF:
            frame += struct.pack("!BH", 126, l)
        else:
            frame += struct.pack("!BQ", 127, l)
        frame += data
        self.stream.write(frame)

    def write_message(self, message, binary=False):
        """Sends the given message to the client of this Web Socket."""
        if binary:
            opcode = 0x2
        else:
            opcode = 0x1
        message = tornado.escape.utf8(message)
        assert isinstance(message, bytes_type)
        self._write_frame(True, opcode, message)

    def _receive_frame(self):
        self.stream.read_bytes(2, self._on_frame_start)

    def _on_frame_start(self, data):
        header, payloadlen = struct.unpack("BB", data)
        self._final_frame = header & 0x80
        reserved_bits = header & 0x70
        self._frame_opcode = header & 0xf
        self._frame_opcode_is_control = self._frame_opcode & 0x8
        if reserved_bits:
            # client is using as-yet-undefined extensions; abort
            self._abort()
            return
        if not (payloadlen & 0x80):
            # Unmasked frame -> abort connection
            self._abort()
            return
        payloadlen = payloadlen & 0x7f
        if self._frame_opcode_is_control and payloadlen >= 126:
            # control frames must have payload < 126
            self._abort()
            return
        if payloadlen < 126:
            self._frame_length = payloadlen
            self.stream.read_bytes(4, self._on_masking_key)
        elif payloadlen == 126:
            self.stream.read_bytes(2, self._on_frame_length_16)
        elif payloadlen == 127:
            self.stream.read_bytes(8, self._on_frame_length_64)

    def _on_frame_length_16(self, data):
        self._frame_length = struct.unpack("!H", data)[0];
        self.stream.read_bytes(4, self._on_masking_key);

    def _on_frame_length_64(self, data):
        self._frame_length = struct.unpack("!Q", data)[0];
        self.stream.read_bytes(4, self._on_masking_key);

    def _on_masking_key(self, data):
        self._frame_mask = array.array("B", data)
        self.stream.read_bytes(self._frame_length, self._on_frame_data)

    def _on_frame_data(self, data):
        unmasked = array.array("B", data)
        for i in xrange(len(data)):
            unmasked[i] = unmasked[i] ^ self._frame_mask[i % 4]

        if self._frame_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 self._final_frame:
                # control frames must not be fragmented
                self._abort()
                return
            opcode = self._frame_opcode
        elif self._frame_opcode == 0:  # continuation frame
            if self._fragmented_message_buffer is None:
                # nothing to continue
                self._abort()
                return
            self._fragmented_message_buffer += unmasked
            if self._final_frame:
                opcode = self._fragmented_message_opcode
                unmasked = 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 self._final_frame:
                opcode = self._frame_opcode
            else:
                self._fragmented_message_opcode = self._frame_opcode
                self._fragmented_message_buffer = unmasked

        if self._final_frame:
            self._handle_message(opcode, unmasked.tostring())

        if not self.client_terminated:
            self._receive_frame()


    def _handle_message(self, opcode, data):
        if self.client_terminated: return

        if opcode == 0x1:
            # UTF-8 data
            try:
                decoded = data.decode("utf-8")
            except UnicodeDecodeError:
                self._abort()
                return
            self.async_callback(self.handler.on_message)(decoded)
        elif opcode == 0x2:
            # Binary data
            self.async_callback(self.handler.on_message)(data)
        elif opcode == 0x8:
            # Close
            self.client_terminated = True
            self.close()
        elif opcode == 0x9:
            # Ping
            self._write_frame(True, 0xA, data)
        elif opcode == 0xA:
            # Pong
            pass
        else:
            self._abort()

    def close(self):
        """Closes the WebSocket connection."""
        if not self.server_terminated:
            if not self.stream.closed():
                self._write_frame(True, 0x8, b(""))
            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(
                time.time() + 5, self._abort)