server.py

import base64
import gzip
import importlib
import io
import logging
import secrets
import urllib
import zlib

from . import exceptions
from . import packet
from . import payload
from . import socket

default_logger = logging.getLogger('engineio.server')


class Server(object):
    """An Engine.IO server.

    This class implements a fully compliant Engine.IO web server with support
    for websocket and long-polling transports.

    :param async_mode: The asynchronous model to use. See the Deployment
                       section in the documentation for a description of the
                       available options. Valid async modes are "threading",
                       "eventlet", "gevent" and "gevent_uwsgi". If this
                       argument is not given, "eventlet" is tried first, then
                       "gevent_uwsgi", then "gevent", and finally "threading".
                       The first async mode that has all its dependencies
                       installed is the one that is chosen.
    :param ping_interval: The interval in seconds at which the server pings
                          the client. The default is 25 seconds. For advanced
                          control, a two element tuple can be given, where
                          the first number is the ping interval and the second
                          is a grace period added by the server.
    :param ping_timeout: The time in seconds that the client waits for the
                         server to respond before disconnecting. The default
                         is 20 seconds.
    :param max_http_buffer_size: The maximum size of a message when using the
                                 polling transport. The default is 1,000,000
                                 bytes.
    :param allow_upgrades: Whether to allow transport upgrades or not. The
                           default is ``True``.
    :param http_compression: Whether to compress packages when using the
                             polling transport. The default is ``True``.
    :param compression_threshold: Only compress messages when their byte size
                                  is greater than this value. The default is
                                  1024 bytes.
    :param cookie: If set to a string, it is the name of the HTTP cookie the
                   server sends back tot he client containing the client
                   session id. If set to a dictionary, the ``'name'`` key
                   contains the cookie name and other keys define cookie
                   attributes, where the value of each attribute can be a
                   string, a callable with no arguments, or a boolean. If set
                   to ``None`` (the default), a cookie is not sent to the
                   client.
    :param cors_allowed_origins: Origin or list of origins that are allowed to
                                 connect to this server. Only the same origin
                                 is allowed by default. Set this argument to
                                 ``'*'`` to allow all origins, or to ``[]`` to
                                 disable CORS handling.
    :param cors_credentials: Whether credentials (cookies, authentication) are
                             allowed in requests to this server. The default
                             is ``True``.
    :param logger: To enable logging set to ``True`` or pass a logger object to
                   use. To disable logging set to ``False``. The default is
                   ``False``. Note that fatal errors are logged even when
                   ``logger`` is ``False``.
    :param json: An alternative json module to use for encoding and decoding
                 packets. Custom json modules must have ``dumps`` and ``loads``
                 functions that are compatible with the standard library
                 versions.
    :param async_handlers: If set to ``True``, run message event handlers in
                           non-blocking threads. To run handlers synchronously,
                           set to ``False``. The default is ``True``.
    :param monitor_clients: If set to ``True``, a background task will ensure
                            inactive clients are closed. Set to ``False`` to
                            disable the monitoring task (not recommended). The
                            default is ``True``.
    :param transports: The list of allowed transports. Valid transports
                       are ``'polling'`` and ``'websocket'``. Defaults to
                       ``['polling', 'websocket']``.
    :param kwargs: Reserved for future extensions, any additional parameters
                   given as keyword arguments will be silently ignored.
    """
    compression_methods = ['gzip', 'deflate']
    event_names = ['connect', 'disconnect', 'message']
    valid_transports = ['polling', 'websocket']
    _default_monitor_clients = True
    sequence_number = 0

    def __init__(self, async_mode=None, ping_interval=25, ping_timeout=20,
                 max_http_buffer_size=1000000, allow_upgrades=True,
                 http_compression=True, compression_threshold=1024,
                 cookie=None, cors_allowed_origins=None,
                 cors_credentials=True, logger=False, json=None,
                 async_handlers=True, monitor_clients=None, transports=None,
                 **kwargs):
        self.ping_timeout = ping_timeout
        if isinstance(ping_interval, tuple):
            self.ping_interval = ping_interval[0]
            self.ping_interval_grace_period = ping_interval[1]
        else:
            self.ping_interval = ping_interval
            self.ping_interval_grace_period = 0
        self.max_http_buffer_size = max_http_buffer_size
        self.allow_upgrades = allow_upgrades
        self.http_compression = http_compression
        self.compression_threshold = compression_threshold
        self.cookie = cookie
        self.cors_allowed_origins = cors_allowed_origins
        self.cors_credentials = cors_credentials
        self.async_handlers = async_handlers
        self.sockets = {}
        self.handlers = {}
        self.log_message_keys = set()
        self.start_service_task = monitor_clients \
            if monitor_clients is not None else self._default_monitor_clients
        if json is not None:
            packet.Packet.json = json
        if not isinstance(logger, bool):
            self.logger = logger
        else:
            self.logger = default_logger
            if self.logger.level == logging.NOTSET:
                if logger:
                    self.logger.setLevel(logging.INFO)
                else:
                    self.logger.setLevel(logging.ERROR)
                self.logger.addHandler(logging.StreamHandler())
        modes = self.async_modes()
        if async_mode is not None:
            modes = [async_mode] if async_mode in modes else []
        self._async = None
        self.async_mode = None
        for mode in modes:
            try:
                self._async = importlib.import_module(
                    'engineio.async_drivers.' + mode)._async
                asyncio_based = self._async['asyncio'] \
                    if 'asyncio' in self._async else False
                if asyncio_based != self.is_asyncio_based():
                    continue  # pragma: no cover
                self.async_mode = mode
                break
            except ImportError:
                pass
        if self.async_mode is None:
            raise ValueError('Invalid async_mode specified')
        if self.is_asyncio_based() and \
                ('asyncio' not in self._async or not
                 self._async['asyncio']):  # pragma: no cover
            raise ValueError('The selected async_mode is not asyncio '
                             'compatible')
        if not self.is_asyncio_based() and 'asyncio' in self._async and \
                self._async['asyncio']:  # pragma: no cover
            raise ValueError('The selected async_mode requires asyncio and '
                             'must use the AsyncServer class')
        if transports is not None:
            if isinstance(transports, str):
                transports = [transports]
            transports = [transport for transport in transports
                          if transport in self.valid_transports]
            if not transports:
                raise ValueError('No valid transports provided')
        self.transports = transports or self.valid_transports
        self.logger.info('Server initialized for %s.', self.async_mode)

    def is_asyncio_based(self):
        return False

    def async_modes(self):
        return ['eventlet', 'gevent_uwsgi', 'gevent', 'threading']

    def on(self, event, handler=None):
        """Register an event handler.

        :param event: The event name. Can be ``'connect'``, ``'message'`` or
                      ``'disconnect'``.
        :param handler: The function that should be invoked to handle the
                        event. When this parameter is not given, the method
                        acts as a decorator for the handler function.

        Example usage::

            # as a decorator:
            @eio.on('connect')
            def connect_handler(sid, environ):
                print('Connection request')
                if environ['REMOTE_ADDR'] in blacklisted:
                    return False  # reject

            # as a method:
            def message_handler(sid, msg):
                print('Received message: ', msg)
                eio.send(sid, 'response')
            eio.on('message', message_handler)

        The handler function receives the ``sid`` (session ID) for the
        client as first argument. The ``'connect'`` event handler receives the
        WSGI environment as a second argument, and can return ``False`` to
        reject the connection. The ``'message'`` handler receives the message
        payload as a second argument. The ``'disconnect'`` handler does not
        take a second argument.
        """
        if event not in self.event_names:
            raise ValueError('Invalid event')

        def set_handler(handler):
            self.handlers[event] = handler
            return handler

        if handler is None:
            return set_handler
        set_handler(handler)

    def send(self, sid, data):
        """Send a message to a client.

        :param sid: The session id of the recipient client.
        :param data: The data to send to the client. Data can be of type
                     ``str``, ``bytes``, ``list`` or ``dict``. If a ``list``
                     or ``dict``, the data will be serialized as JSON.
        """
        try:
            socket = self._get_socket(sid)
        except KeyError:
            # the socket is not available
            self.logger.warning('Cannot send to sid %s', sid)
            return
        socket.send(packet.Packet(packet.MESSAGE, data=data))

    def get_session(self, sid):
        """Return the user session for a client.

        :param sid: The session id of the client.

        The return value is a dictionary. Modifications made to this
        dictionary are not guaranteed to be preserved unless
        ``save_session()`` is called, or when the ``session`` context manager
        is used.
        """
        socket = self._get_socket(sid)
        return socket.session

    def save_session(self, sid, session):
        """Store the user session for a client.

        :param sid: The session id of the client.
        :param session: The session dictionary.
        """
        socket = self._get_socket(sid)
        socket.session = session

    def session(self, sid):
        """Return the user session for a client with context manager syntax.

        :param sid: The session id of the client.

        This is a context manager that returns the user session dictionary for
        the client. Any changes that are made to this dictionary inside the
        context manager block are saved back to the session. Example usage::

            @eio.on('connect')
            def on_connect(sid, environ):
                username = authenticate_user(environ)
                if not username:
                    return False
                with eio.session(sid) as session:
                    session['username'] = username

            @eio.on('message')
            def on_message(sid, msg):
                with eio.session(sid) as session:
                    print('received message from ', session['username'])
        """
        class _session_context_manager(object):
            def __init__(self, server, sid):
                self.server = server
                self.sid = sid
                self.session = None

            def __enter__(self):
                self.session = self.server.get_session(sid)
                return self.session

            def __exit__(self, *args):
                self.server.save_session(sid, self.session)

        return _session_context_manager(self, sid)

    def disconnect(self, sid=None):
        """Disconnect a client.

        :param sid: The session id of the client to close. If this parameter
                    is not given, then all clients are closed.
        """
        if sid is not None:
            try:
                socket = self._get_socket(sid)
            except KeyError:  # pragma: no cover
                # the socket was already closed or gone
                pass
            else:
                socket.close()
                if sid in self.sockets:  # pragma: no cover
                    del self.sockets[sid]
        else:
            for client in self.sockets.values():
                client.close()
            self.sockets = {}

    def transport(self, sid):
        """Return the name of the transport used by the client.

        The two possible values returned by this function are ``'polling'``
        and ``'websocket'``.

        :param sid: The session of the client.
        """
        return 'websocket' if self._get_socket(sid).upgraded else 'polling'

    def handle_request(self, environ, start_response):
        """Handle an HTTP request from the client.

        This is the entry point of the Engine.IO application, using the same
        interface as a WSGI application. For the typical usage, this function
        is invoked by the :class:`Middleware` instance, but it can be invoked
        directly when the middleware is not used.

        :param environ: The WSGI environment.
        :param start_response: The WSGI ``start_response`` function.

        This function returns the HTTP response body to deliver to the client
        as a byte sequence.
        """
        if self.cors_allowed_origins != []:
            # Validate the origin header if present
            # This is important for WebSocket more than for HTTP, since
            # browsers only apply CORS controls to HTTP.
            origin = environ.get('HTTP_ORIGIN')
            if origin:
                allowed_origins = self._cors_allowed_origins(environ)
                if allowed_origins is not None and origin not in \
                        allowed_origins:
                    self._log_error_once(
                        origin + ' is not an accepted origin.', 'bad-origin')
                    r = self._bad_request('Not an accepted origin.')
                    start_response(r['status'], r['headers'])
                    return [r['response']]

        method = environ['REQUEST_METHOD']
        query = urllib.parse.parse_qs(environ.get('QUERY_STRING', ''))
        jsonp = False
        jsonp_index = None

        # make sure the client uses an allowed transport
        transport = query.get('transport', ['polling'])[0]
        if transport not in self.transports:
            self._log_error_once('Invalid transport', 'bad-transport')
            r = self._bad_request('Invalid transport')
            start_response(r['status'], r['headers'])
            return [r['response']]

        # make sure the client speaks a compatible Engine.IO version
        sid = query['sid'][0] if 'sid' in query else None
        if sid is None and query.get('EIO') != ['4']:
            self._log_error_once(
                'The client is using an unsupported version of the Socket.IO '
                'or Engine.IO protocols', 'bad-version')
            r = self._bad_request(
                'The client is using an unsupported version of the Socket.IO '
                'or Engine.IO protocols')
            start_response(r['status'], r['headers'])
            return [r['response']]

        if 'j' in query:
            jsonp = True
            try:
                jsonp_index = int(query['j'][0])
            except (ValueError, KeyError, IndexError):
                # Invalid JSONP index number
                pass

        if jsonp and jsonp_index is None:
            self._log_error_once('Invalid JSONP index number',
                                 'bad-jsonp-index')
            r = self._bad_request('Invalid JSONP index number')
        elif method == 'GET':
            if sid is None:
                # transport must be one of 'polling' or 'websocket'.
                # if 'websocket', the HTTP_UPGRADE header must match.
                upgrade_header = environ.get('HTTP_UPGRADE').lower() \
                    if 'HTTP_UPGRADE' in environ else None
                if transport == 'polling' \
                        or transport == upgrade_header == 'websocket':
                    r = self._handle_connect(environ, start_response,
                                             transport, jsonp_index)
                else:
                    self._log_error_once('Invalid websocket upgrade',
                                         'bad-upgrade')
                    r = self._bad_request('Invalid websocket upgrade')
            else:
                if sid not in self.sockets:
                    self._log_error_once('Invalid session ' + sid, 'bad-sid')
                    r = self._bad_request('Invalid session')
                else:
                    socket = self._get_socket(sid)
                    try:
                        packets = socket.handle_get_request(
                            environ, start_response)
                        if isinstance(packets, list):
                            r = self._ok(packets, jsonp_index=jsonp_index)
                        else:
                            r = packets
                    except exceptions.EngineIOError:
                        if sid in self.sockets:  # pragma: no cover
                            self.disconnect(sid)
                        r = self._bad_request()
                    if sid in self.sockets and self.sockets[sid].closed:
                        del self.sockets[sid]
        elif method == 'POST':
            if sid is None or sid not in self.sockets:
                self._log_error_once(
                    'Invalid session ' + (sid or 'None'), 'bad-sid')
                r = self._bad_request('Invalid session')
            else:
                socket = self._get_socket(sid)
                try:
                    socket.handle_post_request(environ)
                    r = self._ok(jsonp_index=jsonp_index)
                except exceptions.EngineIOError:
                    if sid in self.sockets:  # pragma: no cover
                        self.disconnect(sid)
                    r = self._bad_request()
                except:  # pragma: no cover
                    # for any other unexpected errors, we log the error
                    # and keep going
                    self.logger.exception('post request handler error')
                    r = self._ok(jsonp_index=jsonp_index)
        elif method == 'OPTIONS':
            r = self._ok()
        else:
            self.logger.warning('Method %s not supported', method)
            r = self._method_not_found()

        if not isinstance(r, dict):
            return r or []
        if self.http_compression and \
                len(r['response']) >= self.compression_threshold:
            encodings = [e.split(';')[0].strip() for e in
                         environ.get('HTTP_ACCEPT_ENCODING', '').split(',')]
            for encoding in encodings:
                if encoding in self.compression_methods:
                    r['response'] = \
                        getattr(self, '_' + encoding)(r['response'])
                    r['headers'] += [('Content-Encoding', encoding)]
                    break
        cors_headers = self._cors_headers(environ)
        start_response(r['status'], r['headers'] + cors_headers)
        return [r['response']]

    def start_background_task(self, target, *args, **kwargs):
        """Start a background task using the appropriate async model.

        This is a utility function that applications can use to start a
        background task using the method that is compatible with the
        selected async mode.

        :param target: the target function to execute.
        :param args: arguments to pass to the function.
        :param kwargs: keyword arguments to pass to the function.

        This function returns an object that represents the background task,
        on which the ``join()`` methond can be invoked to wait for the task to
        complete.
        """
        th = self._async['thread'](target=target, args=args, kwargs=kwargs)
        th.start()
        return th  # pragma: no cover

    def sleep(self, seconds=0):
        """Sleep for the requested amount of time using the appropriate async
        model.

        This is a utility function that applications can use to put a task to
        sleep without having to worry about using the correct call for the
        selected async mode.
        """
        return self._async['sleep'](seconds)

    def create_queue(self, *args, **kwargs):
        """Create a queue object using the appropriate async model.

        This is a utility function that applications can use to create a queue
        without having to worry about using the correct call for the selected
        async mode.
        """
        return self._async['queue'](*args, **kwargs)

    def get_queue_empty_exception(self):
        """Return the queue empty exception for the appropriate async model.

        This is a utility function that applications can use to work with a
        queue without having to worry about using the correct call for the
        selected async mode.
        """
        return self._async['queue_empty']

    def create_event(self, *args, **kwargs):
        """Create an event object using the appropriate async model.

        This is a utility function that applications can use to create an
        event without having to worry about using the correct call for the
        selected async mode.
        """
        return self._async['event'](*args, **kwargs)

    def generate_id(self):
        """Generate a unique session id."""
        id = base64.b64encode(
            secrets.token_bytes(12) + self.sequence_number.to_bytes(3, 'big'))
        self.sequence_number = (self.sequence_number + 1) & 0xffffff
        return id.decode('utf-8').replace('/', '_').replace('+', '-')

    def _generate_sid_cookie(self, sid, attributes):
        """Generate the sid cookie."""
        cookie = attributes.get('name', 'io') + '=' + sid
        for attribute, value in attributes.items():
            if attribute == 'name':
                continue
            if callable(value):
                value = value()
            if value is True:
                cookie += '; ' + attribute
            else:
                cookie += '; ' + attribute + '=' + value
        return cookie

    def _handle_connect(self, environ, start_response, transport,
                        jsonp_index=None):
        """Handle a client connection request."""
        if self.start_service_task:
            # start the service task to monitor connected clients
            self.start_service_task = False
            self.start_background_task(self._service_task)

        sid = self.generate_id()
        s = socket.Socket(self, sid)
        self.sockets[sid] = s

        pkt = packet.Packet(packet.OPEN, {
            'sid': sid,
            'upgrades': self._upgrades(sid, transport),
            'pingTimeout': int(self.ping_timeout * 1000),
            'pingInterval': int(
                self.ping_interval + self.ping_interval_grace_period) * 1000})
        s.send(pkt)
        s.schedule_ping()

        # NOTE: some sections below are marked as "no cover" to workaround
        # what seems to be a bug in the coverage package. All the lines below
        # are covered by tests, but some are not reported as such for some
        # reason
        ret = self._trigger_event('connect', sid, environ, run_async=False)
        if ret is not None and ret is not True:  # pragma: no cover
            del self.sockets[sid]
            self.logger.warning('Application rejected connection')
            return self._unauthorized(ret or None)

        if transport == 'websocket':  # pragma: no cover
            ret = s.handle_get_request(environ, start_response)
            if s.closed and sid in self.sockets:
                # websocket connection ended, so we are done
                del self.sockets[sid]
            return ret
        else:  # pragma: no cover
            s.connected = True
            headers = None
            if self.cookie:
                if isinstance(self.cookie, dict):
                    headers = [(
                        'Set-Cookie',
                        self._generate_sid_cookie(sid, self.cookie)
                    )]
                else:
                    headers = [(
                        'Set-Cookie',
                        self._generate_sid_cookie(sid, {
                            'name': self.cookie, 'path': '/', 'SameSite': 'Lax'
                        })
                    )]
            try:
                return self._ok(s.poll(), headers=headers,
                                jsonp_index=jsonp_index)
            except exceptions.QueueEmpty:
                return self._bad_request()

    def _upgrades(self, sid, transport):
        """Return the list of possible upgrades for a client connection."""
        if not self.allow_upgrades or self._get_socket(sid).upgraded or \
                transport == 'websocket':
            return []
        if self._async['websocket'] is None:  # pragma: no cover
            self._log_error_once(
                'The WebSocket transport is not available, you must install a '
                'WebSocket server that is compatible with your async mode to '
                'enable it. See the documentation for details.',
                'no-websocket')
            return []
        return ['websocket']

    def _trigger_event(self, event, *args, **kwargs):
        """Invoke an event handler."""
        run_async = kwargs.pop('run_async', False)
        if event in self.handlers:
            if run_async:
                return self.start_background_task(self.handlers[event], *args)
            else:
                try:
                    return self.handlers[event](*args)
                except:
                    self.logger.exception(event + ' handler error')
                    if event == 'connect':
                        # if connect handler raised error we reject the
                        # connection
                        return False

    def _get_socket(self, sid):
        """Return the socket object for a given session."""
        try:
            s = self.sockets[sid]
        except KeyError:
            raise KeyError('Session not found')
        if s.closed:
            del self.sockets[sid]
            raise KeyError('Session is disconnected')
        return s

    def _ok(self, packets=None, headers=None, jsonp_index=None):
        """Generate a successful HTTP response."""
        if packets is not None:
            if headers is None:
                headers = []
            headers += [('Content-Type', 'text/plain; charset=UTF-8')]
            return {'status': '200 OK',
                    'headers': headers,
                    'response': payload.Payload(packets=packets).encode(
                        jsonp_index=jsonp_index).encode('utf-8')}
        else:
            return {'status': '200 OK',
                    'headers': [('Content-Type', 'text/plain')],
                    'response': b'OK'}

    def _bad_request(self, message=None):
        """Generate a bad request HTTP error response."""
        if message is None:
            message = 'Bad Request'
        message = packet.Packet.json.dumps(message)
        return {'status': '400 BAD REQUEST',
                'headers': [('Content-Type', 'text/plain')],
                'response': message.encode('utf-8')}

    def _method_not_found(self):
        """Generate a method not found HTTP error response."""
        return {'status': '405 METHOD NOT FOUND',
                'headers': [('Content-Type', 'text/plain')],
                'response': b'Method Not Found'}

    def _unauthorized(self, message=None):
        """Generate a unauthorized HTTP error response."""
        if message is None:
            message = 'Unauthorized'
        message = packet.Packet.json.dumps(message)
        return {'status': '401 UNAUTHORIZED',
                'headers': [('Content-Type', 'application/json')],
                'response': message.encode('utf-8')}

    def _cors_allowed_origins(self, environ):
        default_origins = []
        if 'wsgi.url_scheme' in environ and 'HTTP_HOST' in environ:
            default_origins.append('{scheme}://{host}'.format(
                scheme=environ['wsgi.url_scheme'], host=environ['HTTP_HOST']))
            if 'HTTP_X_FORWARDED_PROTO' in environ or \
                    'HTTP_X_FORWARDED_HOST' in environ:
                scheme = environ.get(
                    'HTTP_X_FORWARDED_PROTO',
                    environ['wsgi.url_scheme']).split(',')[0].strip()
                default_origins.append('{scheme}://{host}'.format(
                    scheme=scheme, host=environ.get(
                        'HTTP_X_FORWARDED_HOST', environ['HTTP_HOST']).split(
                            ',')[0].strip()))
        if self.cors_allowed_origins is None:
            allowed_origins = default_origins
        elif self.cors_allowed_origins == '*':
            allowed_origins = None
        elif isinstance(self.cors_allowed_origins, str):
            allowed_origins = [self.cors_allowed_origins]
        elif callable(self.cors_allowed_origins):
            origin = environ.get('HTTP_ORIGIN')
            allowed_origins = [origin] \
                if self.cors_allowed_origins(origin) else []
        else:
            allowed_origins = self.cors_allowed_origins
        return allowed_origins

    def _cors_headers(self, environ):
        """Return the cross-origin-resource-sharing headers."""
        if self.cors_allowed_origins == []:
            # special case, CORS handling is completely disabled
            return []
        headers = []
        allowed_origins = self._cors_allowed_origins(environ)
        if 'HTTP_ORIGIN' in environ and \
                (allowed_origins is None or environ['HTTP_ORIGIN'] in
                 allowed_origins):
            headers = [('Access-Control-Allow-Origin', environ['HTTP_ORIGIN'])]
        if environ['REQUEST_METHOD'] == 'OPTIONS':
            headers += [('Access-Control-Allow-Methods', 'OPTIONS, GET, POST')]
        if 'HTTP_ACCESS_CONTROL_REQUEST_HEADERS' in environ:
            headers += [('Access-Control-Allow-Headers',
                        environ['HTTP_ACCESS_CONTROL_REQUEST_HEADERS'])]
        if self.cors_credentials:
            headers += [('Access-Control-Allow-Credentials', 'true')]
        return headers

    def _gzip(self, response):
        """Apply gzip compression to a response."""
        bytesio = io.BytesIO()
        with gzip.GzipFile(fileobj=bytesio, mode='w') as gz:
            gz.write(response)
        return bytesio.getvalue()

    def _deflate(self, response):
        """Apply deflate compression to a response."""
        return zlib.compress(response)

    def _log_error_once(self, message, message_key):
        """Log message with logging.ERROR level the first time, then log
        with given level."""
        if message_key not in self.log_message_keys:
            self.logger.error(message + ' (further occurrences of this error '
                              'will be logged with level INFO)')
            self.log_message_keys.add(message_key)
        else:
            self.logger.info(message)

    def _service_task(self):  # pragma: no cover
        """Monitor connected clients and clean up those that time out."""
        while True:
            if len(self.sockets) == 0:
                # nothing to do
                self.sleep(self.ping_timeout)
                continue

            # go through the entire client list in a ping interval cycle
            sleep_interval = float(self.ping_timeout) / len(self.sockets)

            try:
                # iterate over the current clients
                for s in self.sockets.copy().values():
                    if not s.closing and not s.closed:
                        s.check_ping_timeout()
                    self.sleep(sleep_interval)
            except (SystemExit, KeyboardInterrupt):
                self.logger.info('service task canceled')
                break
            except:
                # an unexpected exception has occurred, log it and continue
                self.logger.exception('service task exception')