text: """Tornado handlers for kernels.
  
  Preliminary documentation at https://github.com/ipython/ipython/wiki/IPEP-16%3A-Notebook-multi-directory-dashboard-and-URL-mapping#kernels-api
  """
  
  # Copyright (c) Jupyter Development Team.
  # Distributed under the terms of the Modified BSD License.
  
  import json
  import logging
  from textwrap import dedent
  
  from tornado import gen, web
  from tornado.concurrent import Future
  from tornado.ioloop import IOLoop
  
  from jupyter_client import protocol_version as client_protocol_version
  try:
      from jupyter_client.jsonutil import json_default
  except ImportError:
      from jupyter_client.jsonutil import (
          date_default as json_default
      )
  from ipython_genutils.py3compat import cast_unicode
  from notebook.utils import maybe_future, url_path_join, url_escape
  
  from ...base.handlers import APIHandler
  from ...base.zmqhandlers import AuthenticatedZMQStreamHandler, deserialize_binary_message
  
  class MainKernelHandler(APIHandler):
  
      @web.authenticated
      @gen.coroutine
      def get(self):
          km = self.kernel_manager
          kernels = yield maybe_future(km.list_kernels())
          self.finish(json.dumps(kernels, default=json_default))
  
      @web.authenticated
      @gen.coroutine
      def post(self):
          km = self.kernel_manager
          model = self.get_json_body()
          if model is None:
              model = {
                  'name': km.default_kernel_name
              }
          else:
              model.setdefault('name', km.default_kernel_name)
  
          kernel_id = yield maybe_future(km.start_kernel(kernel_name=model['name']))
          model = yield maybe_future(km.kernel_model(kernel_id))
          location = url_path_join(self.base_url, 'api', 'kernels', url_escape(kernel_id))
          self.set_header('Location', location)
          self.set_status(201)
          self.finish(json.dumps(model, default=json_default))
  
  
  class KernelHandler(APIHandler):
  
      @web.authenticated
      @gen.coroutine
      def get(self, kernel_id):
          km = self.kernel_manager
          model = yield maybe_future(km.kernel_model(kernel_id))
          self.finish(json.dumps(model, default=json_default))
  
      @web.authenticated
      @gen.coroutine
      def delete(self, kernel_id):
          km = self.kernel_manager
          yield maybe_future(km.shutdown_kernel(kernel_id))
          self.set_status(204)
          self.finish()
  
  
  class KernelActionHandler(APIHandler):
  
      @web.authenticated
      @gen.coroutine
      def post(self, kernel_id, action):
          km = self.kernel_manager
          if action == 'interrupt':
              yield maybe_future(km.interrupt_kernel(kernel_id))
              self.set_status(204)
          if action == 'restart':
  
              try:
                  yield maybe_future(km.restart_kernel(kernel_id))
              except Exception as e:
                  self.log.error("Exception restarting kernel", exc_info=True)
                  self.set_status(500)
              else:
                  model = yield maybe_future(km.kernel_model(kernel_id))
                  self.write(json.dumps(model, default=json_default))
          self.finish()
  
  
  class ZMQChannelsHandler(AuthenticatedZMQStreamHandler):
      '''There is one ZMQChannelsHandler per running kernel and it oversees all
      the sessions.
      '''
  
      # class-level registry of open sessions
      # allows checking for conflict on session-id,
      # which is used as a zmq identity and must be unique.
      _open_sessions = {}
  
      @property
      def kernel_info_timeout(self):
          km_default = self.kernel_manager.kernel_info_timeout
          return self.settings.get('kernel_info_timeout', km_default)
  
      @property
      def iopub_msg_rate_limit(self):
          return self.settings.get('iopub_msg_rate_limit', 0)
  
      @property
      def iopub_data_rate_limit(self):
          return self.settings.get('iopub_data_rate_limit', 0)
  
      @property
      def rate_limit_window(self):
          return self.settings.get('rate_limit_window', 1.0)
  
      def __repr__(self):
          return f"{self.__class__.__name__}({getattr(self, 'kernel_id', 'uninitialized')})"
  
      def create_stream(self):
          km = self.kernel_manager
          identity = self.session.bsession
          for channel in ("iopub", "shell", "control", "stdin"):
              meth = getattr(km, "connect_" + channel)
              self.channels[channel] = stream = meth(self.kernel_id, identity=identity)
              stream.channel = channel
  
      def nudge(self):
          """Nudge the zmq connections with kernel_info_requests
  
          Returns a Future that will resolve when we have received
          a shell or control reply and at least one iopub message,
          ensuring that zmq subscriptions are established,
          sockets are fully connected, and kernel is responsive.
  
          Keeps retrying kernel_info_request until these are both received.
          """
          kernel = self.kernel_manager.get_kernel(self.kernel_id)
  
          # Do not nudge busy kernels as kernel info requests sent to shell are
          # queued behind execution requests.
          # nudging in this case would cause a potentially very long wait
          # before connections are opened,
          # plus it is *very* unlikely that a busy kernel will not finish
          # establishing its zmq subscriptions before processing the next request.
          if getattr(kernel, "execution_state") == "busy":
              self.log.debug("Nudge: not nudging busy kernel %s", self.kernel_id)
              f = Future()
              f.set_result(None)
              return f
          # Use a transient shell channel to prevent leaking
          # shell responses to the front-end.
          shell_channel = kernel.connect_shell()
          # Use a transient control channel to prevent leaking
          # control responses to the front-end.
          control_channel = kernel.connect_control()
          # The IOPub used by the client, whose subscriptions we are verifying.
          iopub_channel = self.channels["iopub"]
  
          info_future = Future()
          iopub_future = Future()
          both_done = gen.multi([info_future, iopub_future])
  
          def finish(f=None):
              """Ensure all futures are resolved
  
              which in turn triggers cleanup
              """
              for f in (info_future, iopub_future):
                  if not f.done():
                      f.set_result(None)
  
          def cleanup(f=None):
              """Common cleanup"""
              loop.remove_timeout(nudge_handle)
              iopub_channel.stop_on_recv()
              if not shell_channel.closed():
                  shell_channel.close()
              if not control_channel.closed():
                  control_channel.close()
  
          # trigger cleanup when both message futures are resolved
          both_done.add_done_callback(cleanup)
  
          def on_shell_reply(msg):
              self.log.debug("Nudge: shell info reply received: %s", self.kernel_id)
              if not info_future.done():
                  self.log.debug("Nudge: resolving shell future: %s", self.kernel_id)
                  info_future.set_result(None)
  
          def on_control_reply(msg):
              self.log.debug("Nudge: control info reply received: %s", self.kernel_id)
              if not info_future.done():
                  self.log.debug("Nudge: resolving control future: %s", self.kernel_id)
                  info_future.set_result(None)
  
          def on_iopub(msg):
              self.log.debug("Nudge: IOPub received: %s", self.kernel_id)
              if not iopub_future.done():
                  iopub_channel.stop_on_recv()
                  self.log.debug("Nudge: resolving iopub future: %s", self.kernel_id)
                  iopub_future.set_result(None)
  
          iopub_channel.on_recv(on_iopub)
          shell_channel.on_recv(on_shell_reply)
          control_channel.on_recv(on_control_reply)
          loop = IOLoop.current()
  
          # Nudge the kernel with kernel info requests until we get an IOPub message
          def nudge(count):
              count += 1
  
              # NOTE: this close check appears to never be True during on_open,
              # even when the peer has closed the connection
              if self.ws_connection is None or self.ws_connection.is_closing():
                  self.log.debug(
                      "Nudge: cancelling on closed websocket: %s", self.kernel_id
                  )
                  finish()
                  return
  
              # check for stopped kernel
              if self.kernel_id not in self.kernel_manager:
                  self.log.debug(
                      "Nudge: cancelling on stopped kernel: %s", self.kernel_id
                  )
                  finish()
                  return
  
              # check for closed zmq socket
              if shell_channel.closed():
                  self.log.debug("Nudge: cancelling on closed zmq socket: %s", self.kernel_id)
                  finish()
                  return
  
              # check for closed zmq socket
              if control_channel.closed():
                  self.log.debug(
                      "Nudge: cancelling on closed zmq socket: %s", self.kernel_id
                  )
                  finish()
                  return
  
              if not both_done.done():
                  log = self.log.warning if count % 10 == 0 else self.log.debug
                  log(f"Nudge: attempt {count} on kernel {self.kernel_id}")
                  self.session.send(shell_channel, "kernel_info_request")
                  self.session.send(control_channel, "kernel_info_request")
                  nonlocal nudge_handle
                  nudge_handle = loop.call_later(0.5, nudge, count)
  
          nudge_handle = loop.call_later(0, nudge, count=0)
  
          # resolve with a timeout if we get no response
          future = gen.with_timeout(loop.time() + self.kernel_info_timeout, both_done)
          # ensure we have no dangling resources or unresolved Futures in case of timeout
          future.add_done_callback(finish)
          return future
  
      def request_kernel_info(self):
          """send a request for kernel_info"""
          km = self.kernel_manager
          kernel = km.get_kernel(self.kernel_id)
          try:
              # check for previous request
              future = kernel._kernel_info_future
          except AttributeError:
              self.log.debug("Requesting kernel info from %s", self.kernel_id)
              # Create a kernel_info channel to query the kernel protocol version.
              # This channel will be closed after the kernel_info reply is received.
              if self.kernel_info_channel is None:
                  self.kernel_info_channel = km.connect_shell(self.kernel_id)
              self.kernel_info_channel.on_recv(self._handle_kernel_info_reply)
              self.session.send(self.kernel_info_channel, "kernel_info_request")
              # store the future on the kernel, so only one request is sent
              kernel._kernel_info_future = self._kernel_info_future
          else:
              if not future.done():
                  self.log.debug("Waiting for pending kernel_info request")
              future.add_done_callback(lambda f: self._finish_kernel_info(f.result()))
          return self._kernel_info_future
  
      def _handle_kernel_info_reply(self, msg):
          """process the kernel_info_reply
  
          enabling msg spec adaptation, if necessary
          """
          idents,msg = self.session.feed_identities(msg)
          try:
              msg = self.session.deserialize(msg)
          except:
              self.log.error("Bad kernel_info reply", exc_info=True)
              self._kernel_info_future.set_result({})
              return
          else:
              info = msg['content']
              self.log.debug("Received kernel info: %s", info)
              if msg['msg_type'] != 'kernel_info_reply' or 'protocol_version' not in info:
                  self.log.error("Kernel info request failed, assuming current %s", info)
                  info = {}
              self._finish_kernel_info(info)
  
          # close the kernel_info channel, we don't need it anymore
          if self.kernel_info_channel:
              self.kernel_info_channel.close()
          self.kernel_info_channel = None
  
      def _finish_kernel_info(self, info):
          """Finish handling kernel_info reply
  
          Set up protocol adaptation, if needed,
          and signal that connection can continue.
          """
          protocol_version = info.get('protocol_version', client_protocol_version)
          if protocol_version != client_protocol_version:
              self.session.adapt_version = int(protocol_version.split('.')[0])
              self.log.info(f"Adapting from protocol version {protocol_version} (kernel {self.kernel_id}) to {client_protocol_version} (client).")
          if not self._kernel_info_future.done():
              self._kernel_info_future.set_result(info)
  
      def initialize(self):
          super().initialize()
          self.zmq_stream = None
          self.channels = {}
          self.kernel_id = None
          self.kernel_info_channel = None
          self._kernel_info_future = Future()
          self._close_future = Future()
          self.session_key = ''
  
          # Rate limiting code
          self._iopub_window_msg_count = 0
          self._iopub_window_byte_count = 0
          self._iopub_msgs_exceeded = False
          self._iopub_data_exceeded = False
          # Queue of (time stamp, byte count)
          # Allows you to specify that the byte count should be lowered
          # by a delta amount at some point in the future.
          self._iopub_window_byte_queue = []
  
      @gen.coroutine
      def pre_get(self):
          # authenticate first
          super().pre_get()
          # check session collision:
          yield self._register_session()
          # then request kernel info, waiting up to a certain time before giving up.
          # We don't want to wait forever, because browsers don't take it well when
          # servers never respond to websocket connection requests.
          kernel = self.kernel_manager.get_kernel(self.kernel_id)
          self.session.key = kernel.session.key
          future = self.request_kernel_info()
  
          def give_up():
              """Don't wait forever for the kernel to reply"""
              if future.done():
                  return
              self.log.warning("Timeout waiting for kernel_info reply from %s", self.kernel_id)
              future.set_result({})
          loop = IOLoop.current()
          loop.add_timeout(loop.time() + self.kernel_info_timeout, give_up)
          # actually wait for it
          yield future
  
      @gen.coroutine
      def get(self, kernel_id):
          self.kernel_id = cast_unicode(kernel_id, 'ascii')
          yield super().get(kernel_id=kernel_id)
  
      @gen.coroutine
      def _register_session(self):
          """Ensure we aren't creating a duplicate session.
  
          If a previous identical session is still open, close it to avoid collisions.
          This is likely due to a client reconnecting from a lost network connection,
          where the socket on our side has not been cleaned up yet.
          """
          self.session_key = f'{self.kernel_id}:{self.session.session}'
          stale_handler = self._open_sessions.get(self.session_key)
          if stale_handler:
              self.log.warning("Replacing stale connection: %s", self.session_key)
              yield stale_handler.close()
          if self.kernel_id in self.kernel_manager:  # only update open sessions if kernel is actively managed
              self._open_sessions[self.session_key] = self
  
      def open(self, kernel_id):
          super().open()
          km = self.kernel_manager
          km.notify_connect(kernel_id)
  
          # on new connections, flush the message buffer
          buffer_info = km.get_buffer(kernel_id, self.session_key)
          if buffer_info and buffer_info['session_key'] == self.session_key:
              self.log.info("Restoring connection for %s", self.session_key)
              self.channels = buffer_info['channels']
              connected = self.nudge()
  
              def replay(value):
                  replay_buffer = buffer_info['buffer']
                  if replay_buffer:
                      self.log.info("Replaying %s buffered messages", len(replay_buffer))
                      for channel, msg_list in replay_buffer:
                          stream = self.channels[channel]
                          self._on_zmq_reply(stream, msg_list)
  
              connected.add_done_callback(replay)
          else:
              try:
                  self.create_stream()
                  connected = self.nudge()
              except web.HTTPError as e:
                  self.log.error("Error opening stream: %s", e)
                  # WebSockets don't response to traditional error codes so we
                  # close the connection.
                  for channel, stream in self.channels.items():
                      if not stream.closed():
                          stream.close()
                  self.close()
                  return
  
          km.add_restart_callback(self.kernel_id, self.on_kernel_restarted)
          km.add_restart_callback(self.kernel_id, self.on_restart_failed, 'dead')
  
          def subscribe(value):
              for channel, stream in self.channels.items():
                  stream.on_recv_stream(self._on_zmq_reply)
  
          connected.add_done_callback(subscribe)
  
          return connected
  
      def on_message(self, msg):
          if not self.channels:
              # already closed, ignore the message
              self.log.debug("Received message on closed websocket %r", msg)
              return
          if isinstance(msg, bytes):
              msg = deserialize_binary_message(msg)
          else:
              msg = json.loads(msg)
          channel = msg.pop('channel', None)
          if channel is None:
              self.log.warning("No channel specified, assuming shell: %s", msg)
              channel = 'shell'
          if channel not in self.channels:
              self.log.warning("No such channel: %r", channel)
              return
          am = self.kernel_manager.allowed_message_types
          mt = msg['header']['msg_type']
          if am and mt not in am:
              self.log.warning(f'Received message of type "{mt}", which is not allowed. Ignoring.')
          else:
              stream = self.channels[channel]
              self.session.send(stream, msg)
  
      def _on_zmq_reply(self, stream, msg_list):
          idents, fed_msg_list = self.session.feed_identities(msg_list)
          msg = self.session.deserialize(fed_msg_list)
          parent = msg['parent_header']
          def write_stderr(error_message):
              self.log.warning(error_message)
              msg = self.session.msg("stream",
                  content={"text": error_message + '\n', "name": "stderr"},
                  parent=parent
              )
              msg['channel'] = 'iopub'
              self.write_message(json.dumps(msg, default=json_default))
          channel = getattr(stream, 'channel', None)
          msg_type = msg['header']['msg_type']
  
          if channel == 'iopub' and msg_type == 'status' and msg['content'].get('execution_state') == 'idle':
              # reset rate limit counter on status=idle,
              # to avoid 'Run All' hitting limits prematurely.
              self._iopub_window_byte_queue = []
              self._iopub_window_msg_count = 0
              self._iopub_window_byte_count = 0
              self._iopub_msgs_exceeded = False
              self._iopub_data_exceeded = False
  
          if channel == 'iopub' and msg_type not in {'status', 'comm_open', 'execute_input'}:
  
              # Remove the counts queued for removal.
              now = IOLoop.current().time()
              while len(self._iopub_window_byte_queue) > 0:
                  queued = self._iopub_window_byte_queue[0]
                  if (now >= queued[0]):
                      self._iopub_window_byte_count -= queued[1]
                      self._iopub_window_msg_count -= 1
                      del self._iopub_window_byte_queue[0]
                  else:
                      # This part of the queue hasn't be reached yet, so we can
                      # abort the loop.
                      break
  
              # Increment the bytes and message count
              self._iopub_window_msg_count += 1
              if msg_type == 'stream':
                  byte_count = sum(len(x) for x in msg_list)
              else:
                  byte_count = 0
              self._iopub_window_byte_count += byte_count
  
              # Queue a removal of the byte and message count for a time in the
              # future, when we are no longer interested in it.
              self._iopub_window_byte_queue.append((now + self.rate_limit_window, byte_count))
  
              # Check the limits, set the limit flags, and reset the
              # message and data counts.
              msg_rate = float(self._iopub_window_msg_count) / self.rate_limit_window
              data_rate = float(self._iopub_window_byte_count) / self.rate_limit_window
  
              # Check the msg rate
              if self.iopub_msg_rate_limit > 0 and msg_rate > self.iopub_msg_rate_limit:
                  if not self._iopub_msgs_exceeded:
                      self._iopub_msgs_exceeded = True
                      write_stderr(dedent(f"""\
                      IOPub message rate exceeded.
                      The notebook server will temporarily stop sending output
                      to the client in order to avoid crashing it.
                      To change this limit, set the config variable
                      `--NotebookApp.iopub_msg_rate_limit`.
  
                      Current values:
                      NotebookApp.iopub_msg_rate_limit={self.iopub_msg_rate_limit} (msgs/sec)
                      NotebookApp.rate_limit_window={self.rate_limit_window} (secs)
                      """))
              else:
                  # resume once we've got some headroom below the limit
                  if self._iopub_msgs_exceeded and msg_rate < (0.8 * self.iopub_msg_rate_limit):
                      self._iopub_msgs_exceeded = False
                      if not self._iopub_data_exceeded:
                          self.log.warning("iopub messages resumed")
  
              # Check the data rate
              if self.iopub_data_rate_limit > 0 and data_rate > self.iopub_data_rate_limit:
                  if not self._iopub_data_exceeded:
                      self._iopub_data_exceeded = True
                      write_stderr(dedent(f"""\
                      IOPub data rate exceeded.
                      The notebook server will temporarily stop sending output
                      to the client in order to avoid crashing it.
                      To change this limit, set the config variable
                      `--NotebookApp.iopub_data_rate_limit`.
  
                      Current values:
                      NotebookApp.iopub_data_rate_limit={self.iopub_data_rate_limit} (bytes/sec)
                      NotebookApp.rate_limit_window={self.rate_limit_window} (secs)
                      """))
              else:
                  # resume once we've got some headroom below the limit
                  if self._iopub_data_exceeded and data_rate < (0.8 * self.iopub_data_rate_limit):
                      self._iopub_data_exceeded = False
                      if not self._iopub_msgs_exceeded:
                          self.log.warning("iopub messages resumed")
  
              # If either of the limit flags are set, do not send the message.
              if self._iopub_msgs_exceeded or self._iopub_data_exceeded:
                  # we didn't send it, remove the current message from the calculus
                  self._iopub_window_msg_count -= 1
                  self._iopub_window_byte_count -= byte_count
                  self._iopub_window_byte_queue.pop(-1)
                  return
          super()._on_zmq_reply(stream, msg)
  
      def close(self):
          super().close()
          return self._close_future
  
      def on_close(self):
          self.log.debug("Websocket closed %s", self.session_key)
          # unregister myself as an open session (only if it's really me)
          if self._open_sessions.get(self.session_key) is self:
              self._open_sessions.pop(self.session_key)
  
          km = self.kernel_manager
          if self.kernel_id in km:
              km.notify_disconnect(self.kernel_id)
              km.remove_restart_callback(
                  self.kernel_id, self.on_kernel_restarted,
              )
              km.remove_restart_callback(
                  self.kernel_id, self.on_restart_failed, 'dead',
              )
  
              # start buffering instead of closing if this was the last connection
              if km._kernel_connections[self.kernel_id] == 0:
                  km.start_buffering(self.kernel_id, self.session_key, self.channels)
                  self._close_future.set_result(None)
                  return
  
          # This method can be called twice, once by self.kernel_died and once
          # from the WebSocket close event. If the WebSocket connection is
          # closed before the ZMQ streams are setup, they could be None.
          for channel, stream in self.channels.items():
              if stream is not None and not stream.closed():
                  stream.on_recv(None)
                  stream.close()
  
          self.channels = {}
          self._close_future.set_result(None)
  
      def _send_status_message(self, status):
          iopub = self.channels.get('iopub', None)
          if iopub and not iopub.closed():
              # flush IOPub before sending a restarting/dead status message
              # ensures proper ordering on the IOPub channel
              # that all messages from the stopped kernel have been delivered
              iopub.flush()
          msg = self.session.msg("status",
              {'execution_state': status}
          )
          msg['channel'] = 'iopub'
          self.write_message(json.dumps(msg, default=json_default))
  
      def on_kernel_restarted(self):
          logging.warn("kernel %s restarted", self.kernel_id)
          self._send_status_message('restarting')
  
      def on_restart_failed(self):
          logging.error("kernel %s restarted failed!", self.kernel_id)
          self._send_status_message('dead')
  
  
  #-----------------------------------------------------------------------------
  # URL to handler mappings
  #-----------------------------------------------------------------------------
  
  
  _kernel_id_regex = r"(?P<kernel_id>\w+-\w+-\w+-\w+-\w+)"
  _kernel_action_regex = r"(?P<action>restart|interrupt)"
  
  default_handlers = [
      (r"/api/kernels", MainKernelHandler),
      (fr"/api/kernels/{_kernel_id_regex}", KernelHandler),
      (fr"/api/kernels/{_kernel_id_regex}/{_kernel_action_regex}", KernelActionHandler),
      (fr"/api/kernels/{_kernel_id_regex}/channels", ZMQChannelsHandler),
  ]
  