text: """The client and server for a basic ping-pong style heartbeat.
  """
  
  #-----------------------------------------------------------------------------
  #  Copyright (C) 2008-2011  The IPython Development Team
  #
  #  Distributed under the terms of the BSD License.  The full license is in
  #  the file COPYING, distributed as part of this software.
  #-----------------------------------------------------------------------------
  
  #-----------------------------------------------------------------------------
  # Imports
  #-----------------------------------------------------------------------------
  
  import errno
  import os
  import socket
  from threading import Thread
  
  import zmq
  
  from IPython.utils.localinterfaces import localhost
  
  #-----------------------------------------------------------------------------
  # Code
  #-----------------------------------------------------------------------------
  
  
  class Heartbeat(Thread):
      "A simple ping-pong style heartbeat that runs in a thread."
  
      def __init__(self, context, addr=None):
          if addr is None:
              addr = ('tcp', localhost(), 0)
          Thread.__init__(self)
          self.context = context
          self.transport, self.ip, self.port = addr
          if self.port == 0:
              if addr[0] == 'tcp':
                  s = socket.socket()
                  # '*' means all interfaces to 0MQ, which is '' to socket.socket
                  s.bind(('' if self.ip == '*' else self.ip, 0))
                  self.port = s.getsockname()[1]
                  s.close()
              elif addr[0] == 'ipc':
                  self.port = 1
                  while os.path.exists("%s-%s" % (self.ip, self.port)):
                      self.port = self.port + 1
              else:
                  raise ValueError("Unrecognized zmq transport: %s" % addr[0])
          self.addr = (self.ip, self.port)
          self.daemon = True
  
      def run(self):
          self.socket = self.context.socket(zmq.REP)
          self.socket.linger = 1000
          c = ':' if self.transport == 'tcp' else '-'
          self.socket.bind('%s://%s' % (self.transport, self.ip) + c + str(self.port))
          while True:
              try:
                  zmq.device(zmq.FORWARDER, self.socket, self.socket)
              except zmq.ZMQError as e:
                  if e.errno == errno.EINTR:
                      continue
                  else:
                      raise
              else:
                  break
  