class token():
  '''
  Poor man's interprocess lock

  To use from another module:
      with baton.token() as t:
        do_something()

  The token will be release when comming out of the "with".
  '''
  filename = '/var/lock/my.token'
  delay = 2

  def __init__(self):
    self.gotit = False
    self.get()

  def get(self):
    import os
    import sys
    import logging
    import random
    import time
    if not self.gotit:
      pid = os.getpid()
      while True:
        try:
          fd = os.open(self.filename, os.O_WRONLY | os.O_CREAT | os.O_EXCL)
        except OSError as err:
          if err.errno == 17:
            logging.info('waiting for token')
            time.sleep(random.randrange(1,10))
            continue
          else:
            # e.g.: a permission issue
            raise
        break
      f = os.fdopen(fd, 'w')
      f.writelines('pid: ' + str(pid) + '\n')
      f.close()
      self.gotit = True

  def release(self):
    import os
    import time
    import logging
    if self.gotit:
      logging.debug('releasing token')
      time.sleep(self.delay)
      os.unlink(self.filename)
      self.gotit = False

  def __del__(self):
    self.release()

  def __enter__(self):
    return self
    
  def __exit__(self, type, value, traceback):
    self.release()
    # We don't care if / why the calling value failed, we need to
    # get rid of the token.
    return True