Asynchronous workers using a state machine.

• A state machine calls a single-argument worker function in a loop:

  def func(graph):
      pass
  
  state_machine = StateMachine(graph, func)
  state_machine.run()
  

• A worker function can return another callable to cause a state transition:

  def next_func(graph):
      pass
  
  def func(graph):
      return next_func
  

• A worker function can raise SleepNow to cause the state machine to sleep before processing itself again:

  def func(graph):
      raise SleepNow
  

• A worker function can raise an exception. By default, all exceptions except FatalError are swallowed by the state machine's error handler although the state machine can be made to fail fast by configuring the error policy to be strict.

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