future.py Future is not thread safe，may loss callback

[anexplore]

as we can see future.py has no locks, when we use kafka proudcer's method send and add callback, the callback may never called;

when execute success, success callback are called by sender thread, and we add callback in our's work thread;
as 1-5 steps, callback will never be called

# work thread
kafka_producer.send(topic, value).add_both(callback)

 def success(self, value):
       # called by sender
        assert not self.is_done, 'Future is already complete'
       #
       # 1  if sender thread execute here and then stop and switch to work thread
       #
       self.value = value
        self.is_done = True
        if self._callbacks:
            self._call_backs('callback', self._callbacks, self.value)
       # 4  sender thread execute finished and switch to work thread
        return self

    def failure(self, e):
        assert not self.is_done, 'Future is already complete'
        self.exception = e if type(e) is not type else e()
        assert isinstance(self.exception, BaseException), (
            'future failed without an exception')
        self.is_done = True
        self._call_backs('errback', self._errbacks, self.exception)
        return self

    def add_callback(self, f, *args, **kwargs):
        if args or kwargs:
            f = functools.partial(f, *args, **kwargs)
       #
       # 2  work thread execute here and is_done is still False 
       # 
       if self.is_done and not self.exception:
            self._call_backs('callback', [f], self.value)
        else:
            # 3  work thread stop here and been switch to sender thread
            self._callbacks.append(f)
           # 5  work thread add callback , but this call back will never be called
        return self