无法取消 Twisted Deferred 链 (AlreadyCalledError)

Question

每当某些链接的延迟（子）引发错误时，我都会尝试取消 Deferred 链（主）。 但我收到了AlreadyCalledError 并且连锁店继续工作。

代码如下：

from twisted.internet import defer

def asyncText(s, okCallback, errCallback):
  deferred = defer.Deferred()
  deferred.addCallbacks(okCallback, errCallback)
  if s.find('STOP')>=0:
    deferred.errback(ValueError('You are trying to print the unprintable :('))
  else:
    deferred.callback(s)
  return deferred

def asyncChain():
  texts = ['Hello StackOverflow,', 
           'this is an example of Twisted.chainDeferred()',
           'which is raising an AlreadyCalledError',
           'when I try to cancel() it.',
           'STOP => I will raise error and should stop the main deferred',
           'Best regards'
           ]

  mainDeferred= defer.Deferred()

  for text in texts: 
    def onSuccess(res):
      print('>> {}'.format(res))

    def onError(err):
      print('Error!! {}'.format(err))
      mainDeferred.cancel()

    d = asyncText(text, onSuccess, onError)
    mainDeferred.chainDeferred(d)

这是输出：

>>> asyncChain()
- Hello StackOverflow,
- this is an example of Twisted.chainDeferred()
- which is raising an AlreadyCalledError
- when I try to cancel() it.
Error!! [Failure instance: Traceback (failure with no frames): <class 'ValueError'>: You are trying to print the unprintable :(
]
- Best regards
Unhandled error in Deferred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../test.py", line 1, in asyncChain
    mainDeferred.chainDeferred(d)
  File ".../.venvs/p3/lib/python3.6/site-packages/twisted/internet/defer.py", line 435, in chainDeferred
    return self.addCallbacks(d.callback, d.errback)
  File ".../.venvs/p3/lib/python3.6/site-packages/twisted/internet/defer.py", line 311, in addCallbacks
    self._runCallbacks()
--- <exception caught here> ---
  File ".../.venvs/p3/lib/python3.6/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File ".../.venvs/p3/lib/python3.6/site-packages/twisted/internet/defer.py", line 501, in errback
    self._startRunCallbacks(fail)
  File ".../.venvs/p3/lib/python3.6/site-packages/twisted/internet/defer.py", line 561, in _startRunCallbacks
    raise AlreadyCalledError
twisted.internet.defer.AlreadyCalledError: 

我也尝试使用canceller，如下所示：

def asyncOnCancel(d):
  print('------ cancel() called ------')
  d.errback(ValueError('chain seems to be cancelled!'))
def asyncChainOnError(err):
  print('------ ERROR ON Chain {} ------'.format(err))

...
  mainDeferred= defer.Deferred(canceller= asyncOnCancel)
  mainDeferred.addErrback(asyncChainOnError)
...

但结果是一样的。

我还尝试过延迟孩子的.callback(s) 呼叫，或者在.chainDeferred() 之后呼叫他们。但我总是得到相同的行为。

• 真的有可能取消链接的Deferred（并让链接的子节点延迟也被取消）吗？
• 为什么我会收到这个AlreadyCalledError？

我正在使用 python 3.6.6 和 Twisted 18.9.0。

谢谢！

******* 编辑 *******

在Jean-Paul 回答之后，并注意到.chainDeferred() 不是我需要的，我会以更清晰的方式将我想要的（以及我最终是如何做到的）放在这里。

我想要的非常简单：以“同步”方式运行多个 Deferred（它们必须等待前一个完成），尽管它们不需要共享结果。如果一个失败，其余的都不会执行。

事实证明，使用 @defer.inlineCallbacks 和 yield 很容易做到这一点。举个例子：

def asyncText(s):
  deferred = defer.Deferred()
  if s.find('STOP') >= 0:
    deferred.callback(True)
  else:
    deferred.callback(False)
  return deferred

@defer.inlineCallbacks
def asyncChain():
  texts = ['Hello StackOverflow,',
           'this is a simpler way to explain the question.',
           'I need no chainDeferred(). I need no .cancel().',
           'I just need inlineCallbacks decorator.',
           'STOP',
           'Yeah I will not be printed'
           ]
  for text in texts:
    stopHere = yield asyncText(text)
    if stopHere:
      break
    print('- {}'.format(text))

deferred= asyncChain()

>>> asyncChain()
- Hello StackOverflow,
- this is a simpler way to explain the question.
- I need no chainDeferred(). I need no .cancel().
- I just need inlineCallbacks decorator.

Answer 1

目前尚不清楚您实际上要做什么，但我认为您的链条与您的预期相反：

    d = asyncText(text, onSuccess, onError)
    mainDeferred.chainDeferred(d)

d 在asyncText 返回时已经触发。但是mainDeferred.chainDeferred(d) 的意思是“当 mainDeferred 触发时，将其结果传递给 d”。由于d 已经被解雇，这是无效的。 Deferred 只能触发一次。

也许您的意思是d.chainDeferred(mainDeferred)。然后“当 d 触发时，将其结果传递给 mainDeferred”。

但是，仍然存在一个问题，因为如果将d 链接到mainDeferred，那么在d 的回调中取消 mainDeferred 是没有意义的。结果将从d 传播到mainDeferred，因为它们是链式的。取消没有必要也没有用。