无法取消 Twisted Deferred 链（AlreadyCalledError）

Question

每当某些链式延迟 (child) 引发错误时，我都会尝试取消 Deferred 链（主）。但是我得到了 AlreadyCalledError 并且连锁店继续其工作。

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 Whosebug,', 
           '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 Whosebug,
- 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:

我也试过使用 canceller，像这样：

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

我也试过延迟 child .callback(s) 次调用，或者在 .chainDeferred() 之后调用它们。但我总是得到同样的行为。

真的有可能取消链式 Deferred（并且被链式 child 延迟也被取消）？
为什么我收到这个 AlreadyCalledError？

我正在使用 python 3.6.6 和 Twisted 18.9.0。

谢谢！

******* 编辑 *******

在回答后，注意到 .chainDeferred() 不是我需要的，我将在这里更清楚地说明我想要什么（以及我最终是如何做到的）。

我想要的更简单：运行几个 Deferred，以 "synchronous" 的方式（他们必须等待前一个完成），尽管他们不需要分享他们的成果。一个失败，其余的不执行。

原来用 @defer.inlineCallbacks 和 yield 很容易做到。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 Whosebug,',
           '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 Whosebug,
- this is a simpler way to explain the question.
- I need no chainDeferred(). I need no .cancel().
- I just need inlineCallbacks decorator.

Answer 1

不清楚您实际想要做什么，但我认为您的链条与您的预期相反：

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

d 已在 asyncText returns 时触发。但是 mainDeferred.chainDeferred(d) 意味着 "when mainDeferred fires, pass its result to d"。因为 d 已经开火了，所以这是无效的。 Deferred 只能触发一次。

也许您的意思是 d.chainDeferred(mainDeferred)。然后"when d fires, pass its result to mainDeferred".

但是，仍然存在一个问题，因为如果您将 d 链接到 mainDeferred，那么取消 [=17 就没有意义了=] 在 d 的回调中。结果将从 d 传播到 mainDeferred，因为它们是链接在一起的。取消没有必要也没有用。

无法取消 Twisted Deferred 链（AlreadyCalledError）

Unable to cancel Twisted Deferred chain (AlreadyCalledError)

python-3.x

twisted

twisted.internet