Hi Kai

I think it's helpful to keep clear on two different things that cancelation is intended to do: 1) to fire the original deferred so that things relying on it can proceed, and 2) to try to terminate an ongoing action that the deferred might be waiting on.

For 1, I think calling cancel() should *always* result in the deferred being fired (with, as it currently stands, CancelledError being used if a provided cancel function does not fire the deferred itself). Always firing the deferred is very important because the caller of cancel may have set up many deferreds that rely on each other and their entire program may not be able to proceed at all until the offending deferred is actually fired. It's also contractually simple, and easy to document & understand.

For 2, the question is: do we want to also return information to the caller if 2a) the underlying cancel function detects that it cannot, or can no longer, stop the operation, or 2b) there is some kind of exception when cancel calls the cancellation function. I don't think 2a) is really an exception situation, so it makes sense, as you say, just to return False from cancel in this case. It's basically the cancel function saying it was too late to do anything about the underlying operation but not providing more information than that. Internally raising and catching CancellationFailedError (as in your code) in that case seems good to me. In the case of 2b) I would just let the exception bubble up to the calling code. Agreed, it could break some existing code, but isn't that existing code already subject to that exact failure? It's just currently undefined/undocumented.

Terry

On Thu, Aug 29, 2013 at 10:27 AM, zhang kai <kylerzhang11@gmail.com> wrote:

Hi,
As itamar mentioned in ticket #6676, If a cancellation function for a Deferred throws an exception(the cancel() method of Deferred won’t throw exceptions, but the canceller may), behavior is undefined. If the cancellation function throws an exception it is currently not caught, and cancellation does not occur.

We can catch the exception and log it, and fallback to just firing Deferred withCancelledError. This won’t break any old code. But an exception raising from the cancellation function often means the cancellation is failed.

Another option we have is taking this opportunity to make the cancellation being able to fail. There is the motivation:

There are cases where a Deferred is uncancellable. For example, we can call twisted.mail.imap4.IMAP4Client.delete to delete a mailbox. When the operation is waiting in the queue, we can cancel it by removing it from the queue. However, when the operation is already sent and is waiting for the response, it becomes uncancellable.

If we allow the canceller(NOT the cancel() method of the Deferred) to raise an exception, we can tell the user the cancellation is failed and the Deferredwon’t be fired with a CancelledError.

Raising an exception from cancel() may break the old code. So we can catch the exception raised by the canceller, then return a False without firing theDeferred to tell the user that the cancellation is failed.

In order to avoid missing unexpected exceptions, we can create a CancellationFailedError. When the canceller raises CancellationFailedError, we catch it and return False. When the canceller raises others exceptions, we catch it, log it then return False.

Something like this:
def cancel(self):
    if not self.called:
        canceller = self._canceller
        if canceller:
            try:
                canceller(self)
            except CancellationFailedError:
                return False
            except Exception:
                log.err(None, "Unexpected exception from canceller.")
                return False
        else:
            # Arrange to eat the callback that will eventually be fired
            # since there was no real canceller.
            self._suppressAlreadyCalled = True
        if not self.called:
            # There was no canceller, or the canceller didn't call
            # callback or errback.
            self.errback(failure.Failure(CancelledError()))
        return True
    elif isinstance(self.result, Deferred):
        # Waiting for another deferred -- cancel it instead.
        return self.result.cancel()
    else:
        return False
This won’t break any code by raising an exception from cancel(), although some code may rely on cancel() not returning any value.

So, what’s your opinion on raising an exception from the canceller?

Regards,
-Kai
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