Thanks a lot for this clever solution. Even as a temporary fix, this is a big improvement over the previous behavior (and I got to learn about MemoryReactorClock.pump -- cool!).

One thing I noticed here (and please correct me if I'm misunderstanding) is that the result of Poller.target isn't actually being propagated up to the waiting Deferred (and, likewise, your Failure isn't wrapping the exception) resulting in those details being swallowed. Given the currently-very-narrow/singular usage of Poll, however, this isn't a big deal -- and is certainly not worth holding anything up.

Merging now... Thanks again!

Thanks a lot for this clever solution. Even as a temporary fix, this is a big improvement over the previous behavior (and I got to learn about MemoryReactorClock.pump -- cool!).

One thing I noticed here (and please correct me if I'm misunderstanding) is that the result of Poller.target isn't actually being propagated up to the waiting Deferred (and, likewise, your Failure isn't wrapping the exception) resulting in those details being swallowed.

Hm. I didn't test that case so we should assume it is broken. :) However, considering this code from Poller:

        try:
            ready = yield self.target()
            if ready:
                self._completed()
            else:
                self._schedule()
        except Exception:  # pylint: disable=broad-except
            self._deliver_result(Failure())

that propagation is exactly the intent. If self.target() fails then the except Exception: block should execute. Failure() (called with no arguments like that) captures the "current exception state" to initialize the Failure instance. Then _deliver_result will send it to all waiting Deferreds. It does so with Deferred.callback but that is equivalent to Deferred.errback when called with a Failure.

There's plenty of places things could go wrong there so I should have tested it...

Failure() (called with no arguments like that) captures the "current exception state" to initialize the Failure instance.

Ooh... I did not know that a Failure() would bubble everything up like that (and assumed that, given the python mantra of "explicit is better than implicit", the outer exception object would naturally have to be passed in). Then again, I typically use inlineCallbacks with traditional exceptions over the callback/errback methods... Anyway, good to know! Thank you for explaining. :)