Following up on my own post:

When the backoff iterator finishes, the RetryingCall class gives up on trying to get a non-error result from the function. In that case you just get a StopIteration exception in the failure that start() deferred returns (I was originally returning the original failure, but decided to simplify. If you want that, you can keep it yourself in an error tracking class, see below).

Thinking about it a bit more it seems more sensible that the start method's default failureTester function ignores all failures, and that in the case of an eventual failure, you get the first ever (or last ever) failure back. Those changes would make the RetryingCall class more useful (as it stands with the defaults you get a failure back the first time something goes wrong, so in the default case the class doesn't even do what it says). I wont post the changes here, as I don't even know if anyone else is interested. Contact me if you want an updated version. Terry