On 30/09/10 11:48, Chris Withers wrote:
On 30/09/2010 04:45, exarkun@twistedmatrix.com wrote:
On 12:52 am, chris@simplistix.co.uk wrote:
Because I haven't found any permutation that doesn't result in the LoopingCall's calls to `loop` from stopping after the exception.
I would be more than ecstatic to be proved wrong ;-)
You keep saying the LoopingCall stops. It does not stop. It is waiting for a result.
What result is it waiting for and how do I pass that from the `loop` function in my first example when the exception is caught?
It is waiting for the deferred you return to callback/errback. This, I found, was initially non-obvious. I assumed that LoopingCall would be called every N seconds regardless, but it won't. If you return a deferred, the LoopingCall will stop until the deferred callback/errback's, then it will be restarted. This is very useful in some circumstances.
If you provide it with a result, it will proceed.
See above: how do I do that?
Glyph suggested a number of (hackfully) mechanisms you might use to provide it with a result.
Why do you say hackfully? I'm trying to build a scheduler that is
Because this is (apparently) a bug in the underlying Twisted code, so anything you do to fix that is a hack. The quickest way I can describe it is this; as I understand, any Twisted function which returns a deferred should never raise an Exception. The exception should instead flow down the errback. This is because (as you've discovered) there are circumstances when a function is called in Twisted where the exception can't propagate "up" to the caller, because the caller is no longer higher in the call stack. Most prominent is reactor.callLater.
resilient to failure in a scheduled task. Yes, of course I want to fix that failure, but I don't want a failure in one scheduled task to prevent any further scheduled tasks from being executed...
I suggested another way (fix the broken code, or at least supply enough information for someone else to fix it).
For all I know, it's already fixed in a newer version of Twisted (unfortunately, I can't move this project to a newer version of Twisted) but once I have a robust scheduler, I'll certainly do my best to report the underlying failure to you guys properly...
If you can produce a short example that uses the real Twisted API which fails, people might a) be able to find/fix the bug and b) suggest a less hacky way of ensuring you see the error.