[Twisted-Python] A pseudo-deferred class that can be canceled

Tristan Seligmann mithrandi at mithrandi.net
Tue Jan 5 14:12:34 EST 2010

On Tue, Jan 5, 2010 at 6:29 AM, Terry Jones <terry at jon.es> wrote:
> - Once someone has made a function call, gotten a deferred, added
>  call/errbacks to it, etc., it's gone. It's in flight. Forget about it.

The thing is, this attitude isn't always reasonable. Deferred is not
necessarily the place to implement cancellation, but I think the idiom
itself is important. Certainly there are some operations where
cancellation is not feasible; perhaps because the underlying APIs do
not support cancellation, or because allowing the operation to
complete is quicker/easier than trying to interrupt it. But, consider
something like the transfer of a 512 TB file; cancellation /is/
possible, through closing the TCP connection, and the operation is
significantly expensive, so "just forget it and let it finish" is a
somewhat cavalier attitude to take.

>  complex manner. Other code, that the Deferred class itself can't possibly
>  be aware of, may be relying on the deferred firing and at least part of
>  its callback chain being run, etc. The simplest thing to do is to just
>  provide a mechanism whereby the eventual holder of the deferred can opt
>  to trigger their deferred immediately and ignore the final result of the
>  original call (supposing there ever is one).

I would imagine that you would errback the deferred in the general
case of cancellation; if your callback chain doesn't handle errors,
mithrandi, i Ainil en-Balandor, a faer Ambar

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