Many moons ago, while we were frantically trying to make Firefox OS 1.0 run half-decently, I introduced a derived class of nsIRunnable that could be canceled: nsICancelableRunnable. As with many other things that led to the first release of FxOS this was done in a rush and without much thinking. It's only goal at the time was to allow me to cancel a single, simple, stateless runnable that could take a while to run.
Thinking about it afterwards I came to the conclusion that this was a
terrible approach: implementing the Cancel() method was up to the user
and there was no telling on which threads Run() and Cancel() could be
called making it potentially racy. So I thought it could be replaced by
adding a Cancel() method to the event queue that would just remove the
event in a thread-safe way. This seemed a cleaner, safer approach.
But then I looked into our code and discovered that
nsICancelableRunnable has spread like a disease through the codebase and
it's systematically used in workers to let us cancel events posted to
them. Ouch. Some of its uses highlight its problems: in some cases its
only purpose is to force calling the Run() method by the calling thread
so that certain objects get destroyed on that thread (possibly to
prevent leaks). In other cases we have patterns like this:
Run() {
if (mDoSomething) {
DoSomething();
}
}
Cancel() {
mDoSomething = false;
}
Which is fine if both methods are called by the same thread, but most
likely not if they're called on different threads and the interface
itself does nothing to prevent you from doing it.
All in all I'd still like to get rid of it but I'm not sure if my
original approach of moving the cancel functionality into the EventQueue
is the right way to go. Which leads me to the point of this e-mail: to
gather feedback on my idea to get rid of it. Since I don't have enough
knowledge about all the affected areas I'd like to hear from those who
actually used it.
Gabriele
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