TaskSource-specific TaskCanceller

[gterzian]

In order to support stuff like #21114, we need the ability to cancel tasks in a more granular way, at the level of 'per tasksource'.

Currently 'cancelling' tasks is basically dependent on the ignore_further_async_events field on Window, which is the basis for the TaskCanceller returned via task_canceller, which is accessed via the queue method of the TaskSource trait(via the globalscope passed to that function).

One idea I can come up with is adding a new method to TaskSource, something like
fn choose_canceller<T>(&self, global: &GlobalScope) -> TaskCanceller, and then change queue to read something like the below:

let canceller = self.choose_canceller(&global);
self.queue_with_canceller(task, canceller);

Then each task source will need to implement choose_canceller, calling the appropriate method of GlobalScope, which would forward the call to the window, and the window would require a flag similar to ignore_further_async_events for each task source...

Any better ideas?

[jdm]

That implementation makes sense to me!