Cancellation can't be controlled finely enough

[fresheneesz]

If you have multiple levels of cancellable processes in the different promise chains, currently bluebird has no way of specifying what you want to cancel - either the whole thing will be canceled, or none of it.

Conceptual example:

promise x

Process A {
  step 1: Wait on x
  step 2: 
  step 3: 
}
Process B { 
  step 1: Wait on x
  step 2: 
  step 3: 
}

In this example, if you want to cancel process A before promise x has resolved, process B will never complete, even tho it doesn't have anything to do with process A's cancellation.

A concrete example:

var p = Promise.resolve('mooose').delay(1000).cancellable()

p.then(function(x) {
    console.log("Process A done")
}).catch(function(e) {
    console.log("Process A was canceled (or cancelled)! Hopefully because p was cancelled directly?")
})

var processB = p.then(function(x) {
    console.log("Process B started")
})

var x = processB.cancellable().then(function() {
    console.log("Process B done")
}).catch(function(e) {
    console.log("Process B was canceled (or cancelled)!")
})

processB.cancel()
// x.cancel()          // same result

The result of this is that both process A and process B are cancelled when I only wanted to cancel process B. Is there something i'm missing about this API that resolves this?

What I would expect is that instead of cancel cancelling the farthest ancestor promise, that it would propogate to the closest descendant promise that hasn't been resolved. That way I could call processB.cancel() and only expect to see the CancellationException in one place here. Better yet would be to be able to define the boundaries of a cancelable process in some way. For example, with an api like x.cancel(processB) which would mean that the cancellation propogates up to the farthest anscestor promise of x up to processB (at which point it stops and triggers a rejection on that promise). That way cancellable processes can be nested inside other cancellable processes - exactly what you want in a complicated application.

Thoughts?

[fresheneesz]

Closing because there's already much discussion at cancellation overhaul. Also, I've come up with a better proposal i wrote there