Suspicious supervision behaviour during cancellation

[felixSchl]

Hey, I've been running into issues with fork cancellation and supervisors and decided to spend some time narrowing the issue down. Below are two snippets highlighting what I would consider strange behaviour. It's not obvious to me why this behaviour occurs and looks like a bug on the surface.

The following snippet throws off-stack, due to:
https://github.com/slamdata/purescript-aff/blob/3457df9993a78e408ed56f0bb619941911d375d0/src/Control/Monad/Aff.js#L500-L505

        outer <- forkAff do
          bracket
            (pure unit)
            (const $ pure unit)
            \_ -> supervise do
              fiber <- forkAff $ supervise $ supervise do -- notice the double 'supervise'
                delay $ 10.0 # Milliseconds
              joinFiber fiber
        killFiber (error "done") outer
        delay $ 100.0 # Milliseconds

and this blocks forever:

        outer <- forkAff do
          bracket
            (pure unit)
            (const $ pure unit)
            \_ -> supervise do
              fiber <- forkAff $ supervise do
                delay $ 10.0 # Milliseconds
              joinFiber fiber
        killFiber (error "doen") outer
        delay $ 100.0 # Milliseconds

You can play around with these, but from my observations this only happens when inside a bracket, but the problem is probably broader.

[natefaubion]

I think this is because Supervisor does not handle the case where there are no forks, so it never invokes the callback.