Async Future status w/o retaining returned value #109
Description
Is your feature request related to a problem? Please describe.
Currently, a Future cannot simply have its status checked, you must block on a Future either synchronously or via an event barrier within the CUDA stream. Additionally, if one tried to introduce this capability, the first and most obvious way to implement this would involve simply checking the contents of the IORef within the Future however holding onto this IORef would retain the memory backing its Full contents which could potentially be a substantial amount of memory. It should be possible to effectively extract some kind of Future status handle which does not retain this information.
The reason I am requesting this feature is actually this issue I posted in accelerate-fft: AccelerateHS/accelerate-fft#12
Essentially, the simplest solution to that issue would be to maintain a list of plan handles per shape and type of FFT operation and to then rapidly check that either there is no corresponding owner of this FFT plan handle, in which case it can be used without reservation, or it was previous used and contains some way to definitively check that such a necessity has expired and so can be used. If all existing handles are reserved, a new FFT plan handle can then be introduced. It is essential that such expiration tests do not necessitate retaining whole Accelerate Array's since that would introduce prohibitive CUDA memory space leaks.
Describe the solution you'd like
Introduce the following functions to the Async typeclass:
statusHandle :: FutureR arch a -> Par arch (FutureR arch ())
poll :: FutureR arch a -> Par arch (Maybe a)
The Future type, such as that defined for PTX contexts should include a list of status handles which are other Future's over (). Wherever a Future might be set to Full, the list of status handles should be traversed and each should be set to Full (), thus delivering the completion state. poll can simply return Just when the Future is Full otherwise return Nothing.