Since the existing Promise class is designed with deferred tasks on the
main thread only, we need a new class that will ensure we can handle
promises that are resolved/rejected off the main thread.
This new class ensures that the callbacks are only called on the same
thread that the promise is fulfilled from. If the callbacks are not set
before the thread tries to fulfill the promise, it will spin until they
are so that they will run on that thread.
Returning a reference resulted in Mail's use of Promise causing a crash.
Also, awaiting an already-awaited promise is an odd thing to do anyway,
so let's just make it release the resolved/rejected value instead of
returning a reference to it.
Co-Authored-By: Valtteri Koskivuori <vkoskiv@gmail.com>
The previous iteration of this API was somewhat odd and rough in random
places, which degraded usability and made less than perfect sense.
This commit reworks the API to be a little closer to more
conventional promise APIs (a la javascript promises).
Also adds a test to ensure the class even works.
