This is the async-ready equivalent of Semaphore, similar to Stephen Toub's AsyncSempahore. Alternatively, you can use the SemaphoreSlim class, which is async-ready on modern platforms.
An AsyncSemaphore keeps a count, which is the number of open "slots" it has available to satisfy waiters. Any thread may increase the number of slots available by calling Release.
The task returned from WaitAsync will enter the Completed state when the AsyncSemaphore has given it a slot. That same task will enter the Canceled state if the CancellationToken is signaled before the wait is satisfied; in that case, the AsyncSemaphore does not lose a slot.
You can call WaitAsync with an already-cancelled CancellationToken to attempt to acquire a slot from the AsyncSemaphore immediately without actually entering the wait queue.