Thank you for the clarification. You are right, these types of race conditions are possible with Async Ruby.

I find these races relatively easy to spot: yes, global state CAN change when you yield back to the reactor.

IMO thread race conditions are much, much worse. Global state can change AT ANY POINT, because thread scheduler preemptively switches threads. Here's an example:

    global.a = 0

    thread {
      global.a = 1
      print(global.a) // a is 1 or 2
    }

    thread {
      global.a = 2
      print(global.a) // a is 1 or 2
    }

    print(global.a) // a is 0, 1 or 2