FinalizerCancelable.scala

final class FinalizerCancelable(finalizer: () => Unit) extends BooleanCancelable {
  private val state = new AtomicReference(FinalizerCancelable.State())

  def isCanceled: Boolean = state.get().canceled

  def acquire(): BooleanCancelable = {
    state.get() match {
      case FinalizerCancelable.State(true, _) => BooleanCancelable.alreadyCanceled
      case s => state.compareAndSet(s, s.copy(active = s.active + 1)) match {
        case false => acquire()
        case true => BooleanCancelable(release())
      }
    }
  }

  def cancel(): Boolean = state.get match {
    case FinalizerCancelable.State(true, _) => false
    case s => state.compareAndSet(s, s.copy(canceled = true)) match {
      case true =>
        finalizer()
        true
      case false => cancel()
    }
  }

  private def release() = state.get match {
    case FinalizerCancelable.State(true, _) =>
    case s => state.compareAndSet(s, s.copy(active = s.active - 1)) match {
      case true => state.get match {
        case s@FinalizerCancelable.State(false, 0) => state.compareAndSet(s, s.copy(canceled = true)) match {
          case true => finalizer()
          case _ =>
        }
        case _ =>
      }
      case _ =>
    }
  }

}

object FinalizerCancelable {

  def apply(cb: => Unit): FinalizerCancelable = new FinalizerCancelable(() => cb)

  case class State(canceled: Boolean = false, active: Int = 0)

}