* control this will return null. If there is more than a single task ready to execute immediately
   * this will start another thread to handle that work.
   */
  fun awaitTaskToRun(): Task? {
    lock.assertHeld()

    while (true) {
      if (readyQueues.isEmpty()) {
        return null // Nothing to do.
      }

      val now = backend.nanoTime()

          check(waitingCoordinatorTask != null)

          // Queue a task to resume the waiting coordinator.
          serialTaskQueue +=
            object : SerialTask {
              override fun start() {
                taskRunner.assertThreadHoldsLock()
                val coordinatorTask = waitingCoordinatorTask
                if (coordinatorTask != null) {
                  waitingCoordinatorNotified = true