* 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()

    taskRunner.assertThreadHoldsLock()

    val index = serialTaskQueue.indexOfFirst { it.isReady() }
    if (index == -1) return null

    val nextTask = serialTaskQueue.removeAt(index)
    currentTask = nextTask
    contextSwitchCount++
    nextTask.start()
    return nextTask
  }

  private interface SerialTask {
    /** Returns true if this task is ready to start. */
    fun isReady() = true