BeforeOtherTasks,

    /** Resumes after the already-enqueued tasks. */
    AfterEnqueuedTasks,

    /** Resumes after all other tasks, including tasks enqueued while yielding. */
    AfterOtherTasks,
  }

  /** Returns the task that was started, or null if there were no tasks to start. */
  private fun startNextTask(): SerialTask? {
    taskRunner.assertThreadHoldsLock()