override fun coordinatorNotify(taskRunner: TaskRunner) {
          taskRunner.assertThreadHoldsLock()
          check(waitingCoordinatorTask != null)

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