taskRunner.assertLockNotHeld()

    taskRunner.withLock {
      assertThat(activeThreads).isEqualTo(0)
    }
  }

  /** Unblock a waiting task thread. Used by the test thread only. */
  fun interruptCoordinatorThread() {
    taskRunner.assertLockNotHeld()
    require(currentTask == TestThreadSerialTask)

    // Queue a task to interrupt the waiting coordinator.
    serialTaskQueue +=
      object : SerialTask {