block()
          return -1L
        }
      },
      delayNanos,
    )
  }

  /** Returns a latch that reaches 0 when the queue is next idle. */
  fun idleLatch(): CountDownLatch {
    taskRunner.withLock {
      // If the queue is already idle, that's easy.
      if (activeTask == null && futureTasks.isEmpty()) {
        return CountDownLatch(0)
      }

    queue.schedule("task") {
      Thread.sleep(250)
      backend.shutdown()
      return@schedule -1L
    }

    assertThat(queue.idleLatch().await(500L, TimeUnit.MILLISECONDS)).isTrue()
    assertThat(queue.idleLatch().count).isEqualTo(0)
  }