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

    /** Do this task's work, and then start another, such as by calling [startNextTask]. */
    fun start()
  }

  private object TestThreadSerialTask : SerialTask {
    override fun start() = error("unexpected call")
  }

  inner class RunnableSerialTask(
    private val runnable: Runnable,
  ) : SerialTask {

            cancelAll()
          } finally {
            coordinatorWaiting = false
          }
        }
      }
    }
  }

  /** Start another thread, unless a new thread is already scheduled to start. */
  private fun startAnotherThread() {
    lock.assertHeld()
    if (executeCallCount > runCallCount) return // A thread is still starting.

    executeCallCount++

      taskFaker.sleep(100.µs)
      log += "red:finishing@${taskFaker.nanoTime}"
    }
    assertThat(taskFaker.executeCallCount).isEqualTo(1)

    // Enqueueing the blue task doesn't start a thread because the red one is still starting.
    blueQueue.execute("blue task") {
      log += "blue:starting@${taskFaker.nanoTime}"
      taskFaker.sleep(100.µs)
      log += "blue:finishing@${taskFaker.nanoTime}"