assertLockHeld()
    if (executeCallCount > runCallCount) return // A thread is still starting.

    executeCallCount++
    backend.execute(this@TaskRunner, runnable)
  }

  fun newQueue(): TaskQueue {
    val name = this.withLock { nextQueueName++ }
    return TaskQueue(this, "Q$name")
  }

  /**
   * Returns a snapshot of queues that currently have tasks scheduled. The task runner does not