}

    val isRunning = promoteAndExecute()

    if (!isRunning && idleCallback != null) {
      idleCallback.run()
    }
  }

  /** Returns a snapshot of the calls currently awaiting execution. */
  fun queuedCalls(): List<Call> =
    this.withLock {
      return Collections.unmodifiableList(readyAsyncCalls.map { it.call })
    }