val idleCallback: Runnable?
    this.withLock {
      if (!calls.remove(call)) throw AssertionError("Call wasn't in-flight!")
      idleCallback = this.idleCallback
    }

    val isRunning = promoteAndExecute()

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

  /** Returns a snapshot of the calls currently awaiting execution. */
  fun queuedCalls(): List<Call> =