RunnableExecutorPair newHead = new RunnableExecutorPair(runnable, executor);
      RunnableExecutorPair oldHead;
      do {
        oldHead = head;
        if (oldHead == null) {
          // If runnables == null then execute() has been called so we should just execute our
          // listener immediately.
          newHead.execute();
          return;
        }

      RunnableExecutorPair newHead = new RunnableExecutorPair(runnable, executor);
      RunnableExecutorPair oldHead;
      do {
        oldHead = head;
        if (oldHead == null) {
          // If runnables == null then execute() has been called so we should just execute our
          // listener immediately.
          newHead.execute();
          return;
        }