提交之前请查看是否有相关 issue，谢谢。

多人群聊时消息先后顺序乱了，程序中用户与channel都是一对一的关系，群聊的时候使用的for循环发送，替代了原来的ChannelGroup 发送，使用for循环发消息时线程不安全导致了多人发送时消息穿插。是否有更好的办法呢？

Start by tracing the group-message send path described in the issue, comparing the current for-loop behavior with the former ChannelGroup approach. Reproduce concurrent sends and inspect how thread safety affects message ordering; done means multi-user messages no longer interleave, with the chosen approach verified by relevant tests.

Written by the indexing model from the issue text.