how to rewrite attachment

[champly]

背景

在 service mesh环境下，需要在 sidecar 中解析dubbo的attachment，并根据需求添加一些自定义的参数进去，就像SetHeader一样，然后重新构建 dubbo 包

问题

如果使用hessian.NewDecoder，传递的参数没有对象是可以的，如果参数中有对象会报错，参考 #163 的解决办法，使用 hessian.NewDecoderWithSkip可以解决，但是重新构建的时候arg类型是interface{},值是nil

解决

能否多提供一个Decode方法，直接返回[]byte内容，重新构建的时候直接encode.Append(arg)就可以重新构建dubbo包

[champly]

solve

decode

1. use hessian.NewDecoderWithSkip Decode dubboVersion/path/version/method/argsTypes/args/attachment info.
2. create hessian encode object, encode attachment.
3. calc encode len

encode

1. encode.Append(payload[len(payload]-attachment_encode_len)
2. encode.Encode(attachment)

[RAJASHEKARGUNAGANTI]

hello folks ..!
I am good at java and I wanted to resolve this issue so please explain this issue some more detail as this is my first contribution .

[tiltwind]

some materials:

• dubbo rpc protocol: https://cn.dubbo.apache.org/zh/blog/2018/10/05/dubbo-%e5%8d%8f%e8%ae%ae%e8%af%a6%e8%a7%a3/
• dubbo rpc protocol implement : https://github.com/apache/dubbo-go/blob/master/protocol/dubbo/dubbo_protocol.go

dubbo rpc attachments belongs to the Variable Part, but u can't get where it starts from, u must skip the parts before it, include Dubbo version， Service name， Service version， Method name， Method parameter types， Method arguments.
But for hessian2 serialization protocol , u must decode the data to find where it ends with. There is't a length to tell the length of the data. So it costs a lot for the sidecar of mesh to update the attachments.

And I'm not sure whether the MOSN has already implemented this feature.