如何使用json.Decoder解码单个json消息并将连接切换到其他协议？

Question

I am working on a TCP-based proxy that must first do a REQ/REPLY handshake in json on a given connection. Because JSON is a self-delimiting protocol I reach for Go's json.Decoder to pull off this work which does the job nicely.

Here are the steps I take:

1. Dial a connection to a remote server
2. Write a single json request to a remote server (REQ)
3. Read a single json reply from the same remote server (completing the proxy handshake REPLY)
4. Upon a valid json handshake, pass the client connection onto another part of the code which will (going forward) switch to a text based protocol from this point on.

The problem is, when json.Decoder reads data into its internal buffer it can potentially read more data than it needs in which case the json.Decoder has a Buffered() method which gives back an io.Reader with the remainder of the data.

This data (available in the Buffered() method) is now the text-based protocol data which needs to get read from the connection after the json hand-shake did its work. But if I pass the connection forward as is without considering the left over buffer, the connection gets into a locked state because it is waiting to read this data which never comes. The code that deals with the text-based protocol expects a net.Conn going forward and once I pass the connection forward (after the json handshake has been made) the code utilizing the connection understands how to speak the text-based protocol at this point on. So there should be a clear boundary of work.

My question is what is the ideal way to solve this issue so I can still take advantage of the json.Decoder, but ensure that when I pass the connection to a different part of the code in my proxy I know the start of the data for the text-based protocol will still be readable. I somehow need to take the remaining data in the json.Decoder's Buffered() method and put that back in front of the connection so it can be properly read going forward.

Any insight is much appreciated.

Answer 1

You can try

type ConnWithBuffIncluded struct{ //Implement net.Conn so can be passed through pipeline
    net.Conn
    json.Decoder
}

func (x ConnWithBuffIncluded) Read(p []byte) (n int, err error){ //Will Read both sources
    return io.MultiReader(x.Decoder.Buffered(), x.Conn).Read(p)
}