How to Use ServeMux with ServerConn?

Question

Im creating a Networking API and want people to be able to route requests to specific endpoints using a ServeMux. Instead of using a Server instance, I need to use my own low level ServerConn. This is because I am receiving both incoming HTTP requests and plain text data from the same port.

The problem, however, is that if I want to forward a request using my ServeMux, I would use it's ServeHTTP method. For this, I need to provide a ResponseWriter, which I don't know how to create an instance of since it is an interface, not a struct. Should a I create my own ResponseWriter struct? Is there one given by the Golang Standard Library? Or is there an alternate solution to this altogether?

JimB · Accepted Answer · 2015-04-30T14:01:01.587

I would avoid doing this altogether if at all possible. Mixing protocols on the same connection is bound to lead to hard-to-trace bugs, and unexpected behavior. If you really want to do it, and have all the http/1.1 mechanisms work correctly, leave as much as possible to the http package.

Since ResponseWriter is an interface, you would implement your own type to satisfy it. Look at the unexported response type in the http package for a full example. There's a lot to get right, and using it in combination with a ServerConn (which is documented as "do no use") is probably not a good idea.

The place to do this at a lower level would be in Accept inside the Server's net.Listener. Since you're going to have to parse the start of every request twice, you would need a net.Conn that can be "rewound" partly.

Make yourself a net.Listener that checks the start of the stream on a new connection, and if it looks like an http request, return a net.Conn that replays the first chunk you read off the wire on its first Reads. Something like:

type replayConn struct {
    net.Conn
    buf []byte
    pos int
}

func (c *replayConn) Read(b []byte) (int, error) {
    if c.pos < len(c.buf) {
        n := copy(b, c.buf[c.pos:])
        c.pos += n
        return n, nil
    }
    return c.Conn.Read(b)
}

If the connection isn't http, then send the connection off to your other type of handler, and continue blocking on Accept.

Agreed, thanks for such a comprehensive answer! And yeah, I realize that this is certainly not best practice for Go code or just Networking in general, but at this point Im just trying to make a quick refactor for some existing code. I think Ill go with creating a custom net.Conn interface and a custom net.Listener so I can just use a net.Server and have it listening on my custom net.Listener. That way I don't need to deal with creating a custom ResponseWriter, which may be problematic. But all things said, I agree, this is a total hack. Good thing this isn't production code or anything though — sourdesi, Apr 30 '15 at 15:31

How to Use ServeMux with ServerConn?

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