[go-nuts] question: subclassing http.ResponseWriter

Hello,

Here is a beginner's question. Sorry that it's a bit long. I'd be happy
if somebody could help.

I am trying to turn an existing http.ResponseWriter into a new object, also
implementing the http.ResponseWriter interface. The new object should do
the following:
- in the WriteHeader method, set a session cookie before writing the header
- pass on everything else to the given parent ResponseWriter.

The following is a failed attempt to implement this:

--------------------------------------------------------------------------
type SessionResponseWriter struct {
     http.ResponseWriter
     SessionId []byte
}

func (w *SessionResponseWriter) WriteHeader(code int) {
     setSessionCookie(w, w.SessionId, sessionTimeoutSeconds)
     w.ResponseWriter.WriteHeader(code)
}
--------------------------------------------------------------------------

The above compiles, but does not work. The Write method seems to not call
my version of the WriteHeader method and thus the session cookie das not
get set if the user calls .Write().

I managed to fix the problem, using the following code:

--------------------------------------------------------------------------
type SessionResponseWriter struct {
     parent http.ResponseWriter
     wroteHeader bool
     SessionId []byte
}

func (w *SessionResponseWriter) Header() http.Header {
     return w.parent.Header()
}

func (w *SessionResponseWriter) Write(data []byte) (int, error) {
     if !w.wroteHeader {
         w.WriteHeader(http.StatusOK)
     }
     n, err := w.parent.Write(data)
     return n, err
}

func (w *SessionResponseWriter) WriteHeader(code int) {
     w.wroteHeader = true
     setSessionCookie(w.parent, w.SessionId, sessionTimeoutSeconds)
     w.parent.WriteHeader(code)
}
--------------------------------------------------------------------------

This works, but looks ugly:
- The wroteHeader logic is now present both in w and in w.parent. This
looks like unnecessary duplication to me.
- The new Header() method doesn't do anything interesting. Do I really
need to write this out?

My question: Is there a better way of solving this problem?

Many thanks,
Jochen