golang 获取statuscode

type doneWriter struct {
    http.ResponseWriter
    done bool
}

func (w *doneWriter) WriteHeader(status int) {
    w.done = true
    w.ResponseWriter.WriteHeader(status)
}

func (w *doneWriter) Write(b []byte) (int, error) {
    w.done = true
    return w.ResponseWriter.Write(b)
}

func myMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        dw := &doneWriter{ResponseWriter: w}
        next.ServeHTTP(dw, r)
        if dw.done {
            // Something already wrote a response
            return
        }
        // Nothing else wrote a response
        w.WriteHeader(http.StatusOK)
        // Whatever you want here
    }
}