How can I tell if net/http's ResponseWriter.Write() has been called?

Question

Suppose I have a chain of net/http Handlers, and an early one responds with an HTTP error (http.StatusInternalServerError, for instance). How can I detect this in

Answer 1

http.ResponseWriter is an interface. So just compose a new instance of it:

type MyResponseWriter struct {
    http.ResponseWriter
    WroteHeader bool
}

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

func (w *MyResponseWriter) WriteHeader(code int) {
    w.WroteHeader = true
    w.ResponseWriter.WriteHeader(code)
}

And in your handlers:

//...
if w, ok := w.(*MyResponseWriter); ok && w.WroteHeader {
    log.Println("Already wrote, skipping")
    return
}

---

EDIT: Another thing to consider. Most of the time if you have a "chain" of handlers that means that a handler is called inside a handler. So if you have something like

type Handler1 struct { http.Handler }
type Handler2 struct { http.Handler }
type Handler3 struct { http.Handler }
var MyHandler http.Handler = Handler1{Handler2{Handler3{h}}}

as long as each of those call the inner handler as the last thing they do with w and r, you should be fine because then w and r won't even reach the inner handler. E.g.

func (h Handler2) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    if somethingBadHappened() {
        w.WriteHeader(http.StatusInternalServerError)
        return
    }
    h.ServeHTTP(w, r) // Not called if somethingBadHappened().
}