http handler factory to proxy requests in go

proxy-handler-factory.go

func newProxyHandler(client *http.Client, backend *url.URL) http.Handler {
	return http.HandlerFunc(func(ow http.ResponseWriter, r *http.Request) {
		w := &loggedResponseWriter{ResponseWriter: ow}
		defer func() {
			log.Printf("%s %s %d %d Bytes", r.Method, r.URL.Path, w.code, w.size)
		}()
		req, err := http.NewRequest(r.Method, fmt.Sprintf("%s://%s", backend.Scheme, backend.Host), r.Body)
		if err != nil {
			log.Println("failed to call backend:", err.Error())
			w.WriteHeader(http.StatusInternalServerError)
			return
		}
		req.Host = r.Host
		// r.RequestURI contains the full value of GET http://host/path?query HTTP/1.1
		// When sending the request, request.URL.RequestURI() is used to fill GET <what> HTTP/1.1
		// Although this does not work when using proxy:
		// https://github.com/golang/go/blob/c0547476f342665514904cf2581a62135d2366c3/src/net/http/request.go#L524
		// This behaviour is achieved by providing only Opaque:
		// https://github.com/golang/go/blob/c0547476f342665514904cf2581a62135d2366c3/src/net/url/url.go#L1002
		req.URL.Opaque = r.URL.RequestURI()

		// Implement the X-Forwarded headers as defined in
		// https://docs.aws.amazon.com/elasticloadbalancing/latest/classic/x-forwarded-headers.html
		reqURL := url.URL{Host: r.RemoteAddr}
		req.Header.Set("X-Forwarded-Port", reqURL.Port())
		if r.TLS == nil {
			req.Header.Set("X-Forwarded-Proto", "http")
		} else {
			req.Header.Set("X-Forwarded-Proto", "https")
		}
		req.Header["X-Forwarded-For"] = []string{}
		// Forward all original request headers
		for key, value := range r.Header {
			req.Header[key] = value
		}
		req.Header["X-Forwarded-For"] = append(req.Header["X-Forwarded-For"], reqURL.Hostname())

		resp, err := client.Do(req)
		if err != nil {
			log.Println("failed to write response:", err.Error())
			w.WriteHeader(http.StatusInternalServerError)
		} else {
			// Write the response back with all headers
			for key, value := range resp.Header {
				w.Header()[key] = value
			}
			w.WriteHeader(resp.StatusCode)
			_, err = io.Copy(w, resp.Body)
			if err != nil {
				log.Println("failed to write response:", err.Error())
			}
		}
	})
}