Go 404 Handler

not_found.go

// CatchNotFound is used to apply specific handler for code 404; it receives two handlers:
// if the requested path is not handled by the first, the second is called
func CatchNotFound(foundHandler, notFoundHandler http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		writer := &notFoundWriter{ResponseWriter: w}
		foundHandler.ServeHTTP(writer, r)

		if writer.isNotFound {
			notFoundHandler.ServeHTTP(w, r)
		}
	})
}

// notFoundWriter is a ResponseWriter used to implement CatchNotFound
type notFoundWriter struct {
	http.ResponseWriter
	isNotFound bool
}

func (w *notFoundWriter) WriteHeader(status int) {
	if status == http.StatusNotFound {
		// Don't actually write the 404 header, just set a flag:
		w.isNotFound = true
	} else {
		w.ResponseWriter.WriteHeader(status)
	}
}

func (w *notFoundWriter) Write(p []byte) (int, error) {
	if w.isNotFound {
		return 0, nil
	}

	return w.ResponseWriter.Write(p)
}