feat: ensure that 404 responses indicate that they originate from go-httpbin (#107)

This can help debug misconfigurations, where it's useful to know whether a 404 response is due to a configuration issue in some upstream proxy (load balancer, gateway, etc) or due to an actual bad request to go-httpbin.

Author: chinaran

httpbin/handlers.go

@@ -23,7 +23,8 @@ func notImplementedHandler(w http.ResponseWriter, r *http.Request) {
 // Index renders an HTML index page
 func (h *HTTPBin) Index(w http.ResponseWriter, r *http.Request) {
 	if r.URL.Path != "/" {
-		http.Error(w, "Not Found", http.StatusNotFound)
+		msg := fmt.Sprintf("Not Found (go-httpbin does not handle the path %s)", r.URL.Path)
+		http.Error(w, msg, http.StatusNotFound)
 		return
 	}
 	w.Header().Set("Content-Security-Policy", "default-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' camo.githubusercontent.com")

httpbin/handlers_test.go

@@ -116,6 +116,7 @@ func TestIndex__NotFound(t *testing.T) {
 	w := httptest.NewRecorder()
 	app.ServeHTTP(w, r)
 	assertStatusCode(t, w, http.StatusNotFound)
+	assertBodyContains(t, w, "/foo")
 }
 
 func TestFormsPost(t *testing.T) {