How to check Mux router on allowed methods for specific endpoint? - go

Question

I have a requirement of informing user the allowed methods for specific endpoint. This information will be shown in case there is 405 response from server (I'm using gorilla/mux).
I've trying using custom handler by mux for 405, but I can't find any info in the Request object and ResponseWriter.
After reading the docs and SO, I can't find any. May I know if anyone has been doing same thing before?
Code is below. I only allow GET apparently.
router.HandleFunc("/users/{id}",).Methods(http.MethodGet)
In my handler for 405, the response header is empty apparently. There is no info on allowed methods for this endpoint.
func MethodNotAllowedHandler() http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
logrus.Debugln("Header Writer: ", w.Header())
})
Thanks!

Answer 1

You can modify your MethodNotAllowed handler such below;
It simply walks through the routes and applies routematch function. If path is matched and method is not, it returns mux.ErrMethodMismatch error. Then you can obtain allowed methods for the route and send it in the headers
func notAllowedHandler(x *mux.Router) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
//next.ServeHTTP(w, r)
x.Walk(func(route *mux.Route, router *mux.Router, ancestors []*mux.Route) error {
var routeMatch mux.RouteMatch
if route.Match(r, &routeMatch) || routeMatch.MatchErr == mux.ErrMethodMismatch {
m, _ := route.GetMethods()
w.Header().Set("Access-Control-Allow-Methods", strings.Join(m, ", "))
}
return nil
})
http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
})
}
Edit: I forgot to mention. You need to pass router parameter to your custom method not allowed handler;
You can find the complete example below;
func main() {
r := mux.NewRouter()
r.HandleFunc("/test", func(rw http.ResponseWriter, r *http.Request) {}).Methods(http.MethodPost)
r.HandleFunc("/test2", func(rw http.ResponseWriter, r *http.Request) {}).Methods(http.MethodPut)
r.MethodNotAllowedHandler = notAllowedHandler(r)
http.ListenAndServe(":8089", r)
}
func notAllowedHandler(x *mux.Router) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
x.Walk(func(route *mux.Route, router *mux.Router, ancestors []*mux.Route) error {
var routeMatch mux.RouteMatch
if route.Match(r, &routeMatch) || routeMatch.MatchErr == mux.ErrMethodMismatch {
m, _ := route.GetMethods()
w.Header().Set("Access-Control-Allow-Methods", strings.Join(m, ", "))
}
return nil
})
http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
})
}