[bug] setting custom MethodNotAllowedHandler for subrouter returns 404s

[jonasdebeukelaer]

Describe the bug

When setting a custom MethodNotAllowedHandler for a Subrouter, a default 404 page not found error is returned instead of the defined handler. Removing the custom handler makes a 405 method not found return as normal.

…

Versions

Go version: go1.12 darwin/amd64
package version: 6137e19

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Steps to Reproduce

Run a server with a subrouter, whose MethodNotAllowedHandler has been set a custom one. sending a POST to http://localhost:3000/api/test responses with 404 page not found

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Expected behavior

the custom response custom method not allowed should be returned when trying to POST to http://localhost:3000/api/test

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Code Snippets
here's a whole minimal server for your convenience:

package main

import (
	"fmt"
	"net/http"

	"github.com/gorilla/mux"
)

func main() {
	router := mux.NewRouter()
	h := handler{}
	nah := notAllowedHandler{}

	apiRouter := router.PathPrefix("/api").Subrouter()
	apiRouter.MethodNotAllowedHandler = nah
	apiRouter.Handle("/test", h).Methods("GET")

	server := &http.Server{
		Addr:    ":3000",
		Handler: router,
	}

	if err := server.ListenAndServe(); err != http.ErrServerClosed {
		fmt.Println("server shutdown:", err)
	}
}

type handler struct {
}

func (h handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	fmt.Fprint(w, "ok")
}

type notAllowedHandler struct {
}

func (h notAllowedHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	w.WriteHeader(http.StatusMethodNotAllowed)
	fmt.Fprint(w, "custom method not allowed")
}

…

[elithrar]

Reproduced on d83b6ff

Fix is likely here:

mux/mux.go

Lines 136 to 167 in eab9c4f

	func (r *Router) Match(req *http.Request, match *RouteMatch) bool {
	for _, route := range r.routes {
	if route.Match(req, match) {
	// Build middleware chain if no error was found
	if match.MatchErr == nil {
	for i := len(r.middlewares) - 1; i >= 0; i-- {
	match.Handler = r.middlewares[i].Middleware(match.Handler)
	}
	}
	return true
	}
	}
	if match.MatchErr == ErrMethodMismatch {
	if r.MethodNotAllowedHandler != nil {
	match.Handler = r.MethodNotAllowedHandler
	return true
	}
	return false
	}
	// Closest match for a router (includes sub-routers)
	if r.NotFoundHandler != nil {
	match.Handler = r.NotFoundHandler
	match.MatchErr = ErrNotFound
	return true
	}
	match.MatchErr = ErrNotFound
	return false
	}

Are you willing to submit a PR? I'm travelling through until next week.

[jonasdebeukelaer]

Hey, yeah I had an initial dig around and agree the issue is around there.
Sure I'll submit one, hopefully today