Ignore ErrNotFound while matching Subrouters (#438)

MatchErr is set by the router to ErrNotFound if no route matches. If
no route of a Subrouter matches the error can by safely ignored. This
implementation only ignores these errors and does not ignore other
errors like ErrMethodMismatch.

mux_test.go

@@ -2715,6 +2715,38 @@ func Test_copyRouteConf(t *testing.T) {
 	}
 }
 
+func TestMethodNotAllowed(t *testing.T) {
+	handler := func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(http.StatusOK) }
+	router := NewRouter()
+	router.HandleFunc("/thing", handler).Methods(http.MethodGet)
+	router.HandleFunc("/something", handler).Methods(http.MethodGet)
+
+	w := NewRecorder()
+	req := newRequest(http.MethodPut, "/thing")
+
+	router.ServeHTTP(w, req)
+
+	if w.Code != 405 {
+		t.Fatalf("Expected status code 405 (got %d)", w.Code)
+	}
+}
+
+func TestSubrouterNotFound(t *testing.T) {
+	handler := func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(http.StatusOK) }
+	router := NewRouter()
+	router.Path("/a").Subrouter().HandleFunc("/thing", handler).Methods(http.MethodGet)
+	router.Path("/b").Subrouter().HandleFunc("/something", handler).Methods(http.MethodGet)
+
+	w := NewRecorder()
+	req := newRequest(http.MethodPut, "/not-present")
+
+	router.ServeHTTP(w, req)
+
+	if w.Code != 404 {
+		t.Fatalf("Expected status code 404 (got %d)", w.Code)
+	}
+}
+
 // mapToPairs converts a string map to a slice of string pairs
 func mapToPairs(m map[string]string) []string {
 	var i int

route.go

@@ -43,11 +43,6 @@ func (r *Route) Match(req *http.Request, match *RouteMatch) bool {
 		return false
 	}
 
-	// Set MatchErr to nil to prevent
-	// subsequent matching subrouters from failing to run middleware.
-	// If not reset, the middleware would see a non-nil MatchErr and be skipped,
-	// even when there was a matching route.
-	match.MatchErr = nil
 	var matchErr error
 
 	// Match everything.
@@ -57,6 +52,18 @@ func (r *Route) Match(req *http.Request, match *RouteMatch) bool {
 				matchErr = ErrMethodMismatch
 				continue
 			}
+
+			// Ignore ErrNotFound errors. These errors arise from match call
+			// to Subrouters.
+			//
+			// This prevents subsequent matching subrouters from failing to
+			// run middleware. If not ignored, the middleware would see a
+			// non-nil MatchErr and be skipped, even when there was a
+			// matching route.
+			if match.MatchErr == ErrNotFound {
+				match.MatchErr = nil
+			}
+
 			matchErr = nil
 			return false
 		}