Fix issue #410 (#450)

Co-authored-by: Pierre Fenoll <pierrefenoll@gmail.com>

routers/gorillamux/router.go

@@ -35,10 +35,16 @@ func NewRouter(doc *openapi3.T) (routers.Router, error) {
 	servers := make([]srv, 0, len(doc.Servers))
 	for _, server := range doc.Servers {
 		serverURL := server.URL
-		scheme0 := strings.Split(serverURL, "://")[0]
-		schemes := permutePart(scheme0, server)
-
-		u, err := url.Parse(bEncode(strings.Replace(serverURL, scheme0+"://", schemes[0]+"://", 1)))
+		var schemes []string
+		var u *url.URL
+		var err error
+		if strings.Contains(serverURL, "://") {
+			scheme0 := strings.Split(serverURL, "://")[0]
+			schemes = permutePart(scheme0, server)
+			u, err = url.Parse(bEncode(strings.Replace(serverURL, scheme0+"://", schemes[0]+"://", 1)))
+		} else {
+			u, err = url.Parse(bEncode(serverURL))
+		}
 		if err != nil {
 			return nil, err
 		}

routers/gorillamux/router_test.go

@@ -219,3 +219,26 @@ func TestServerPath(t *testing.T) {
 	})
 	require.NoError(t, err)
 }
+
+func TestRelativeURL(t *testing.T) {
+	helloGET := &openapi3.Operation{Responses: openapi3.NewResponses()}
+	doc := &openapi3.T{
+		Servers: openapi3.Servers{
+			&openapi3.Server{
+				URL: "/api/v1",
+			},
+		},
+		Paths: openapi3.Paths{
+			"/hello": &openapi3.PathItem{
+				Get: helloGET,
+			},
+		},
+	}
+	router, err := NewRouter(doc)
+	require.NoError(t, err)
+	req, err := http.NewRequest(http.MethodGet, "https://example.com/api/v1/hello", nil)
+	require.NoError(t, err)
+	route, _, err := router.FindRoute(req)
+	require.NoError(t, err)
+	require.Equal(t, "/hello", route.Path)
+}