pkg/util/net: use a more descriptive error in getAllDefaultRoutes()

Change the error output of getAllDefaultRoutes() so that it includes
information on which files were probed for the IP routing tables
even if such files are obvious.

Introduce a new error type which can be used to figure out of this
error is exactly of the "no routes" type.

staging/src/k8s.io/apimachinery/pkg/util/net/interface.go

@@ -53,6 +53,28 @@ type RouteFile struct {
 	parse func(input io.Reader) ([]Route, error)
 }
 
+// noRoutesError can be returned by ChooseBindAddress() in case of no routes
+type noRoutesError struct {
+	message string
+}
+
+func (e noRoutesError) Error() string {
+	return e.message
+}
+
+// IsNoRoutesError checks if an error is of type noRoutesError
+func IsNoRoutesError(err error) bool {
+	if err == nil {
+		return false
+	}
+	switch err.(type) {
+	case noRoutesError:
+		return true
+	default:
+		return false
+	}
+}
+
 var (
 	v4File = RouteFile{name: ipv4RouteFile, parse: getIPv4DefaultRoutes}
 	v6File = RouteFile{name: ipv6RouteFile, parse: getIPv6DefaultRoutes}
@@ -347,7 +369,9 @@ func getAllDefaultRoutes() ([]Route, error) {
 	v6Routes, _ := v6File.extract()
 	routes = append(routes, v6Routes...)
 	if len(routes) == 0 {
-		return nil, fmt.Errorf("No default routes.")
+		return nil, noRoutesError{
+			message: fmt.Sprintf("no default routes found in %q or %q", v4File.name, v6File.name),
+		}
 	}
 	return routes, nil
 }

staging/src/k8s.io/apimachinery/pkg/util/net/interface_test.go

@@ -669,7 +669,7 @@ func TestGetAllDefaultRoutes(t *testing.T) {
 		expected   []Route
 		errStrFrag string
 	}{
-		{"no routes", noInternetConnection, v6noDefaultRoutes, 0, nil, "No default routes"},
+		{"no routes", noInternetConnection, v6noDefaultRoutes, 0, nil, "no default routes"},
 		{"only v4 route", gatewayfirst, v6noDefaultRoutes, 1, routeV4, ""},
 		{"only v6 route", noInternetConnection, v6gatewayfirst, 1, routeV6, ""},
 		{"v4 and v6 routes", gatewayfirst, v6gatewayfirst, 2, bothRoutes, ""},