supporting exposing master loopback client

pkg/builder/builder_misc.go

@@ -2,6 +2,7 @@ package builder
 
 import (
 	"k8s.io/apimachinery/pkg/runtime"
+	genericapiserver "k8s.io/apiserver/pkg/server"
 	openapicommon "k8s.io/kube-openapi/pkg/common"
 	"sigs.k8s.io/apiserver-runtime/internal/sample-apiserver/pkg/cmd/server"
 	"sigs.k8s.io/apiserver-runtime/pkg/util/loopback"
@@ -56,6 +57,15 @@ func (a *Server) ExposeLoopbackAuthorizer() *Server {
 	})
 }
 
+// ExposeLoopbackMasterClientConfig exposes loopback client config for accessing the
+// configured master cluster's kube-apiserver as an external singleton.
+func (a *Server) ExposeLoopbackMasterClientConfig() *Server {
+	return a.WithConfigFns(func(config *genericapiserver.RecommendedConfig) *genericapiserver.RecommendedConfig {
+		loopback.SetLoopbackMasterClientConfig(config.ClientConfig)
+		return config
+	})
+}
+
 // WithoutEtcd removes etcd related settings from apiserver.
 func (a *Server) WithoutEtcd() *Server {
 	return a.WithOptionsFns(func(o *ServerOptions) *ServerOptions {

pkg/util/loopback/master.go

@@ -0,0 +1,39 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package loopback
+
+import (
+	"sync"
+
+	"k8s.io/client-go/rest"
+)
+
+var setLoopbackMasterClientOnce sync.Once
+var loopbackMasterClientConfig *rest.Config
+
+// SetLoopbackMasterClientConfig provides loopback client config for one time
+func SetLoopbackMasterClientConfig(c *rest.Config) {
+	setLoopbackMasterClientOnce.Do(func() {
+		loopbackMasterClientConfig = c
+	})
+}
+
+// GetLoopbackMasterClientConfig gets loopback client config for the
+// master kube-apiserver.
+func GetLoopbackMasterClientConfig() *rest.Config {
+	return loopbackMasterClientConfig
+}