add an admission decorator chain

staging/src/k8s.io/apiserver/pkg/admission/BUILD

@@ -32,6 +32,7 @@ go_library(
         "attributes.go",
         "chain.go",
         "config.go",
+        "decorator.go",
         "errors.go",
         "handler.go",
         "interfaces.go",

staging/src/k8s.io/apiserver/pkg/admission/decorator.go

@@ -0,0 +1,39 @@
+/*
+Copyright 2018 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 admission
+
+type Decorator interface {
+	Decorate(handler Interface, name string) Interface
+}
+
+type DecoratorFunc func(handler Interface, name string) Interface
+
+func (d DecoratorFunc) Decorate(handler Interface, name string) Interface {
+	return d(handler, name)
+}
+
+type Decorators []Decorator
+
+// Decorate applies the decorator in inside-out order, i.e. the first decorator in the slice is first applied to the given handler.
+func (d Decorators) Decorate(handler Interface, name string) Interface {
+	result := handler
+	for _, d := range d {
+		result = d.Decorate(result, name)
+	}
+
+	return result
+}

staging/src/k8s.io/apiserver/pkg/admission/plugins.go

@@ -122,8 +122,6 @@ func splitStream(config io.Reader) (io.Reader, io.Reader, error) {
 	return bytes.NewBuffer(configBytes), bytes.NewBuffer(configBytes), nil
 }
 
-type Decorator func(handler Interface, name string) Interface
-
 // NewFromPlugins returns an admission.Interface that will enforce admission control decisions of all
 // the given plugins.
 func (ps *Plugins) NewFromPlugins(pluginNames []string, configProvider ConfigProvider, pluginInitializer PluginInitializer, decorator Decorator) (Interface, error) {
@@ -140,7 +138,7 @@ func (ps *Plugins) NewFromPlugins(pluginNames []string, configProvider ConfigPro
 		}
 		if plugin != nil {
 			if decorator != nil {
-				handlers = append(handlers, decorator(plugin, pluginName))
+				handlers = append(handlers, decorator.Decorate(plugin, pluginName))
 			} else {
 				handlers = append(handlers, plugin)
 			}

staging/src/k8s.io/apiserver/pkg/server/options/admission.go

@@ -141,7 +141,7 @@ func (a *AdmissionOptions) ApplyTo(
 	pluginInitializers = append(pluginInitializers, genericInitializer)
 	initializersChain = append(initializersChain, pluginInitializers...)
 
-	admissionChain, err := a.Plugins.NewFromPlugins(pluginNames, pluginsConfigProvider, initializersChain, admissionmetrics.WithControllerMetrics)
+	admissionChain, err := a.Plugins.NewFromPlugins(pluginNames, pluginsConfigProvider, initializersChain, admission.DecoratorFunc(admissionmetrics.WithControllerMetrics))
 	if err != nil {
 		return err
 	}