support watch namespaces matched namespace selector (#7472)

skip caching namespaces at cluster scope if only watching single namespace

add --watch-namespace-selector in user guide

add e2e test

internal/ingress/controller/store/namespace.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 store
+
+import (
+	apiv1 "k8s.io/api/core/v1"
+	"k8s.io/client-go/tools/cache"
+)
+
+// NamespaceLister makes a Store that lists Namespaces.
+type NamespaceLister struct {
+	cache.Store
+}
+
+// ByKey returns the Namespace matching key in the local Namespace Store.
+func (cml *NamespaceLister) ByKey(key string) (*apiv1.Namespace, error) {
+	s, exists, err := cml.GetByKey(key)
+	if err != nil {
+		return nil, err
+	}
+	if !exists {
+		return nil, NotExistsError(key)
+	}
+	return s.(*apiv1.Namespace), nil
+}