kubernetes: Improve namespace usage (#4767)

* Use GetByKey instead of List in GetNamespaceByName.
* Add ToNamespace to reduce memory for namespace cache.

Signed-off-by: Mat Lowery <mlowery@ebay.com>

plugin/kubernetes/object/namespace.go

@@ -0,0 +1,61 @@
+package object
+
+import (
+	"fmt"
+
+	api "k8s.io/api/core/v1"
+	meta "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+)
+
+// Namespace is a stripped down api.Namespace with only the items we need for CoreDNS.
+type Namespace struct {
+	// Don't add new fields to this struct without talking to the CoreDNS maintainers.
+	Version   string
+	Name      string
+
+	*Empty
+}
+
+// ToNamespace returns a function that converts an api.Namespace to a *Namespace.
+func ToNamespace(obj meta.Object) (meta.Object, error) {
+	ns, ok := obj.(*api.Namespace)
+	if !ok {
+		return nil, fmt.Errorf("unexpected object %v", obj)
+	}
+	n := &Namespace{
+		Version: ns.GetResourceVersion(),
+		Name:    ns.GetName(),
+	}
+	*ns = api.Namespace{}
+	return n, nil
+}
+
+var _ runtime.Object = &Namespace{}
+
+// DeepCopyObject implements the ObjectKind interface.
+func (n *Namespace) DeepCopyObject() runtime.Object {
+	n1 := &Namespace{
+		Version: n.Version,
+		Name:    n.Name,
+	}
+	return n1
+}
+
+// GetNamespace implements the metav1.Object interface.
+func (n *Namespace) GetNamespace() string { return "" }
+
+// SetNamespace implements the metav1.Object interface.
+func (n *Namespace) SetNamespace(namespace string) {}
+
+// GetName implements the metav1.Object interface.
+func (n *Namespace) GetName() string { return n.Name }
+
+// SetName implements the metav1.Object interface.
+func (n *Namespace) SetName(name string) {}
+
+// GetResourceVersion implements the metav1.Object interface.
+func (n *Namespace) GetResourceVersion() string { return n.Version }
+
+// SetResourceVersion implements the metav1.Object interface.
+func (n *Namespace) SetResourceVersion(version string) {}