garbagecollector.go

/*
Copyright 2016 The Kubernetes Authors All rights reserved.

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 garbagecollector

import (
	"fmt"
	"sync"
	"time"

	"github.com/golang/glog"

	"k8s.io/kubernetes/pkg/api"
	"k8s.io/kubernetes/pkg/api/errors"
	"k8s.io/kubernetes/pkg/api/meta"
	"k8s.io/kubernetes/pkg/api/meta/metatypes"
	"k8s.io/kubernetes/pkg/api/unversioned"
	"k8s.io/kubernetes/pkg/api/v1"
	"k8s.io/kubernetes/pkg/apimachinery/registered"
	"k8s.io/kubernetes/pkg/client/cache"
	"k8s.io/kubernetes/pkg/client/typed/dynamic"
	"k8s.io/kubernetes/pkg/controller/framework"
	"k8s.io/kubernetes/pkg/runtime"
	"k8s.io/kubernetes/pkg/types"
	utilerrors "k8s.io/kubernetes/pkg/util/errors"
	utilruntime "k8s.io/kubernetes/pkg/util/runtime"
	"k8s.io/kubernetes/pkg/util/sets"
	"k8s.io/kubernetes/pkg/util/wait"
	"k8s.io/kubernetes/pkg/util/workqueue"
	"k8s.io/kubernetes/pkg/watch"
)

const ResourceResyncTime = 60 * time.Second

type monitor struct {
	store      cache.Store
	controller *framework.Controller
}

type objectReference struct {
	metatypes.OwnerReference
	// This is needed by the dynamic client
	Namespace string
}

func (s objectReference) String() string {
	return fmt.Sprintf("[%s/%s, namespace: %s, name: %s, uid: %s]", s.APIVersion, s.Kind, s.Namespace, s.Name, s.UID)
}

// node does not require a lock to protect. The single-threaded
// Propagator.processEvent() is the sole writer of the nodes. The multi-threaded
// GarbageCollector.processItem() reads the nodes, but it only reads the fields
// that never get changed by Propagator.processEvent().
type node struct {
	identity objectReference
	// dependents will be read by the orphan() routine, we need to protect it with a lock.
	dependentsLock *sync.RWMutex
	dependents     map[*node]struct{}
	// When processing an Update event, we need to compare the updated
	// ownerReferences with the owners recorded in the graph.
	owners []metatypes.OwnerReference
}

func (ownerNode *node) addDependent(dependent *node) {
	ownerNode.dependentsLock.Lock()
	defer ownerNode.dependentsLock.Unlock()
	ownerNode.dependents[dependent] = struct{}{}
}

func (ownerNode *node) deleteDependent(dependent *node) {
	ownerNode.dependentsLock.Lock()
	defer ownerNode.dependentsLock.Unlock()
	delete(ownerNode.dependents, dependent)
}

type eventType int

const (
	addEvent eventType = iota
	updateEvent
	deleteEvent
)

type event struct {
	eventType eventType
	obj       interface{}
	// the update event comes with an old object, but it's not used by the garbage collector.
	oldObj interface{}
}

type concurrentUIDToNode struct {
	*sync.RWMutex
	uidToNode map[types.UID]*node
}

func (m *concurrentUIDToNode) Write(node *node) {
	m.Lock()
	defer m.Unlock()
	m.uidToNode[node.identity.UID] = node
}

func (m *concurrentUIDToNode) Read(uid types.UID) (*node, bool) {
	m.RLock()
	defer m.RUnlock()
	n, ok := m.uidToNode[uid]
	return n, ok
}

func (m *concurrentUIDToNode) Delete(uid types.UID) {
	m.Lock()
	defer m.Unlock()
	delete(m.uidToNode, uid)
}

type Propagator struct {
	eventQueue *workqueue.Type
	// uidToNode doesn't require a lock to protect, because only the
	// single-threaded Propagator.processEvent() reads/writes it.
	uidToNode *concurrentUIDToNode
	gc        *GarbageCollector
}

// addDependentToOwners adds n to owners' dependents list. If the owner does not
// exist in the p.uidToNode yet, a "virtual" node will be created to represent
// the owner. The "virtual" node will be enqueued to the dirtyQueue, so that
// processItem() will verify if the owner exists according to the API server.
func (p *Propagator) addDependentToOwners(n *node, owners []metatypes.OwnerReference) {
	for _, owner := range owners {
		ownerNode, ok := p.uidToNode.Read(owner.UID)
		if !ok {
			// Create a "virtual" node in the graph for the owner if it doesn't
			// exist in the graph yet. Then enqueue the virtual node into the
			// dirtyQueue. The garbage processor will enqueue a virtual delete
			// event to delete it from the graph if API server confirms this
			// owner doesn't exist.
			ownerNode = &node{
				identity: objectReference{
					OwnerReference: owner,
					Namespace:      n.identity.Namespace,
				},
				dependentsLock: &sync.RWMutex{},
				dependents:     make(map[*node]struct{}),
			}
			p.uidToNode.Write(ownerNode)
			p.gc.dirtyQueue.Add(ownerNode)
		}
		ownerNode.addDependent(n)
	}
}

// insertNode insert the node to p.uidToNode; then it finds all owners as listed
// in n.owners, and adds the node to their dependents list.
func (p *Propagator) insertNode(n *node) {
	p.uidToNode.Write(n)
	p.addDependentToOwners(n, n.owners)
}

// removeDependentFromOwners remove n from owners' dependents list.
func (p *Propagator) removeDependentFromOwners(n *node, owners []metatypes.OwnerReference) {
	for _, owner := range owners {
		ownerNode, ok := p.uidToNode.Read(owner.UID)
		if !ok {
			continue
		}
		ownerNode.deleteDependent(n)
	}
}

// removeNode removes the node from p.uidToNode, then finds all
// owners as listed in n.owners, and removes n from their dependents list.
func (p *Propagator) removeNode(n *node) {
	p.uidToNode.Delete(n.identity.UID)
	p.removeDependentFromOwners(n, n.owners)
}

// TODO: profile this function to see if a naive N^2 algorithm performs better
// when the number of references is small.
func referencesDiffs(old []metatypes.OwnerReference, new []metatypes.OwnerReference) (added []metatypes.OwnerReference, removed []metatypes.OwnerReference) {
	oldUIDToRef := make(map[string]metatypes.OwnerReference)
	for i := 0; i < len(old); i++ {
		oldUIDToRef[string(old[i].UID)] = old[i]
	}
	oldUIDSet := sets.StringKeySet(oldUIDToRef)
	newUIDToRef := make(map[string]metatypes.OwnerReference)
	for i := 0; i < len(new); i++ {
		newUIDToRef[string(new[i].UID)] = new[i]
	}
	newUIDSet := sets.StringKeySet(newUIDToRef)

	addedUID := newUIDSet.Difference(oldUIDSet)
	removedUID := oldUIDSet.Difference(newUIDSet)

	for uid := range addedUID {
		added = append(added, newUIDToRef[uid])
	}
	for uid := range removedUID {
		removed = append(removed, oldUIDToRef[uid])
	}
	return added, removed
}

func shouldOrphanDependents(e event, accessor meta.Object) bool {
	// The delta_fifo may combine the creation and update of the object into one
	// event, so we need to check AddEvent as well.
	if e.oldObj == nil {
		if accessor.GetDeletionTimestamp() == nil {
			return false
		}
	} else {
		oldAccessor, err := meta.Accessor(e.oldObj)
		if err != nil {
			utilruntime.HandleError(fmt.Errorf("cannot access oldObj: %v", err))
			return false
		}
		// ignore the event if it's not updating DeletionTimestamp from non-nil to nil.
		if accessor.GetDeletionTimestamp() == nil || oldAccessor.GetDeletionTimestamp() != nil {
			return false
		}
	}
	finalizers := accessor.GetFinalizers()
	for _, finalizer := range finalizers {
		if finalizer == api.FinalizerOrphan {
			return true
		}
	}
	return false
}

// dependents are copies of pointers to the owner's dependents, they don't need to be locked.
func (gc *GarbageCollector) orhpanDependents(owner objectReference, dependents []*node) error {
	var failedDependents []objectReference
	var errorsSlice []error
	for _, dependent := range dependents {
		// the dependent.identity.UID is used as precondition
		deleteOwnerRefPatch := fmt.Sprintf(`{"metadata":{"ownerReferences":[{"$patch":"delete","uid":"%s"}],"uid":"%s"}}`, owner.UID, dependent.identity.UID)
		_, err := gc.patchObject(dependent.identity, []byte(deleteOwnerRefPatch))
		// note that if the target ownerReference doesn't exist in the
		// dependent, strategic merge patch will NOT return an error.
		if err != nil && !errors.IsNotFound(err) {
			errorsSlice = append(errorsSlice, fmt.Errorf("orphaning %s failed with %v", dependent.identity, err))
		}
	}
	if len(failedDependents) != 0 {
		return fmt.Errorf("failed to orphan dependents of owner %s, got errors: %s", owner, utilerrors.NewAggregate(errorsSlice).Error())
	}
	fmt.Println("CHAO: successfully updated all dependents")
	return nil
}

// TODO: Using Patch when strategicmerge supports deleting an entry from a
// slice of a base type.
func (gc *GarbageCollector) removeOrphanFinalizer(owner *node) error {
	const retries = 5
	for count := 0; count < retries; count++ {
		ownerObject, err := gc.getObject(owner.identity)
		if err != nil {
			return fmt.Errorf("cannot finalize owner %s, because cannot get it. The garbage collector will retry later.", owner.identity)
		}
		accessor, err := meta.Accessor(ownerObject)
		if err != nil {
			return fmt.Errorf("cannot access the owner object: %v. The garbage collector will retry later.", err)
		}
		finalizers := accessor.GetFinalizers()
		var newFinalizers []string
		found := false
		for _, f := range finalizers {
			if f == api.FinalizerOrphan {
				found = true
			} else {
				newFinalizers = append(newFinalizers, f)
			}
		}
		if !found {
			glog.V(6).Infof("the orphan finalizer is already removed from object %s", owner.identity)
			return nil
		}
		// remove the owner from dependent's OwnerReferences
		ownerObject.SetFinalizers(newFinalizers)
		_, err = gc.updateObject(owner.identity, ownerObject)
		if err == nil {
			return nil
		}
		if err != nil && !errors.IsConflict(err) {
			return fmt.Errorf("cannot update the finalizers of owner %s, with error: %v, tried %d times", owner.identity, err, count+1)
		}
		// retry if it's a conflict
		glog.V(6).Infof("got conflict updating the owner object %s, tried %d times", owner.identity, count+1)
	}
	return fmt.Errorf("updateMaxRetries(%d) has reached. The garbage collector will retry later for owner %v.", retries, owner.identity)
}

// orphanFinalizer dequeues a node from the orphanQueue, then finds its dependents
// based on the graph maintained by the GC, then removes it from the
// OwnerReferences of its dependents, and finally updates the owner to remove
// the "Orphan" finalizer. The node is add back into the orphanQueue if any of
// these steps fail.
func (gc *GarbageCollector) orphanFinalizer() {
	key, quit := gc.orphanQueue.Get()
	if quit {
		return
	}
	defer gc.orphanQueue.Done(key)
	owner, ok := key.(*node)
	if !ok {
		utilruntime.HandleError(fmt.Errorf("expect *node, got %#v", key))
	}
	// we don't need to lock each element, because they never get updated
	owner.dependentsLock.RLock()
	dependents := make([]*node, 0, len(owner.dependents))
	for dependent := range owner.dependents {
		dependents = append(dependents, dependent)
	}
	owner.dependentsLock.RUnlock()

	err := gc.orhpanDependents(owner.identity, dependents)
	if err != nil {
		glog.V(6).Infof("orphanDependents for %s failed with %v", owner.identity, err)
		gc.orphanQueue.Add(owner)
		return
	}
	// update the owner, remove "orphaningFinalizer" from its finalizers list
	err = gc.removeOrphanFinalizer(owner)
	if err != nil {
		glog.V(6).Infof("removeOrphanFinalizer for %s failed with %v", owner.identity, err)
		gc.orphanQueue.Add(owner)
	}
}

// Dequeueing an event from eventQueue, updating graph, populating dirty_queue.
func (p *Propagator) processEvent() {
	key, quit := p.eventQueue.Get()
	if quit {
		return
	}
	defer p.eventQueue.Done(key)
	event, ok := key.(event)
	if !ok {
		utilruntime.HandleError(fmt.Errorf("expect an event, got %v", key))
		return
	}
	obj := event.obj
	accessor, err := meta.Accessor(obj)
	if err != nil {
		utilruntime.HandleError(fmt.Errorf("cannot access obj: %v", err))
		return
	}
	typeAccessor, err := meta.TypeAccessor(obj)
	if err != nil {
		utilruntime.HandleError(fmt.Errorf("cannot access obj: %v", err))
		return
	}
	glog.V(6).Infof("Propagator process object: %s/%s, namespace %s, name %s, event type %s", typeAccessor.GetAPIVersion(), typeAccessor.GetKind(), accessor.GetNamespace(), accessor.GetName(), event.eventType)
	// Check if the node already exsits
	existingNode, found := p.uidToNode.Read(accessor.GetUID())
	switch {
	case (event.eventType == addEvent || event.eventType == updateEvent) && !found:
		newNode := &node{
			identity: objectReference{
				OwnerReference: metatypes.OwnerReference{
					APIVersion: typeAccessor.GetAPIVersion(),
					Kind:       typeAccessor.GetKind(),
					UID:        accessor.GetUID(),
					Name:       accessor.GetName(),
				},
				Namespace: accessor.GetNamespace(),
			},
			dependentsLock: &sync.RWMutex{},
			dependents:     make(map[*node]struct{}),
			owners:         accessor.GetOwnerReferences(),
		}
		p.insertNode(newNode)
		// the underlying delta_fifo may combine a creation and deletion into one event
		if shouldOrphanDependents(event, accessor) {
			glog.V(6).Infof("add %s to the orphanQueue", newNode.identity)
			p.gc.orphanQueue.Add(newNode)
		}
	case (event.eventType == addEvent || event.eventType == updateEvent) && found:
		// caveat: if GC observes the creation of the dependents later than the
		// deletion of the owner, then the orphaning finalizer won't be effective.
		if shouldOrphanDependents(event, accessor) {
			glog.V(6).Infof("add %s to the orphanQueue", existingNode.identity)
			p.gc.orphanQueue.Add(existingNode)
		}
		// add/remove owner refs
		added, removed := referencesDiffs(existingNode.owners, accessor.GetOwnerReferences())
		if len(added) == 0 && len(removed) == 0 {
			glog.V(6).Infof("The updateEvent %#v doesn't change node references, ignore", event)
			return
		}
		// update the node itself
		existingNode.owners = accessor.GetOwnerReferences()
		// Add the node to its new owners' dependent lists.
		p.addDependentToOwners(existingNode, added)
		// remove the node from the dependent list of node that are no long in
		// the node's owners list.
		p.removeDependentFromOwners(existingNode, removed)
	case event.eventType == deleteEvent:
		if !found {
			glog.V(6).Infof("%v doesn't exist in the graph, this shouldn't happen", accessor.GetUID())
			return
		}
		p.removeNode(existingNode)
		existingNode.dependentsLock.RLock()
		defer existingNode.dependentsLock.RUnlock()
		for dep := range existingNode.dependents {
			p.gc.dirtyQueue.Add(dep)
		}
	}
}

// GarbageCollector is responsible for carrying out cascading deletion, and
// removing ownerReferences from the dependents if the owner is deleted with
// DeleteOptions.OrphanDependents=true.
type GarbageCollector struct {
	restMapper  meta.RESTMapper
	clientPool  dynamic.ClientPool
	dirtyQueue  *workqueue.Type
	orphanQueue *workqueue.Type
	monitors    []monitor
	propagator  *Propagator
}

func monitorFor(p *Propagator, clientPool dynamic.ClientPool, resource unversioned.GroupVersionResource) (monitor, error) {
	// TODO: consider store in one storage.
	glog.V(6).Infof("create storage for resource %s", resource)
	var monitor monitor
	client, err := clientPool.ClientForGroupVersion(resource.GroupVersion())
	if err != nil {
		return monitor, err
	}
	monitor.store, monitor.controller = framework.NewInformer(
		// TODO: make special List and Watch function that removes fields other
		// than ObjectMeta.
		&cache.ListWatch{
			ListFunc: func(options api.ListOptions) (runtime.Object, error) {
				// APIResource.Kind is not used by the dynamic client, so
				// leave it empty. We want to list this resource in all
				// namespaces if it's namespace scoped, so leave
				// APIResource.Namespaced as false is all right.
				apiResource := unversioned.APIResource{Name: resource.Resource}
				return client.Resource(&apiResource, api.NamespaceAll).List(&options)
			},
			WatchFunc: func(options api.ListOptions) (watch.Interface, error) {
				// APIResource.Kind is not used by the dynamic client, so
				// leave it empty. We want to list this resource in all
				// namespaces if it's namespace scoped, so leave
				// APIResource.Namespaced as false is all right.
				apiResource := unversioned.APIResource{Name: resource.Resource}
				return client.Resource(&apiResource, api.NamespaceAll).Watch(&options)
			},
		},
		nil,
		ResourceResyncTime,
		framework.ResourceEventHandlerFuncs{
			// add the event to the propagator's eventQueue.
			AddFunc: func(obj interface{}) {
				event := event{
					eventType: addEvent,
					obj:       obj,
				}
				p.eventQueue.Add(event)
			},
			UpdateFunc: func(oldObj, newObj interface{}) {
				event := event{updateEvent, newObj, oldObj}
				p.eventQueue.Add(event)
			},
			DeleteFunc: func(obj interface{}) {
				// delta fifo may wrap the object in a cache.DeletedFinalStateUnknown, unwrap it
				if deletedFinalStateUnknown, ok := obj.(cache.DeletedFinalStateUnknown); ok {
					obj = deletedFinalStateUnknown.Obj
				}
				event := event{
					eventType: deleteEvent,
					obj:       obj,
				}
				p.eventQueue.Add(event)
			},
		},
	)
	return monitor, nil
}

var ignoredResources = map[unversioned.GroupVersionResource]struct{}{
	unversioned.GroupVersionResource{Group: "extensions", Version: "v1beta1", Resource: "replicationcontrollers"}: {},
	unversioned.GroupVersionResource{Group: "", Version: "v1", Resource: "bindings"}:                              {},
	unversioned.GroupVersionResource{Group: "", Version: "v1", Resource: "componentstatuses"}:                     {},
	unversioned.GroupVersionResource{Group: "", Version: "v1", Resource: "events"}:                                {},
}

func NewGarbageCollector(clientPool dynamic.ClientPool, resources []unversioned.GroupVersionResource) (*GarbageCollector, error) {
	gc := &GarbageCollector{
		clientPool:  clientPool,
		dirtyQueue:  workqueue.New(),
		orphanQueue: workqueue.New(),
		// TODO: should use a dynamic RESTMapper built from the discovery results.
		restMapper: registered.RESTMapper(),
	}
	gc.propagator = &Propagator{
		eventQueue: workqueue.New(),
		uidToNode: &concurrentUIDToNode{
			RWMutex:   &sync.RWMutex{},
			uidToNode: make(map[types.UID]*node),
		},
		gc: gc,
	}
	for _, resource := range resources {
		if _, ok := ignoredResources[resource]; ok {
			glog.V(6).Infof("ignore resource %#v", resource)
			continue
		}
		monitor, err := monitorFor(gc.propagator, gc.clientPool, resource)
		if err != nil {
			return nil, err
		}
		gc.monitors = append(gc.monitors, monitor)
	}
	return gc, nil
}

func (gc *GarbageCollector) worker() {
	key, quit := gc.dirtyQueue.Get()
	if quit {
		return
	}
	defer gc.dirtyQueue.Done(key)
	err := gc.processItem(key.(*node))
	if err != nil {
		utilruntime.HandleError(fmt.Errorf("Error syncing item %v: %v", key, err))
	}
}

// apiResource consults the REST mapper to translate an <apiVersion, kind,
// namespace> tuple to a unversioned.APIResource struct.
func (gc *GarbageCollector) apiResource(apiVersion, kind string, namespaced bool) (*unversioned.APIResource, error) {
	fqKind := unversioned.FromAPIVersionAndKind(apiVersion, kind)
	mapping, err := gc.restMapper.RESTMapping(fqKind.GroupKind(), apiVersion)
	if err != nil {
		return nil, fmt.Errorf("unable to get REST mapping for kind: %s, version: %s", kind, apiVersion)
	}
	glog.V(6).Infof("map kind %s, version %s to resource %s", kind, apiVersion, mapping.Resource)
	resource := unversioned.APIResource{
		Name:       mapping.Resource,
		Namespaced: namespaced,
		Kind:       kind,
	}
	return &resource, nil
}

func (gc *GarbageCollector) deleteObject(item objectReference) error {
	fqKind := unversioned.FromAPIVersionAndKind(item.APIVersion, item.Kind)
	client, err := gc.clientPool.ClientForGroupVersion(fqKind.GroupVersion())
	resource, err := gc.apiResource(item.APIVersion, item.Kind, len(item.Namespace) != 0)
	if err != nil {
		return err
	}
	uid := item.UID
	preconditions := v1.Preconditions{UID: &uid}
	deleteOptions := v1.DeleteOptions{Preconditions: &preconditions}
	return client.Resource(resource, item.Namespace).Delete(item.Name, &deleteOptions)
}

func (gc *GarbageCollector) getObject(item objectReference) (*runtime.Unstructured, error) {
	fqKind := unversioned.FromAPIVersionAndKind(item.APIVersion, item.Kind)
	client, err := gc.clientPool.ClientForGroupVersion(fqKind.GroupVersion())
	resource, err := gc.apiResource(item.APIVersion, item.Kind, len(item.Namespace) != 0)
	if err != nil {
		return nil, err
	}
	return client.Resource(resource, item.Namespace).Get(item.Name)
}

func (gc *GarbageCollector) updateObject(item objectReference, obj *runtime.Unstructured) (*runtime.Unstructured, error) {
	fqKind := unversioned.FromAPIVersionAndKind(item.APIVersion, item.Kind)
	client, err := gc.clientPool.ClientForGroupVersion(fqKind.GroupVersion())
	resource, err := gc.apiResource(item.APIVersion, item.Kind, len(item.Namespace) != 0)
	if err != nil {
		return nil, err
	}
	return client.Resource(resource, item.Namespace).Update(obj)
}

func (gc *GarbageCollector) patchObject(item objectReference, patch []byte) (*runtime.Unstructured, error) {
	fqKind := unversioned.FromAPIVersionAndKind(item.APIVersion, item.Kind)
	client, err := gc.clientPool.ClientForGroupVersion(fqKind.GroupVersion())
	resource, err := gc.apiResource(item.APIVersion, item.Kind, len(item.Namespace) != 0)
	if err != nil {
		return nil, err
	}
	return client.Resource(resource, item.Namespace).Patch(item.Name, api.StrategicMergePatchType, patch)
}

func objectReferenceToUnstructured(ref objectReference) *runtime.Unstructured {
	ret := &runtime.Unstructured{}
	ret.SetKind(ref.Kind)
	ret.SetAPIVersion(ref.APIVersion)
	ret.SetUID(ref.UID)
	ret.SetNamespace(ref.Namespace)
	ret.SetName(ref.Name)
	return ret
}

func (gc *GarbageCollector) processItem(item *node) error {
	// Get the latest item from the API server
	latest, err := gc.getObject(item.identity)
	if err != nil {
		if errors.IsNotFound(err) {
			// the Propagator can add "virtual" node for an owner that doesn't
			// exist yet, so we need to enqueue a virtual Delete event to remove
			// the virtual node from Propagator.uidToNode.
			glog.V(6).Infof("item %v not found, generating a virtual delete event", item.identity)
			event := event{
				eventType: deleteEvent,
				obj:       objectReferenceToUnstructured(item.identity),
			}
			gc.propagator.eventQueue.Add(event)
			return nil
		}
		return err
	}
	if latest.GetUID() != item.identity.UID {
		glog.V(6).Infof("UID doesn't match, item %v not found, ignore it", item.identity)
		return nil
	}
	ownerReferences := latest.GetOwnerReferences()
	if len(ownerReferences) == 0 {
		glog.V(6).Infof("object %s's doesn't have an owner, continue on next item", item.identity)
		return nil
	}
	// TODO: we need to remove dangling references if the object is not to be
	// deleted.
	for _, reference := range ownerReferences {
		// TODO: we need to verify the reference resource is supported by the
		// system. If it's not a valid resource, the garbage collector should i)
		// ignore the reference when decide if the object should be deleted, and
		// ii) should update the object to remove such references. This is to
		// prevent objects having references to an old resource from being
		// deleted during a cluster upgrade.
		fqKind := unversioned.FromAPIVersionAndKind(reference.APIVersion, reference.Kind)
		client, err := gc.clientPool.ClientForGroupVersion(fqKind.GroupVersion())
		if err != nil {
			return err
		}
		resource, err := gc.apiResource(reference.APIVersion, reference.Kind, len(item.identity.Namespace) != 0)
		if err != nil {
			return err
		}
		owner, err := client.Resource(resource, item.identity.Namespace).Get(reference.Name)
		if err == nil {
			if owner.GetUID() != reference.UID {
				glog.V(6).Infof("object %s's owner %s/%s, %s is not found, UID mismatch", item.identity.UID, reference.APIVersion, reference.Kind, reference.Name)
				continue
			}
			glog.V(6).Infof("object %s has at least an existing owner, will not garbage collect", item.identity.UID)
			return nil
		} else if errors.IsNotFound(err) {
			glog.V(6).Infof("object %s's owner %s/%s, %s is not found", item.identity.UID, reference.APIVersion, reference.Kind, reference.Name)
		} else {
			return err
		}
	}
	glog.V(2).Infof("none of object %s's owners exist any more, will garbage collect it", item.identity)
	return gc.deleteObject(item.identity)
}

func (gc *GarbageCollector) Run(workers int, stopCh <-chan struct{}) {
	for _, monitor := range gc.monitors {
		go monitor.controller.Run(stopCh)
	}

	// worker
	go wait.Until(gc.propagator.processEvent, 0, stopCh)

	for i := 0; i < workers; i++ {
		go wait.Until(gc.worker, 0, stopCh)
		go wait.Until(gc.orphanFinalizer, 0, stopCh)
	}
	<-stopCh
	glog.Infof("Shutting down garbage collector")
	gc.dirtyQueue.ShutDown()
	gc.orphanQueue.ShutDown()
	gc.propagator.eventQueue.ShutDown()
}

// QueueDrained returns if the dirtyQueue and eventQueue are drained. It's
// useful for debugging. Note that it doesn't guarantee the workers are idle.
func (gc *GarbageCollector) QueuesDrained() bool {
	return gc.dirtyQueue.Len() == 0 && gc.propagator.eventQueue.Len() == 0 && gc.orphanQueue.Len() == 0
}

// *FOR TEST USE ONLY* It's not safe to call this function when the GC is still
// busy.
// GraphHasUID returns if the Propagator has a particular UID store in its
// uidToNode graph. It's useful for debugging.
func (gc *GarbageCollector) GraphHasUID(UIDs []types.UID) bool {
	for _, u := range UIDs {
		if _, ok := gc.propagator.uidToNode.Read(u); ok {
			return true
		}
	}
	return false
}