事件处理中的 DeletedFinalStateUnknown 是什么

sharedInformer.Informer().AddEventHandler(cache.ResourceEventHandlerFuncs{
	AddFunc: func(obj interface{}) {
		// handle add 
	},
	UpdateFunc: func(oldObj, newObj interface{}) {
		// handle update		
	},
	DeleteFunc: func(obj interface{}) {
		pv, ok := obj.(*v1.PersistentVolume)
		if !ok {
			klog.Warningf("Deleted object is not a v1.PersistentVolume type")
			// When a delete is dropped, the relist will notice a pv in the local cache but not
			// in the list, leading to the insertion of a tombstone object which contains
			// the deleted pv.
			tombstone, ok := obj.(cache.DeletedFinalStateUnknown)
			if !ok {
				klog.Errorf("Unknown object type in delete event %+v", obj)
				return
			}
			pv, ok = tombstone.Obj.(*v1.PersistentVolume)
			if !ok {
				klog.Errorf("Tombstone contained object is not a v1.PersistentVolume %+v", obj)
				return
			}
		}
		// 仍然要处理删除事件
		p.handlePVDelete(pv)
	},
})

func (f *DeltaFIFO) Replace(list []interface{}, resourceVersion string) error {
	f.lock.Lock()
	defer f.lock.Unlock()
	keys := make(sets.String, len(list))
	
	// 省去其他代码... 

	if f.knownObjects == nil {
		// Do deletion detection against our own list.
		queuedDeletions := 0
		for k, oldItem := range f.items {
			if keys.Has(k) {
				continue
			}
			var deletedObj interface{}
			if n := oldItem.Newest(); n != nil {
				deletedObj = n.Object
			}
			queuedDeletions++
			// 添加DeletedFinalStateUnknown
			if err := f.queueActionLocked(Deleted, DeletedFinalStateUnknown{k, deletedObj}); err != nil {
				return err
			}
		}
		return nil
	}

	// Detect deletions not already in the queue.
	knownKeys := f.knownObjects.ListKeys()
	queuedDeletions := 0
	for _, k := range knownKeys {
		if keys.Has(k) {
			continue
		}

		deletedObj, exists, err := f.knownObjects.GetByKey(k)
		if err != nil {
			deletedObj = nil
			klog.Errorf("Unexpected error %v during lookup of key %v, placing DeleteFinalStateUnknown marker without object", err, k)
		} else if !exists {
			deletedObj = nil
			klog.Infof("Key %v does not exist in known objects store, placing DeleteFinalStateUnknown marker without object", k)
		}
		queuedDeletions++
		// 添加DeletedFinalStateUnknown
		if err := f.queueActionLocked(Deleted, DeletedFinalStateUnknown{k, deletedObj}); err != nil {
			return err
		}
	}
	return nil
}

var (
	// We try to spread the load on apiserver by setting timeouts for
	// watch requests - it is random in [minWatchTimeout, 2*minWatchTimeout].
	minWatchTimeout = 5 * time.Minute
)

timeoutSeconds := int64(minWatchTimeout.Seconds() * (rand.Float64() + 1.0))

options = metav1.ListOptions{
    ResourceVersion: resourceVersion,
    // We want to avoid situations of hanging watchers. Stop any wachers that do not
    // receive any events within the timeout window.
    TimeoutSeconds: &timeoutSeconds,
}