allow handler to join after the informer has started

pkg/controller/framework/shared_informer.go

@@ -88,6 +88,10 @@ type sharedIndexInformer struct {
 
 	started     bool
 	startedLock sync.Mutex
+
+	// blockDeltas gives a way to stop all event distribution so that a late event handler
+	// can safely join the shared informer.
+	blockDeltas sync.Mutex
 }
 
 // dummyController hides the fact that a SharedInformer is different from a dedicated one
@@ -199,16 +203,35 @@ func (s *sharedIndexInformer) AddEventHandler(handler ResourceEventHandler) erro
 	s.startedLock.Lock()
 	defer s.startedLock.Unlock()
 
-	if s.started {
-		return fmt.Errorf("informer has already started")
+	if !s.started {
+		listener := newProcessListener(handler)
+		s.processor.listeners = append(s.processor.listeners, listener)
+		return nil
 	}
 
+	// in order to safely join, we have to
+	// 1. stop sending add/update/delete notifications
+	// 2. do a list against the store
+	// 3. send synthetic "Add" events to the new handler
+	// 4. unblock
+	s.blockDeltas.Lock()
+	defer s.blockDeltas.Unlock()
+
 	listener := newProcessListener(handler)
 	s.processor.listeners = append(s.processor.listeners, listener)
+
+	items := s.indexer.List()
+	for i := range items {
+		listener.add(addNotification{newObj: items[i]})
+	}
+
 	return nil
 }
 
 func (s *sharedIndexInformer) HandleDeltas(obj interface{}) error {
+	s.blockDeltas.Lock()
+	defer s.blockDeltas.Unlock()
+
 	// from oldest to newest
 	for _, d := range obj.(cache.Deltas) {
 		switch d.Type {