kube-proxy: initialization wait for service and endpoint handlers synced

Ensure kube-proxy waits for the services/endpointslices informer
caches to be synced *and* all pre-sync events delivered before
setting isInitialized=true. Otherwise, in clusters with many services,
some services may be missing from svcPortMap when kube-proxy starts
(e.g. during daemonset rollout). This can cause kube-proxy to temporarily
remove service DNAT rules and then skip cleanup of UDP conntrack entries
to a service VIP.

Resolves: https://github.com/kubernetes/kubernetes/issues/126468

pkg/proxy/config/config.go

@@ -78,11 +78,10 @@ type EndpointSliceConfig struct {
 // NewEndpointSliceConfig creates a new EndpointSliceConfig.
 func NewEndpointSliceConfig(ctx context.Context, endpointSliceInformer discoveryv1informers.EndpointSliceInformer, resyncPeriod time.Duration) *EndpointSliceConfig {
 	result := &EndpointSliceConfig{
-		listerSynced: endpointSliceInformer.Informer().HasSynced,
 		logger: klog.FromContext(ctx),
 	}
 
-	_, _ = endpointSliceInformer.Informer().AddEventHandlerWithResyncPeriod(
+	handlerRegistration, _ := endpointSliceInformer.Informer().AddEventHandlerWithResyncPeriod(
 		cache.ResourceEventHandlerFuncs{
 			AddFunc:    result.handleAddEndpointSlice,
 			UpdateFunc: result.handleUpdateEndpointSlice,
@@ -91,6 +90,8 @@ func NewEndpointSliceConfig(ctx context.Context, endpointSliceInformer discovery
 		resyncPeriod,
 	)
 
+	result.listerSynced = handlerRegistration.HasSynced
+
 	return result
 }
 
@@ -171,11 +172,10 @@ type ServiceConfig struct {
 // NewServiceConfig creates a new ServiceConfig.
 func NewServiceConfig(ctx context.Context, serviceInformer v1informers.ServiceInformer, resyncPeriod time.Duration) *ServiceConfig {
 	result := &ServiceConfig{
-		listerSynced: serviceInformer.Informer().HasSynced,
 		logger: klog.FromContext(ctx),
 	}
 
-	_, _ = serviceInformer.Informer().AddEventHandlerWithResyncPeriod(
+	handlerRegistration, _ := serviceInformer.Informer().AddEventHandlerWithResyncPeriod(
 		cache.ResourceEventHandlerFuncs{
 			AddFunc:    result.handleAddService,
 			UpdateFunc: result.handleUpdateService,
@@ -184,6 +184,8 @@ func NewServiceConfig(ctx context.Context, serviceInformer v1informers.ServiceIn
 		resyncPeriod,
 	)
 
+	result.listerSynced = handlerRegistration.HasSynced
+
 	return result
 }