webhook config manager: HasSynced returns true when the manager is synced with existing webhookconfig objects at startup

staging/src/k8s.io/apiserver/pkg/admission/configuration/mutating_webhook_manager.go

@@ -36,6 +36,11 @@ type mutatingWebhookConfigurationManager struct {
 	configuration *atomic.Value
 	lister        admissionregistrationlisters.MutatingWebhookConfigurationLister
 	hasSynced     func() bool
+	// initialConfigurationSynced stores a boolean value, which tracks if
+	// the existing webhook configs have been synced (honored) by the
+	// manager at startup-- the informer has synced and either has no items
+	// or has finished executing updateConfiguration() once.
+	initialConfigurationSynced *atomic.Value
 }
 
 var _ generic.Source = &mutatingWebhookConfigurationManager{}
@@ -43,13 +48,15 @@ var _ generic.Source = &mutatingWebhookConfigurationManager{}
 func NewMutatingWebhookConfigurationManager(f informers.SharedInformerFactory) generic.Source {
 	informer := f.Admissionregistration().V1().MutatingWebhookConfigurations()
 	manager := &mutatingWebhookConfigurationManager{
-		configuration: &atomic.Value{},
+		configuration:              &atomic.Value{},
-		lister:        informer.Lister(),
+		lister:                     informer.Lister(),
-		hasSynced:     informer.Informer().HasSynced,
+		hasSynced:                  informer.Informer().HasSynced,
+		initialConfigurationSynced: &atomic.Value{},
 	}
 
 	// Start with an empty list
 	manager.configuration.Store([]webhook.WebhookAccessor{})
+	manager.initialConfigurationSynced.Store(false)
 
 	// On any change, rebuild the config
 	informer.Informer().AddEventHandler(cache.ResourceEventHandlerFuncs{
@@ -66,8 +73,27 @@ func (m *mutatingWebhookConfigurationManager) Webhooks() []webhook.WebhookAccess
 	return m.configuration.Load().([]webhook.WebhookAccessor)
 }
 
+// HasSynced returns true when the manager is synced with existing webhookconfig
+// objects at startup-- which means the informer is synced and either has no items
+// or updateConfiguration() has completed.
 func (m *mutatingWebhookConfigurationManager) HasSynced() bool {
-	return m.hasSynced()
+	if !m.hasSynced() {
+		return false
+	}
+	if m.initialConfigurationSynced.Load().(bool) {
+		// the informer has synced and configuration has been updated
+		return true
+	}
+	if configurations, err := m.lister.List(labels.Everything()); err == nil && len(configurations) == 0 {
+		// the empty list we initially stored is valid to use.
+		// Setting initialConfigurationSynced to true, so subsequent checks
+		// would be able to take the fast path on the atomic boolean in a
+		// cluster without any admission webhooks configured.
+		m.initialConfigurationSynced.Store(true)
+		// the informer has synced and we don't have any items
+		return true
+	}
+	return false
 }
 
 func (m *mutatingWebhookConfigurationManager) updateConfiguration() {
@@ -77,6 +103,7 @@ func (m *mutatingWebhookConfigurationManager) updateConfiguration() {
 		return
 	}
 	m.configuration.Store(mergeMutatingWebhookConfigurations(configurations))
+	m.initialConfigurationSynced.Store(true)
 }
 
 func mergeMutatingWebhookConfigurations(configurations []*v1.MutatingWebhookConfiguration) []webhook.WebhookAccessor {

staging/src/k8s.io/apiserver/pkg/admission/configuration/validating_webhook_manager.go

@@ -36,6 +36,11 @@ type validatingWebhookConfigurationManager struct {
 	configuration *atomic.Value
 	lister        admissionregistrationlisters.ValidatingWebhookConfigurationLister
 	hasSynced     func() bool
+	// initialConfigurationSynced stores a boolean value, which tracks if
+	// the existing webhook configs have been synced (honored) by the
+	// manager at startup-- the informer has synced and either has no items
+	// or has finished executing updateConfiguration() once.
+	initialConfigurationSynced *atomic.Value
 }
 
 var _ generic.Source = &validatingWebhookConfigurationManager{}
@@ -43,13 +48,15 @@ var _ generic.Source = &validatingWebhookConfigurationManager{}
 func NewValidatingWebhookConfigurationManager(f informers.SharedInformerFactory) generic.Source {
 	informer := f.Admissionregistration().V1().ValidatingWebhookConfigurations()
 	manager := &validatingWebhookConfigurationManager{
-		configuration: &atomic.Value{},
+		configuration:              &atomic.Value{},
-		lister:        informer.Lister(),
+		lister:                     informer.Lister(),
-		hasSynced:     informer.Informer().HasSynced,
+		hasSynced:                  informer.Informer().HasSynced,
+		initialConfigurationSynced: &atomic.Value{},
 	}
 
 	// Start with an empty list
 	manager.configuration.Store([]webhook.WebhookAccessor{})
+	manager.initialConfigurationSynced.Store(false)
 
 	// On any change, rebuild the config
 	informer.Informer().AddEventHandler(cache.ResourceEventHandlerFuncs{
@@ -66,9 +73,28 @@ func (v *validatingWebhookConfigurationManager) Webhooks() []webhook.WebhookAcce
 	return v.configuration.Load().([]webhook.WebhookAccessor)
 }
 
-// HasSynced returns true if the shared informers have synced.
+// HasSynced returns true when the manager is synced with existing webhookconfig
+// objects at startup-- which means the informer is synced and either has no items
+// or updateConfiguration() has completed.
 func (v *validatingWebhookConfigurationManager) HasSynced() bool {
-	return v.hasSynced()
+	if !v.hasSynced() {
+		return false
+	}
+	if v.initialConfigurationSynced.Load().(bool) {
+		// the informer has synced and configuration has been updated
+		return true
+	}
+	if configurations, err := v.lister.List(labels.Everything()); err == nil && len(configurations) == 0 {
+		// the empty list we initially stored is valid to use.
+		// Setting initialConfigurationSynced to true, so subsequent checks
+		// would be able to take the fast path on the atomic boolean in a
+		// cluster without any admission webhooks configured.
+		v.initialConfigurationSynced.Store(true)
+		// the informer has synced and we don't have any items
+		return true
+	}
+	return false
+
 }
 
 func (v *validatingWebhookConfigurationManager) updateConfiguration() {
@@ -78,6 +104,7 @@ func (v *validatingWebhookConfigurationManager) updateConfiguration() {
 		return
 	}
 	v.configuration.Store(mergeValidatingWebhookConfigurations(configurations))
+	v.initialConfigurationSynced.Store(true)
 }
 
 func mergeValidatingWebhookConfigurations(configurations []*v1.ValidatingWebhookConfiguration) []webhook.WebhookAccessor {