Debounce kubernetes service endpoint updates

Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

pkg/agent/tunnel/tunnel.go

@@ -35,6 +35,10 @@ import (
 	"k8s.io/kubernetes/pkg/cluster/ports"
 )
 
+var (
+	endpointDebounceDelay = time.Second
+)
+
 type agentTunnel struct {
 	client      kubernetes.Interface
 	cidrs       cidranger.Ranger
@@ -306,9 +310,14 @@ func (a *agentTunnel) watchEndpoints(ctx context.Context, apiServerReady <-chan
 		<-done
 	}()
 
+	var cancelUpdate context.CancelFunc
+
 	for {
 		select {
 		case <-ctx.Done():
+			if cancelUpdate != nil {
+				cancelUpdate()
+			}
 			return
 		case ev, ok := <-watch.ResultChan():
 			endpoint, ok := ev.Object.(*v1.Endpoints)
@@ -317,28 +326,49 @@ func (a *agentTunnel) watchEndpoints(ctx context.Context, apiServerReady <-chan
 				continue
 			}
 
-			newAddresses := util.GetAddresses(endpoint)
-			if reflect.DeepEqual(newAddresses, proxy.SupervisorAddresses()) {
-				continue
+			if cancelUpdate != nil {
+				cancelUpdate()
 			}
-			proxy.Update(newAddresses)
 
-			validEndpoint := map[string]bool{}
+			var debounceCtx context.Context
+			debounceCtx, cancelUpdate = context.WithCancel(ctx)
+
+			// When joining the cluster, the apiserver adds, removes, and then readds itself to
+			// the endpoint list several times.  This causes a bit of thrashing if we react to
+			// endpoint changes immediately.  Instead, perform the endpoint update in a
+			// goroutine that sleeps for a short period before checking for changes and updating
+			// the proxy addresses.  If another update occurs, the previous update operation
+			// will be cancelled and a new one queued.
+			go func() {
+				select {
+				case <-time.After(endpointDebounceDelay):
+				case <-debounceCtx.Done():
+					return
+				}
 
-			for _, address := range proxy.SupervisorAddresses() {
-				validEndpoint[address] = true
-				if _, ok := disconnect[address]; !ok {
-					disconnect[address] = a.connect(ctx, nil, address, tlsConfig)
+				newAddresses := util.GetAddresses(endpoint)
+				if reflect.DeepEqual(newAddresses, proxy.SupervisorAddresses()) {
+					return
 				}
-			}
+				proxy.Update(newAddresses)
 
-			for address, cancel := range disconnect {
-				if !validEndpoint[address] {
-					cancel()
-					delete(disconnect, address)
-					logrus.Infof("Stopped tunnel to %s", address)
+				validEndpoint := map[string]bool{}
+
+				for _, address := range proxy.SupervisorAddresses() {
+					validEndpoint[address] = true
+					if _, ok := disconnect[address]; !ok {
+						disconnect[address] = a.connect(ctx, nil, address, tlsConfig)
+					}
 				}
-			}
+
+				for address, cancel := range disconnect {
+					if !validEndpoint[address] {
+						cancel()
+						delete(disconnect, address)
+						logrus.Infof("Stopped tunnel to %s", address)
+					}
+				}
+			}()
 		}
 	}
 }