Userspace Proxy: allow check for endpoints on svc

This commit adds a method to the `LoadBalancer` interface in the
userspace proxy which allows consumers of the `LoadBalancer` to check if
it thinks a given service has endpoints available.

pkg/proxy/userspace/loadbalancer.go

@@ -31,4 +31,5 @@ type LoadBalancer interface {
 	NewService(service proxy.ServicePortName, sessionAffinityType api.ServiceAffinity, stickyMaxAgeMinutes int) error
 	DeleteService(service proxy.ServicePortName)
 	CleanupStaleStickySessions(service proxy.ServicePortName)
+	ServiceHasEndpoints(service proxy.ServicePortName) bool
 }

pkg/proxy/userspace/roundrobin.go

@@ -120,6 +120,16 @@ func isSessionAffinity(affinity *affinityPolicy) bool {
 	return true
 }
 
+// ServiceHasEndpoints checks whether a service entry has endpoints.
+func (lb *LoadBalancerRR) ServiceHasEndpoints(svcPort proxy.ServicePortName) bool {
+	lb.lock.Lock()
+	defer lb.lock.Unlock()
+	state, exists := lb.services[svcPort]
+	// TODO: while nothing ever assigns nil to the map, *some* of the code using the map
+	// checks for it.  The code should all follow the same convention.
+	return exists && state != nil && len(state.endpoints) > 0
+}
+
 // NextEndpoint returns a service endpoint.
 // The service endpoint is chosen using the round-robin algorithm.
 func (lb *LoadBalancerRR) NextEndpoint(svcPort proxy.ServicePortName, srcAddr net.Addr, sessionAffinityReset bool) (string, error) {