Ignore empty Consul instance updates (#119)

Updating current instances slice to empty one may lock whole mechanism.
It is better to have several network errors in the metrics than to
complicate the whole mechanism.

xnet/writer.go

@@ -94,9 +94,13 @@ func DiscoveryServiceInstanceProvider(serviceName string, interval time.Duration
 				return newInstances[i] < newInstances[j]
 			})
 			if !reflect.DeepEqual(currInstances, newInstances) {
-				log.WithField("instances", newInstances).Infof("Service %q instances in discovery changed - sending update", serviceName)
-				currInstances = newInstances
-				instancesChan <- newInstances
+				if len(newInstances) > 0 {
+					log.WithField("instances", newInstances).Infof("Service %q instances in discovery changed - sending update", serviceName)
+					currInstances = newInstances
+					instancesChan <- newInstances
+				} else {
+					log.WithField("instances", newInstances).Infof("Service %q instances in discovery changed - ignoring empty update", serviceName)
+				}
 			}
 		}
 	}()

xnet/writer_test.go

@@ -76,28 +76,32 @@ func TestDiscoveryServiceInstanceProviderShouldPeriodicallyUpdatesInstances(t *t
 	assert.Empty(t, ret)
 }
 
-func TestDiscoveryServiceInstanceProviderShouldUpdateInstancesWhenTheyAreEmpty(t *testing.T) {
-	ret := make(chan []Address, 1)
-	client := &StubDiscoveryServiceClient{returns: ret}
+func TestIfUpdatesAddressesOnlyIfTheyChanged(t *testing.T) {
+	returns := make(chan []Address, 5)
+	discoveryServiceClient := &StubDiscoveryServiceClient{returns}
 
 	// setup expectations
-	ret <- []Address{}
+	returns <- []Address{"127.0.0.1:1234", "127.0.0.1:4321"} // initial instances
+	returns <- []Address{"127.0.0.1:1234", "127.0.0.1:4321"} // same as before but different order
+	returns <- []Address{"127.0.0.1:1234", "127.0.0.1:4321"} // same as before
+	returns <- []Address{"127.0.0.1:4321", "127.0.0.1:1234"} // same as before
+	returns <- []Address{"127.0.0.1:5678", "127.0.0.1:8765"} // different ports
 
-	provider := DiscoveryServiceInstanceProvider("service name", 1, client)
+	instanceProvider := DiscoveryServiceInstanceProvider("service-name", 1, discoveryServiceClient)
 
-	assert.Equal(t, []Address{}, <-provider)
-	assert.Empty(t, ret)
+	assert.Equal(t, []Address{"127.0.0.1:1234", "127.0.0.1:4321"}, <-instanceProvider)
+	assert.Equal(t, []Address{"127.0.0.1:5678", "127.0.0.1:8765"}, <-instanceProvider)
+	assert.Empty(t, returns)
 }
 
-func TestIfUpdatesAddressesOnlyIfTheyChanged(t *testing.T) {
-	returns := make(chan []Address, 5)
+func TestIfNotUpdatesEmptyAddresses(t *testing.T) {
+	returns := make(chan []Address, 4)
 	discoveryServiceClient := &StubDiscoveryServiceClient{returns}
 
 	// setup expectations
 	returns <- []Address{"127.0.0.1:1234", "127.0.0.1:4321"} // initial instances
-	returns <- []Address{"127.0.0.1:1234", "127.0.0.1:4321"} // same as before but different order
+	returns <- []Address{}                                   // empty pool
 	returns <- []Address{"127.0.0.1:1234", "127.0.0.1:4321"} // same as before
-	returns <- []Address{"127.0.0.1:4321", "127.0.0.1:1234"} // same as before
 	returns <- []Address{"127.0.0.1:5678", "127.0.0.1:8765"} // different ports
 
 	instanceProvider := DiscoveryServiceInstanceProvider("service-name", 1, discoveryServiceClient)