Don't clobber backends inserted by other controllers.

controllers/gce/backends/backends.go

@@ -220,12 +220,19 @@ func (b *Backends) edgeHop(be *compute.BackendService, igs []*compute.InstanceGr
 	for _, igToBE := range igs {
 		igLinks.Insert(igToBE.SelfLink)
 	}
-	if igLinks.Equal(beIGs) {
+	if beIGs.IsSuperset(igLinks) {
 		return nil
 	}
 	glog.Infof("Backend %v has a broken edge, expected igs %+v, current igs %+v",
 		be.Name, igLinks.List(), beIGs.List())
-	be.Backends = getBackendsForIGs(igs)
+
+	newBackends := []*compute.Backend{}
+	for _, b := range getBackendsForIGs(igs) {
+		if !beIGs.Has(b.Group) {
+			newBackends = append(newBackends, b)
+		}
+	}
+	be.Backends = append(be.Backends, newBackends...)
 	if err := b.cloud.UpdateBackendService(be); err != nil {
 		return err
 	}

controllers/gce/backends/backends_test.go

@@ -75,7 +75,9 @@ func TestBackendPoolAdd(t *testing.T) {
 	// This simulates a user doing foolish things through the UI.
 	f.calls = []int{}
 	be, err = f.GetBackendService(beName)
-	be.Backends[0].Group = "test edge hop"
+	be.Backends = []*compute.Backend{
+		{Group: "test edge hop"},
+	}
 	f.UpdateBackendService(be)
 
 	pool.Add(nodePort)
@@ -92,10 +94,14 @@ func TestBackendPoolAdd(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to find instance group %v", namer.IGName())
 	}
-	if gotBackend.Backends[0].Group != gotGroup.SelfLink {
+	backendLinks := sets.NewString()
+	for _, be := range gotBackend.Backends {
+		backendLinks.Insert(be.Group)
+	}
+	if !backendLinks.Has(gotGroup.SelfLink) {
 		t.Fatalf(
-			"Broken instance group link: %v %v",
-			gotBackend.Backends[0].Group,
+			"Broken instance group link, got: %+v expected: %v",
+			backendLinks.List(),
 			gotGroup.SelfLink)
 	}
 }
@@ -178,5 +184,49 @@ func TestBackendPoolShutdown(t *testing.T) {
 	if _, err := f.GetBackendService(namer.BeName(80)); err == nil {
 		t.Fatalf("%v", err)
 	}
-
+}
+
+func TestBackendInstanceGroupClobbering(t *testing.T) {
+	f := NewFakeBackendServices()
+	fakeIGs := instances.NewFakeInstanceGroups(sets.NewString())
+	pool := newBackendPool(f, fakeIGs, false)
+	namer := utils.Namer{}
+
+	// This will add the instance group k8s-ig to the instance pool
+	pool.Add(80)
+
+	be, err := f.GetBackendService(namer.BeName(80))
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+	// Simulate another controller updating the same backend service with
+	// a different instance group
+	newGroups := []*compute.Backend{
+		{Group: "k8s-ig-bar"},
+		{Group: "k8s-ig-foo"},
+	}
+	be.Backends = append(be.Backends, newGroups...)
+	if err := f.UpdateBackendService(be); err != nil {
+		t.Fatalf("Failed to update backend service %v", be.Name)
+	}
+
+	// Make sure repeated adds don't clobber the inserted instance group
+	pool.Add(80)
+	be, err = f.GetBackendService(namer.BeName(80))
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+	gotGroups := sets.NewString()
+	for _, g := range be.Backends {
+		gotGroups.Insert(g.Group)
+	}
+
+	// seed expectedGroups with the first group native to this controller
+	expectedGroups := sets.NewString("k8s-ig")
+	for _, newGroup := range newGroups {
+		expectedGroups.Insert(newGroup.Group)
+	}
+	if !expectedGroups.Equal(gotGroups) {
+		t.Fatalf("Expected %v Got %v", expectedGroups, gotGroups)
+	}
 }