Make forward compatible with pre-scale

pkg/core/test_helpers.go

@@ -99,6 +99,14 @@ func (f *testStackFactory) pendingRemoval() *testStackFactory {
 	return f
 }
 
+func (f *testStackFactory) trafficForward() *testStackFactory {
+	if f.container.Stack.Annotations == nil {
+		f.container.Stack.Annotations = map[string]string{}
+	}
+	f.container.Stack.Annotations[forwardBackendAnnotation] = forwardBackendName
+	return f
+}
+
 func (f *testStackFactory) prescaling(replicas int32, desiredTrafficWeight float64, lastTrafficIncrease time.Time) *testStackFactory {
 	f.container.prescalingActive = true
 	f.container.prescalingReplicas = replicas

pkg/core/traffic_prescaling.go

@@ -41,6 +41,13 @@ func (r PrescalingTrafficReconciler) Reconcile(stacks map[string]*StackContainer
 
 	// Prescale stacks if needed
 	for _, stack := range stacks {
+		// Stacks configured to forward traffic to another cluster don't
+		// have a deployment of their own, so prescaling doesn't apply
+		// to them.
+		if stack.TrafficForward() {
+			continue
+		}
+
 		// If traffic needs to be increased
 		if stack.desiredTrafficWeight > stack.actualTrafficWeight {
 			// If prescaling is not active, or desired weight changed since the last prescaling attempt, update
@@ -86,7 +93,11 @@ func (r PrescalingTrafficReconciler) Reconcile(stacks map[string]*StackContainer
 	actualWeights := make(map[string]float64, len(stacks))
 	for stackName, stack := range stacks {
 		// Check if we're increasing traffic but the stack is not ready
-		if stack.desiredTrafficWeight > stack.actualTrafficWeight {
+		if stack.desiredTrafficWeight > stack.actualTrafficWeight && !stack.TrafficForward() {
+			// Stacks configured to forward traffic to another cluster
+			// don't have a deployment of their own here, so replica
+			// based readiness/prescaling checks don't apply to them
+			// (see stack.IsReady() for the same exception).
 			var desiredReplicas = stack.deploymentReplicas
 			if stack.prescalingActive {
 				desiredReplicas = stack.prescalingReplicas

pkg/core/traffic_test.go

@@ -874,6 +874,21 @@ func TestTrafficSwitchPrescaling(t *testing.T) {
 			},
 			expectedError: "stacks not ready: foo-v1, foo-v3",
 		},
+		{
+			name: "forwarded stacks are not subject to prescaling readiness checks",
+			stacks: map[types.UID]*StackContainer{
+				"foo-v1": testStack("foo-v1").traffic(0, 100).ready(3).stack(),
+				"foo-v2": testStack("foo-v2").traffic(100, 0).trafficForward().stack(),
+			},
+			expectedDesiredWeights: map[string]float64{
+				"foo-v1": 0,
+				"foo-v2": 100,
+			},
+			expectedActualWeights: map[string]float64{
+				"foo-v1": 0,
+				"foo-v2": 100,
+			},
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			c := StackSetContainer{