feat(controller): Capping the replica count during a canary deployment

USERS.md

@@ -28,3 +28,4 @@ Organizations below are **officially** using Argo Rollouts. Please send a PR wit
 1. [Twilio SendGrid](https://sendgrid.com)
 1. [Ubie](https://ubie.life/)
 1. [VISITS Technologies](https://visits.world/en)
+1. [ironSource](https://is.com)

test/e2e/canary_test.go

@@ -1,3 +1,4 @@
+//go:build e2e
 // +build e2e
 
 package e2e
@@ -456,7 +457,7 @@ spec:
 		WaitForRolloutStatus("Degraded").
 		WaitForRolloutStatus("Paused").
 		Then().
-		ExpectCanaryStablePodCount(1, 3).
+		ExpectCanaryStablePodCount(1, 2).
 		When().
 		PromoteRollout().
 		WaitForRolloutStatus("Degraded").
@@ -591,3 +592,46 @@ func (s *CanarySuite) TestCanaryDynamicStableScale() {
 		Then().
 		ExpectRevisionPodCount("1", 4)
 }
+
+func (s *CanarySuite) TestReplicasCapping() {
+	s.Given().
+		HealthyRollout(`
+apiVersion: argoproj.io/v1alpha1
+kind: Rollout
+metadata:
+  name: replicas-capping
+spec:
+  replicas: 15
+  strategy:
+    canary:
+      steps:
+      - setWeight: 1
+      - pause: {}
+      - setWeight: 50
+      - pause: {}
+  selector:
+    matchLabels:
+      app: replicas-capping
+  template:
+    metadata:
+      labels:
+        app: replicas-capping
+    spec:
+      containers:
+      - name: replicas-capping
+        image: nginx:1.19-alpine
+        resources:
+          requests:
+            memory: 16Mi
+            cpu: 1m`).
+		When().
+		UpdateSpec().
+		WaitForRolloutStatus("Paused").
+		Then().
+		ExpectCanaryStablePodCount(1, 14).
+		When().
+		PromoteRollout().
+		WaitForRolloutStatus("Paused").
+		Then().
+		ExpectCanaryStablePodCount(7, 8)
+}

utils/replicaset/canary.go

@@ -128,12 +128,35 @@ func DesiredReplicaCountsForCanary(rollout *v1alpha1.Rollout, newRS, stableRS *a
 // replicas 1 currentWeight 5 NewRS 0 stableRS 1 max unavailable 0, surge 1 - should return newRS 1 stableRS 1
 // replicas 1 currentWeight 95 NewRS 0 stableRS 1 max unavailable 0, surge 1 - should return newRS 1 stableRS 1
 // For more examples, check the CalculateReplicaCountsForBasicCanary test in canary/canary_test.go
+
+func calculateRolloutSteps(rolloutSpecReplica int, desiredWeight float64) (int32, int32) {
+	if int32(desiredWeight) == 0 {
+		desiredStableRSReplicaCount := int32(rolloutSpecReplica)
+		desiredNewRSReplicaCount := int32(0)
+		return desiredNewRSReplicaCount, desiredStableRSReplicaCount
+	} else {
+		log.Infof("Total replica count is: %d", rolloutSpecReplica)
+		desiredStableRSReplicaCount := int32(math.Ceil(float64(rolloutSpecReplica) * (1 - (float64(desiredWeight) / 100))))
+		desiredNewRSReplicaCount := int32(int32(rolloutSpecReplica) - desiredStableRSReplicaCount)
+		if (desiredNewRSReplicaCount == 0) && (desiredStableRSReplicaCount > 1) {
+			desiredNewRSReplicaCount++
+			desiredStableRSReplicaCount--
+		} else if desiredNewRSReplicaCount == 0 {
+			desiredNewRSReplicaCount++
+		}
+		return desiredNewRSReplicaCount, desiredStableRSReplicaCount
+	}
+}
+
 func CalculateReplicaCountsForBasicCanary(rollout *v1alpha1.Rollout, newRS *appsv1.ReplicaSet, stableRS *appsv1.ReplicaSet, oldRSs []*appsv1.ReplicaSet) (int32, int32) {
 	rolloutSpecReplica := defaults.GetReplicasOrDefault(rollout.Spec.Replicas)
 	_, desiredWeight := GetCanaryReplicasOrWeight(rollout)
 
-	desiredStableRSReplicaCount := int32(math.Ceil(float64(rolloutSpecReplica) * (1 - (float64(desiredWeight) / 100))))
-	desiredNewRSReplicaCount := int32(math.Ceil(float64(rolloutSpecReplica) * (float64(desiredWeight) / 100)))
+	desiredNewRSReplicaCount, desiredStableRSReplicaCount := calculateRolloutSteps(int(rolloutSpecReplica), float64(desiredWeight))
+	log.Infof("Desired Weight: %d", desiredWeight)
+	log.Infof("Spec Replica: %d", rolloutSpecReplica)
+	log.Infof("desiredStableRSReplicaCount: %d", desiredStableRSReplicaCount)
+	log.Infof("desiredNewRSReplicaCount: %d", desiredNewRSReplicaCount)
 
 	stableRSReplicaCount := int32(0)
 	newRSReplicaCount := int32(0)