advanced deployment scale down old unhealthy pods firstly

Signed-off-by: mingzhou.swx <mingzhou.swx@alibaba-inc.com>

pkg/controller/deployment/sync.go

@@ -318,6 +318,27 @@ func (dc *DeploymentController) scale(ctx context.Context, deployment *apps.Depl
 			scalingOperation = "down"
 		}
 
+		// Scale down the unhealthy replicas in old replica sets firstly to avoid some bad cases.
+		// For example:
+		//       _______________________________________________________________________
+		//       | ReplicaSet    |       oldRS-1      |   oldRS-2            |   newRS  |
+		//       | --------------| -------------------|----------------------|----------|
+		//       | Replicas      |  5 healthy Pods    |  1 unhealthy Pods 1  |    0     |
+		//       ------------------------------------------------------------------------
+		// If we want to scale down these replica sets from 6 to 5, we expect to scale down the oldRS-2
+		// from 1 to 0 firstly.
+		var err error
+		var cleanupCount int32
+		if deploymentReplicasToAdd < 0 {
+			oldRSs, cleanupCount, err = dc.cleanupUnhealthyReplicas(ctx, oldRSs, deployment, -deploymentReplicasToAdd)
+			if err != nil {
+				return err
+			}
+			klog.V(4).Infof("Cleaned up unhealthy replicas from old RSes by %d during scaling", cleanupCount)
+			deploymentReplicasToAdd += cleanupCount
+			allRSs = deploymentutil.FilterActiveReplicaSets(append(oldRSs, newRS))
+		}
+
 		// Iterate over all active replica sets and estimate proportions for each of them.
 		// The absolute value of deploymentReplicasAdded should never exceed the absolute
 		// value of deploymentReplicasToAdd.