fix: Handle placeholder instance decommission safely in AWS ASGs

This merge resolves the issue where the Kubernetes Cluster Autoscaler incorrectly decommissions actual instances instead of placeholders within AWS ASGs. The fix ensures that only placeholders are considered for scaling down when recent scaling activities fail, thereby preventing the accidental removal of active nodes. Enhanced unit tests and checks are included to ensure robustness.

Fixes #5829

cluster-autoscaler/cloudprovider/aws/auto_scaling_groups.go

@@ -308,14 +308,37 @@ func (m *asgCache) DeleteInstances(instances []*AwsInstanceRef) error {
 		}
 	}
 
-	for _, instance := range instances {
-		// check if the instance is a placeholder - a requested instance that was never created by the node group
-		// if it is, just decrease the size of the node group, as there's no specific instance we can remove
-		if m.isPlaceholderInstance(instance) {
-			klog.V(4).Infof("instance %s is detected as a placeholder, decreasing ASG requested size instead "+
-				"of deleting instance", instance.Name)
-			m.decreaseAsgSizeByOneNoLock(commonAsg)
-		} else {
+    // Initialize the success flag for recent scaling activity.
+    var recentScalingActivitySuccess = false
+    var err error
+
+    // Check if there are any placeholder instances in the list.
+    if m.HasPlaceholder(instances) {
+        // Log the check for placeholders in the ASG.
+        klog.V(4).Infof("Detected a placeholder instance, checking recent scaling activity for ASG %s", commonAsg.Name)
+
+        // Retrieve the most recent scaling activity to determine its success state.
+        recentScalingActivitySuccess, err = m.getMostRecentScalingActivity(commonAsg)
+
+        // Handle errors from retrieving scaling activity.
+        if err != nil {
+            // Log the error if the scaling activity check fails and return the error.
+            klog.Errorf("Error retrieving scaling activity for ASG %s: %v", commonAsg.Name, err)
+            return err  // Return error to prevent further processing with uncertain state information.
+        }
+    }
+
+    for _, instance := range instances {
+        if m.isPlaceholderInstance(instance) {
+            if !recentScalingActivitySuccess {
+                // Log that scaling down due to unsuccessful recent activity
+                klog.V(4).Infof("Recent scaling activity unsuccessful; reducing ASG size for placeholder %s in ASG %s", instance.Name, commonAsg.Name)
+                m.decreaseAsgSizeByOneNoLock(commonAsg)
+                continue  // Continue to the next iteration after handling placeholder
+            }
+            klog.V(4).Infof("Skipping actions for placeholder %s in ASG %s due to successful recent scaling", instance.Name, commonAsg.Name)
+            continue
+        } else {
 			// check if the instance is already terminating - if it is, don't bother terminating again
 			// as doing so causes unnecessary API calls and can cause the curSize cached value to decrement
 			// unnecessarily.
@@ -352,6 +375,45 @@ func (m *asgCache) DeleteInstances(instances []*AwsInstanceRef) error {
 	return nil
 }
 
+func (m *asgCache) getMostRecentScalingActivity(asg *asg) (bool, error) {
+    input := &autoscaling.DescribeScalingActivitiesInput{
+        AutoScalingGroupName: aws.String(asg.Name),
+        MaxRecords:           aws.Int64(1),
+    }
+
+    var response *autoscaling.DescribeScalingActivitiesOutput
+    var err error
+    attempts := 3
+
+    for i := 0; i < attempts; i++ {
+        response, err = m.awsService.DescribeScalingActivities(input)
+        if err == nil {
+            break
+        }
+        klog.V(2).Infof("Failed to describe scaling activities, attempt %d/%d: %v", i+1, attempts, err)
+        time.Sleep(time.Second * 2)
+    }
+
+    if err != nil {
+        klog.Errorf("All attempts failed for DescribeScalingActivities: %v", err)
+        return false, err
+    }
+
+    if len(response.Activities) == 0 {
+        klog.Info("No scaling activities found for ASG:", asg.Name)
+        return false, nil
+    }
+
+    lastActivity := response.Activities[0]
+    if *lastActivity.StatusCode == "Successful" {
+        klog.Infof("Most recent scaling activity for ASG %s was successful", asg.Name)
+        return true, nil
+    } else {
+        klog.Infof("Most recent scaling activity for ASG %s was not successful: %s", asg.Name, *lastActivity.StatusMessage)
+        return false, fmt.Errorf("most recent scaling activity for ASG %s was not successful: %s", asg.Name, *lastActivity.StatusMessage)
+    }
+}
+
 // isPlaceholderInstance checks if the given instance is only a placeholder
 func (m *asgCache) isPlaceholderInstance(instance *AwsInstanceRef) bool {
 	return strings.HasPrefix(instance.Name, placeholderInstanceNamePrefix)
@@ -620,7 +682,19 @@ func (m *asgCache) buildInstanceRefFromAWS(instance *autoscaling.Instance) AwsIn
 	}
 }
 
+func (m *asgCache) HasPlaceholder(instances []*AwsInstanceRef) bool {
+	// check if there's a placeholder instance and verify most recent scaling activity before terminating it
+	var containsPlaceholder bool = false
+	for _, instance := range instances {
+		if strings.HasPrefix(instance.Name, placeholderInstanceNamePrefix) {
+			containsPlaceholder = true
+			break
+		}
+	}
+	return containsPlaceholder
+}
+
 // Cleanup closes the channel to signal the go routine to stop that is handling the cache
 func (m *asgCache) Cleanup() {
 	close(m.interrupt)
-}
+}

cluster-autoscaler/cloudprovider/aws/auto_scaling_groups_test.go

@@ -166,3 +166,54 @@ func TestCreatePlaceholders(t *testing.T) {
 		})
 	}
 }
+
+func TestGetMostRecentScalingActivity(t *testing.T) {
+    a := &autoScalingMock{}
+    asgCache := &asgCache{
+        awsService: &awsWrapper{
+            autoScalingI: a,
+        },
+    }
+
+    asg := &asg{AwsRef: AwsRef{Name: "test-asg"}}
+
+    // Test case: Successful scaling activity
+    a.On("DescribeScalingActivities", &autoscaling.DescribeScalingActivitiesInput{
+        AutoScalingGroupName: aws.String("test-asg"),
+        MaxRecords:           aws.Int64(1),
+    }).Return(&autoscaling.DescribeScalingActivitiesOutput{
+        Activities: []*autoscaling.Activity{{StatusCode: aws.String("Successful")}},
+    }, nil).Once()
+
+    success, err := asgCache.getMostRecentScalingActivity(asg)
+    assert.NoError(t, err)
+    assert.True(t, success)
+
+    // Test case: Failed scaling activity
+    a.On("DescribeScalingActivities", &autoscaling.DescribeScalingActivitiesInput{
+        AutoScalingGroupName: aws.String("test-asg"),
+        MaxRecords:           aws.Int64(1),
+    }).Return(&autoscaling.DescribeScalingActivitiesOutput{
+        Activities: []*autoscaling.Activity{{StatusCode: aws.String("Failed")}},
+    }, nil).Once()
+
+    success, err = asgCache.getMostRecentScalingActivity(asg)
+    assert.NoError(t, err)
+    assert.False(t, success)
+
+    // Test case: No activities found
+    a.On("DescribeScalingActivities", &autoscaling.DescribeScalingActivitiesInput{
+        AutoScalingGroupName: aws.String("test-asg"),
+        MaxRecords:           aws.Int64(1),
+    }).Return(&autoscaling.DescribeScalingActivitiesOutput{
+        Activities: []*autoscaling.Activity{},
+    }, nil).Once()
+
+    success, err = asgCache.getMostRecentScalingActivity(asg)
+    assert.NoError(t, err)
+    assert.False(t, success)
+
+    // Verify that all expectations are met
+    a.AssertExpectations(t)
+}
+

cluster-autoscaler/cloudprovider/aws/aws_cloud_provider_test.go

@@ -17,8 +17,6 @@ limitations under the License.
 package aws
 
 import (
-	"testing"
-
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/mock"
 	apiv1 "k8s.io/api/core/v1"
@@ -27,6 +25,8 @@ import (
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/aws/aws-sdk-go/aws"
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/aws/aws-sdk-go/service/autoscaling"
 	"k8s.io/autoscaler/cluster-autoscaler/config"
+	"testing"
+	"time"
 )
 
 var testAwsManager = &AwsManager{
@@ -589,25 +589,44 @@ func TestDeleteNodesWithPlaceholder(t *testing.T) {
 		},
 	).Return(&autoscaling.DescribeScalingActivitiesOutput{}, nil)
 
+    a.On("DescribeScalingActivities",
+        &autoscaling.DescribeScalingActivitiesInput{
+            AutoScalingGroupName: aws.String("test-asg"),
+            MaxRecords:           aws.Int64(1),
+        },
+    ).Return(
+        &autoscaling.DescribeScalingActivitiesOutput{
+            Activities: []*autoscaling.Activity{
+                {
+                    StatusCode: aws.String("Successful"),
+                    StartTime:  aws.Time(time.Now().Add(-10 * time.Minute)),
+                },
+                {
+                    StatusCode: aws.String("Failed"),
+                    StartTime:  aws.Time(time.Now().Add(-30 * time.Minute)),
+                },
+            },
+        }, nil)
+
 	provider.Refresh()
 
 	initialSize, err := asgs[0].TargetSize()
 	assert.NoError(t, err)
 	assert.Equal(t, 2, initialSize)
-
 	node := &apiv1.Node{
 		Spec: apiv1.NodeSpec{
 			ProviderID: "aws:///us-east-1a/i-placeholder-test-asg-1",
 		},
 	}
 	err = asgs[0].DeleteNodes([]*apiv1.Node{node})
 	assert.NoError(t, err)
-	a.AssertNumberOfCalls(t, "SetDesiredCapacity", 1)
-	a.AssertNumberOfCalls(t, "DescribeAutoScalingGroupsPages", 1)
+        a.AssertNumberOfCalls(t, "SetDesiredCapacity", 1)
+        a.AssertNumberOfCalls(t, "DescribeAutoScalingGroupsPages", 1)
+
+        newSize, err := asgs[0].TargetSize()
+        assert.NoError(t, err)
+        assert.Equal(t, 1, newSize)
 
-	newSize, err := asgs[0].TargetSize()
-	assert.NoError(t, err)
-	assert.Equal(t, 1, newSize)
 }
 
 func TestDeleteNodesAfterMultipleRefreshes(t *testing.T) {
@@ -738,4 +757,4 @@ func TestHasInstance(t *testing.T) {
 	present, err = provider.HasInstance(node4)
 	assert.NoError(t, err)
 	assert.False(t, present)
-}
+}