fix: support Kubernetes 1.21 by handling missing availableReplicas field (#950)

The operator claims to support Kubernetes 1.21+ but fails to set
RisingWave clusters to running=true on K8s 1.21.

Root cause: The availableReplicas field was added to StatefulSet
in Kubernetes 1.22 as an alpha feature (KEP-2599). On K8s 1.21,
this field doesn't exist and defaults to 0, causing readiness checks
to always fail.

Solution: Fall back to readyReplicas (available since K8s 1.9) when
availableReplicas is not available. This ensures backward compatibility
with K8s 1.21 while maintaining optimal behavior on K8s 1.22+.

Changes:
- pkg/utils/apps.go: Add fallback logic in IsStatefulSetRolledOut()
- pkg/utils/apps.go: Add fallback logic in IsAdvancedStatefulSetRolledOut()

Tested on Kubernetes 1.21.13 - clusters now correctly show running=true.

Fixes: #949

Author: ZhicongZheng

pkg/utils/apps.go

@@ -73,7 +73,15 @@ func IsStatefulSetRolledOut(statefulSet *appsv1.StatefulSet) bool {
 		return false
 	}
 
-	if statefulSet.Status.AvailableReplicas < statefulSet.Status.UpdatedReplicas {
+	// Use availableReplicas if available (K8s 1.22+), otherwise fall back to readyReplicas.
+	// The availableReplicas field was added in K8s 1.22 alpha (KEP-2599).
+	// For backward compatibility with K8s 1.21, we check readyReplicas instead.
+	readyCount := statefulSet.Status.AvailableReplicas
+	if readyCount == 0 && statefulSet.Status.ReadyReplicas > 0 {
+		// availableReplicas is not available (K8s 1.21), use readyReplicas
+		readyCount = statefulSet.Status.ReadyReplicas
+	}
+	if readyCount < statefulSet.Status.UpdatedReplicas {
 		return false
 	}
 
@@ -135,7 +143,14 @@ func IsAdvancedStatefulSetRolledOut(statefulSet *kruiseappsv1beta1.StatefulSet)
 		return false
 	}
 
-	if statefulSet.Status.AvailableReplicas < statefulSet.Status.UpdatedReplicas {
+	// Use availableReplicas if available, otherwise fall back to readyReplicas.
+	// This ensures compatibility with different OpenKruise versions.
+	readyCount := statefulSet.Status.AvailableReplicas
+	if readyCount == 0 && statefulSet.Status.ReadyReplicas > 0 {
+		// availableReplicas is not available, use readyReplicas
+		readyCount = statefulSet.Status.ReadyReplicas
+	}
+	if readyCount < statefulSet.Status.UpdatedReplicas {
 		return false
 	}