What happened

When setting/changing spec.volumeClaimTemplates[*].spec.volumeAttributesClassName for ClickHouse Keeper, the operator attempts to update the Keeper StatefulSet spec.volumeClaimTemplates, which is immutable in Kubernetes.

This fails with:
StatefulSet.apps "keeper-primary-0-0" is invalid: spec: Forbidden: updates to statefulset spec for fields other than ... are forbidden

After that, the operator switches from Update to Recreate, and the reconciliation panics with a nil pointer dereference originating from WaitHostStatefulSetReady poller.

Expected behavior

• Operator should NOT attempt to update StatefulSet.spec.volumeClaimTemplates just because volumeAttributesClassName changed.
• Instead, it should reconcile the existing PVC(s) directly (which is allowed for volumeAttributesClassName) and leave StatefulSet VCT untouched.
• No panic during delete/recreate flows.

Actual behavior

• Update fails due to forbidden StatefulSet spec changes.
• Operator falls back to recreate.
• Operator panics (nil pointer dereference) while waiting for StatefulSet readiness.

Reproduction steps (high level)

1. Deploy operator and a ClickHouseKeeperInstallation that creates a Keeper StatefulSet.
2. Change spec.volumeClaimTemplates[0].spec.volumeAttributesClassName (example: gp3-medium-throughput).
3. Observe operator reconciliation.

Relevant logs (from 2025-12-15)

E1215 13:20:22.818837 ... doUpdateStatefulSet(): ... StatefulSet update failed. err:
StatefulSet.apps "keeper-primary-0-0" is invalid: spec: Forbidden: updates to statefulset spec for fields other than 'replicas', 'ordinals', 'template', 'updateStrategy', 'persistentVolumeClaimRetentionPolicy' and 'minReadySeconds' are forbidden

... diff includes:
'.VolumeClaimTemplates[0].Spec.VolumeAttributesClassName' = '"gp3-medium-throughput"'

I1215 13:20:22.819669 ... switch from Update to Recreate

2025-12-15T13:20:22Z INFO Observed a panic in reconciler: runtime error: invalid memory address or nil pointer dereference
...
github.com/altinity/clickhouse-operator/pkg/controller/common/poller/domain.(*HostObjectsPoller).WaitHostStatefulSetReady.func1
pkg/controller/common/poller/domain/poller-host-objects.go:98

Why this looks like an operator bug

The operator already has PVC-level reconciliation for VolumeAttributesClassName:

• pkg/controller/common/storage/storage-reconciler.go calls reconcileVolumeAttributeClass() which sets pvc.Spec.VolumeAttributesClassName when specified.

So the operator can support the change by patching the PVC, without modifying the StatefulSet’s volumeClaimTemplates.

Proposed fix

1. In StatefulSet update logic: do not attempt to update StatefulSet.Spec.VolumeClaimTemplates when only PVC-reconcilable fields changed (at least volumeAttributesClassName, plus labels/resources if applicable). Keep the current StatefulSet’s VCT as-is.

2. Fix nil deref in poller:
   WaitHostStatefulSetReady() has a second poll callback that doesn’t check sts == nil before calling k8s.IsStatefulSetReady(sts).