// managedcluster_controller.go

...
func (r *ManagedClusterReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
    ...
    return r.reconcileUpdate(ctx, managedCluster)
}

func (r *ManagedClusterReconciler) reconcileUpdate(ctx context.Context, mc *hmc.ManagedCluster) (_ ctrl.Result, err error) {
...

    if controllerutil.AddFinalizer(mc, hmc.ManagedClusterFinalizer) {
        if err := r.Client.Update(ctx, mc); err != nil {
            return ctrl.Result{}, fmt.Errorf("failed to update managedCluster %s/%s: %w", mc.Namespace, mc.Name, err)
        }
        return ctrl.Result{}, nil
    }
...