Add finalizer to worker pods to ensure min replica

[YichengWang12]

Why are these changes needed?

Currently all worker pods can be deleted without protection, regardless of minReplica. This change adds a protection of minReplica using finalizer.

Related issue number

Fix #1740

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(

[andrewsykim]

if !controllerutil.ContainsFinalizer(&workerPod, utils.WorkerDeletionFinalizer) {
    continue
}

if int32(len(workerPods.Items)) <= *worker.MinReplicas {
    // If the number of worker Pods is less than or equal to the minimum number of replicas, we should not delete the 
    Pod.
    logger.Info("reconcilePods", "worker Pod", workerPod.Name, "shouldDelete", false, "reason", "Number of worker Pods is less than or equal to the minimum number of replicas")
} else {
    controllerutil.RemoveFinalizer(&workerPod, utils.WorkerDeletionFinalizer)
    if err := r.Update(ctx, &workerPod); err != nil {
  	return err
    }
}

[andrewsykim]

I'm not sure how much this check helps.

Even with a finalizer configured, if a pod has been deleted with a deletion timestamp it is already terminating. We will keep the Pod object around but the worker will not actually be available.

[YichengWang12]

That's true. Would admission webhook be better?

[andrewsykim]

I think we are trying to avoid admission webhooks as much as possible. @kevin85421 thoughts?