Repository: kubernetes
Issue: ReplicaSet controller may create extra Pods when expectations expire during informer delays
Labels: kind/bug,sig/apps,needs-triage

### What happened?

The ReplicaSet controller may create more Pods than desired under the following conditions:

1. ReplicaSet controller creates N Pods and sets expectations (+N)
2. Due to network issues or high latency, PodInformer hasn't received the Pod creation events
3. After 5 minutes, the expectations expire (isExpired() == true)
4. The `SatisfiedExpectations` function returns `true` immediately when expectations expire, without checking if they are actually fulfilled
5. This causes the controller to create additional Pods, even though the previously created Pods may still exist

Code path ：https://github.com/kubernetes/kubernetes/blob/f6f06806cc43ed9f7eb2f68368c90a8239884118/pkg/controller/controller_utils.go#L193

```
func (r *ControllerExpectations) SatisfiedExpectations(logger klog.Logger, controllerKey string) bool {
	if exp, exists, err := r.GetExpectations(controllerKey); exists {
		if exp.Fulfilled() {
			logger.V(4).Info("Controller expectations fulfilled", "expectations", exp)
			return true
		} else if exp.isExpired() {
			logger.V(4).Info("Controller expectations expired", "expectations", exp)
			return true  // There is an issue here. !!! 
		} else {
			logger.V(4).Info("Controller still waiting on expectations", "expectations", exp)
			return false
		}
	} else if err != nil {
		logger.V(2).Info("Error encountered while checking expectations, forcing sync", "err", err)
	} else {
		// When a new controller is created, it doesn't have expectations.
		// When it doesn't see expected watch events for > TTL, the expectations expire.
		//	- In this case it wakes up, creates/deletes controllees, and sets expectations again.
		// When it has satisfied expectations and no controllees need to be created/destroyed > TTL, the expectations expire.
		//	- In this case it continues without setting expectations till it needs to create/delete controllees.
		logger.V(4).Info("Controller either never recorded expectations, or the ttl expired", "controller", controllerKey)
	}
	// Trigger a sync if we either encountered and error (which shouldn't happen since we're
	// getting from local store) or this controller hasn't established expectations.
	return true
}
```

If the PodInformer is delayed, manageReplicas will create too many Pods.
https://github.com/kubernetes/kubernetes/blob/f6f06806cc43ed9f7eb2f68368c90a8239884118/pkg/controller/replicaset/replica_set.go#L572

```
	if diff < 0 {
		diff *= -1
		if diff > rsc.burstReplicas {
			diff = rsc.burstReplicas
		}
		rsc.expectations.ExpectCreations(logger, rsKey, diff)
		successfulCreations, err := slowStartBatch(diff, controller.SlowStartInitialBatchSize, func() error {
			err := rsc.podControl.CreatePods(ctx, rs.Namespace, &rs.Spec.Template, rs, metav1.NewControllerRef(rs, rsc.GroupVersionKind))
			if err != nil {
				if apierrors.HasStatusCause(err, v1.NamespaceTerminatingCause) {
					return nil
				}
			}
			return err
		})

		if skippedPods := diff - successfulCreations; skippedPods > 0 {
			logger.V(2).Info("Slow-start failure. Skipping creation of pods, decrementing expectations", "podsSkipped", skippedPods, "kind", rsc.Kind, "replicaSet", klog.KObj(rs))
			for i := 0; i < skippedPods; i++ {
				rsc.expectations.CreationObserved(logger, rsKey)
			}
		}
		return err
	} 
``` 


### What did you expect to happen?

When controller expectations expire, the ReplicaSet controller should still ensure the actual Pod count matches the desired count, even if there are informer delays. Specifically:

1. If expectations expire but are not fulfilled (add/delete operations not observed), the controller should wait for the informer to catch up rather than immediately proceeding with new Pod creations.

2. The `SatisfiedExpectations` function should return `false` when expectations are expired but not fulfilled, preventing potential Pod over-provisioning during informer delays.

This would maintain the safety guarantee that a ReplicaSet never creates more Pods than desired, even in the presence of informer delays.

### How can we reproduce it (as minimally and precisely as possible)?

NONE

### Anything else we need to know?

_No response_

### Kubernetes version

<details>

```console
$ kubectl version
# paste output here
```

</details>


### Cloud provider

<details>

</details>


### OS version

<details>

```console
# On Linux:
$ cat /etc/os-release
# paste output here
$ uname -a
# paste output here

# On Windows:
C:\> wmic os get Caption, Version, BuildNumber, OSArchitecture
# paste output here
```

</details>


### Install tools

<details>

</details>


### Container runtime (CRI) and version (if applicable)

<details>

</details>


### Related plugins (CNI, CSI, ...) and versions (if applicable)

<details>

</details>