fix(cluster-autoscaler): prevent panic in SimulateNodeRemoval by handling missing node info

[kincoy]

What type of PR is this?

/kind bug

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What this PR does / why we need it:

This PR fixes a potential nil dereference issue in SimulateNodeRemoval when a node is missing from the clusterSnapshot.

Previously, if clusterSnapshot.GetNodeInfo failed, the function would continue and potentially panic when accessing nodeInfo.Pods().

This fix introduces:

• A new UnremovableReason: NoNodeInfo, used to mark such nodes as unremovable.
• An early return from SimulateNodeRemoval when the node is missing.
• A defensive placeholder node (with only .Name set) to maintain observability and event compatibility.
• A dedicated unit test case to verify this scenario is correctly handled.

---

Which issue(s) this PR fixes:

N/A

---

Special notes for your reviewer:

This PR fixes a potential nil dereference in SimulateNodeRemoval when a node is missing from the cluster snapshot.
It adds a new UnremovableReason (NoNodeInfo) to capture this edge case, and add the test coverage.

---

Does this PR introduce a user-facing change?

NONE

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[k8s-ci-robot]

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[elmiko]

thank you for the PR and adding the test too.

the changes here look generally good to me, but i am not overly familiar with the simulator code. in specific, the returns for the failure condition.

would it be possible to add a unit test for the other return value as well? (i see the test for the unremovable node with the NoNodeInfo, but can we also test for UnexpectedError?)

[kincoy]

thank you for the PR and adding the test too.

the changes here look generally good to me, but i am not overly familiar with the simulator code. in specific, the returns for the failure condition.

would it be possible to add a unit test for the other return value as well? (i see the test for the unremovable node with the NoNodeInfo, but can we also test for UnexpectedError?)

Thanks! I looked into GetNodeInfo — aside from ErrNodeNotFound, other errors only happen when draEnabled is true and WrapSchedulerNodeInfo fails, which is rare and hard to simulate without artificial mocks.

If you have a clean way to test this case, I’m happy to give it a try!

[elmiko]

Thanks! I looked into GetNodeInfo — aside from ErrNodeNotFound, other errors only happen when draEnabled is true and WrapSchedulerNodeInfo fails, which is rare and hard to simulate without artificial mocks.

thanks for investigating, i was worried it might take a complicated mock to make it work. i don't think it's worth the effort to create a test with a mock just for the error condition.

/lgtm

would be good to get another review from a maintainer

[jackfrancis]

I have a slight preference for doing something like this

		unremovableReason := UnexpectedError
		if errors.Is(err, clustersnapshot.ErrNodeNotFound) {
			unremovableReason = NoNodeInfo
		}
		unremovableNode := &UnremovableNode{Node: &apiv1.Node{ObjectMeta: metav1.ObjectMeta{Name: nodeName}}, Reason: unremovableReason}
		return nil, unremovableNode

1. to avoid the multiple return statements
2. to actuate what the the if condition is really trying to determine
3. to avoid wrapping the node object in an "opinonated" variable name like "ghostNode", which may indicate something more interesting going on than actually is

[kincoy]

Makes sense — updated as you suggested. Also applied consistent naming in both implementation and tests.Thanks for the helpful review!

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[kincoy]

Friendly ping @jackfrancis — this PR has been idle for a while. Would appreciate a review when convenient 🙏