Fix runtime admission flaky test due to race condition

[jesusha123]

What type of PR is this?

/kind flake

What this PR does / why we need it:

This change fixes a problem in the RuntimeClass admission plugin rejecting pods with a RuntimeClass not found message caused by a race condition.

Which issue(s) this PR fixes:

Fixes #90653

Special notes for your reviewer:

RuntimeClass admission plugin uses a cache to determine if a pod should be admitted. This cache is updated when a RuntimeClass is created in an informer thread.

The cache updating thread runs in a different thread than the admission validation. If a RuntimeClass is created, and a pod with a RuntimeClass spec is immediately created afterwards, there's a window of time where the cache hasn't been updated yet, so the plugin just throws a RuntimeClass not found message and error out.

With this change, if the cache lookup fails, we do a live lookup instead of waiting for the cache to be updated.

Unit tests were added that cover the exported methods ValidationInitialization and Admit, which are used in the modified code.

I was also able to reproduce this problem consistently by adding a delay in the informer updating thread, in method sharedIndexInformer.HandleDeltas, and confirmed this change fixes the reported problem.

Does this PR introduce a user-facing change?:

NONE

/cc @liggitt

[k8s-ci-robot]

Hi @jherrera123. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[liggitt]

/sig node
/priority important-soon
/ok-to-test

[jesusha123]

/retest

[liggitt]

see the verify errors that need fixing in https://prow.k8s.io/view/gcs/kubernetes-jenkins/pr-logs/pull/91315/pull-kubernetes-verify/1263304978068410368

[jesusha123]

/retest

[liggitt]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jherrera123, liggitt

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• plugin/pkg/admission/runtimeclass/OWNERS [liggitt]

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[jesusha123]

/retest