event.DeleteEvent struct's "DeleteStateUnknown" field is never written

[seh]

The event.DeleteEvent type has a field called "DeleteStateUnknown," documented as follows:

DeleteStateUnknown is true if the Delete event was missed but we identified the object as having been deleted.

At present, this field is never written to, and nothing in the controller-runtime library reads it either. This field entered the source back in #2, on 7 June 2018, copying the code from the kubernetes-sigs/kubebuilder project.

Per discussion in the "kubebuilder" channel in the "Kubernetes" Slack team, @pwittrock suggested that perhaps we should use this field, writing to it when appropriate, rather than removing it from the library, which would require a major version change to avoid breaking current Predicates.

Not being sure of the original intention for this field, it looks like the source/internal.EventHandler.OnDelete method might be a fitting place, to indicate that we found a cache.DeleteFinalStateUnknown object in the cache, indicating that we missed a deleting watch event and might therefore have only a stale object available.

For Reconcilers, if I understand the contract correctly, the distinction seems immaterial, as by the time they would go to inspect the object, it would be gone anyway; only its name and possibly its containing namespace would be presented to the Reconciler. However, other event handlers—and Predicates that precede them—would get a chance to see this tombstone object.

Should we populate this field? If so, where and when? Does its documentation adequately explain its meaning?

[DirectXMan12]

can we get some context from k/k? I don't remember what our intention was with that field, but the tombstone logic sits in a bit of a minefield of weird artifacts ported from k/k.

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[vincepri]

/priority awaiting-more-evidence
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[seh]

/remove-lifecycle rotten

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[vincepri]

@seh Were you able to get more information from k/k?

[seh]

No, sorry, I didn't find anything telling when I last looked into it.

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[Mitsuwa]

Does this field actually do anything? I have also noticed that it doesn't seem to populate