KEP-2876: Expose existing object state to CEL CRD validation rules.

[benluddy]

Updates the proposal with a mechanism for accessing the existing object state from CEL expressions when validating updates.

/sig api-machinery

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[benluddy]

/assign @jpbetz

[liggitt]

how disruptive to the current openapi validation code is it to locate/correlate old array items in lists with x-kubernetes-list-type: set or x-kubernetes-list-type: map? would oldSelf be set to the item at the same index in the old object or the item with the same key?

this sort of gets into the same weeds the CRD immutability KEP got into in https://github.com/kubernetes/enhancements/blob/master/keps/sig-api-machinery/1101-immutable-fields/README.md#openapi-extension-x-kubernetes-immutable-keys

[jpbetz]

For x-kubernetes-list-type: map: We can correlate using map key.

x-kubernetes-list-type: set: I don't think we need to correlate since "set applies to lists that include only scalar elements" all validation can be declared one the parent (the entire set) and there is no need correlate the individual elements of the set and allow for per element rules. E.g. if you want to prevent the set to be append only, the rule check that none of the existing elements are removed is defined on the entire set.

[liggitt]

/lgtm

[liggitt]

/ok-to-test

[deads2k]

Thanks for the updates, this looks good.

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: benluddy, deads2k

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