Distinguish PDB error separately in eviction API about kubernetes HOT 9 OPEN

In response to this:

What would you like to be added?

Currently in eviction API, when violating PDB, we'll have two common errors, see

kubernetes/pkg/registry/core/pod/storage/eviction.go

Lines 423 to 432 in eb68409

	if pdb.Status.DisruptionsAllowed < 0 {
	return errors.NewForbidden(policy.Resource("poddisruptionbudget"), pdb.Name, fmt.Errorf("pdb disruptions allowed is negative"))
	}
	if len(pdb.Status.DisruptedPods) > MaxDisruptedPodSize {
	return errors.NewForbidden(policy.Resource("poddisruptionbudget"), pdb.Name, fmt.Errorf("DisruptedPods map too big - too many evictions not confirmed by PDB controller"))
	}
	if pdb.Status.DisruptionsAllowed == 0 {
	err := errors.NewTooManyRequests("Cannot evict pod as it would violate the pod's disruption budget.", 0)
	err.ErrStatus.Details.Causes = append(err.ErrStatus.Details.Causes, metav1.StatusCause{Type: policyv1.DisruptionBudgetCause, Message: fmt.Sprintf("The disruption budget %s needs %d healthy pods and has %d currently", pdb.Name, pdb.Status.DesiredHealthy, pdb.Status.CurrentHealthy)})
	return err

We can't easily distinguish them with other forbidden or tooManyRequests errors, we do have different messages but that's not an appropriate criteria for judgement.

When detecting update conflicts, we have a conflict reason for it, maybe we can do the same thing here like A Forbidden status code with different Reasons, have no idea whether this is acceptable. but at least it's feasible because we maintain the status reasons ourself in kubernetes.

/kind api-machineary

Why is this needed?

It would be great if we can tell them when calling eviction API, like in kubernetes/enhancements#4329 (comment), this can be an important metric for users to know the reason why Pods can't be evicted, because of PDB constraints or others.

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In response to this:

/triage accepted
/help

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In response to this:

/assign
/honk

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