Add spec.gatewayClassName selectable field to Gateway

[pmalek]

What would you like to be added:

Kubernetes 1.31 has promoted CRD selectable field to beta and enabled it by default. 1.32 has promoted it to GA.

• Official docs: https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/#custom-resource-field-selectors
• KEP KEP: Custom Resource Field Selectors kubernetes/enhancements#4359

This proposal is about adding a selectable field (1 or more if the community finds that there's appetite for more) to Gateway, specifically: spec.gatewayClassName.

This proposal can be followed up by separate issues for other types / fields if deemed worthy of doing.

Why this is needed:

This will allow listing Gateways that match a particular GatewayClass more efficient (useful when used with controller-runtime's EnqueueRequestsFromMapFunc

	var gateways gatewayv1.GatewayList
	if err := r.Client.List(ctx, &gateways); err != nil {
		// handle
		return
	}
        var recs []reconcile.Request
	for _, gateway := range gateways.Items {
		if gateway.Spec.GatewayClassName == gatewayv1.ObjectName(gatewayClass.Name) {
			recs = append(recs, reconcile.Request{
				NamespacedName: types.NamespacedName{
					Namespace: gateway.Namespace,
					Name:      gateway.Name,
				},
			})
		}
	}

Becomes:

	var gateways gatewayv1.GatewayList
	if err := r.Client.List(ctx, gateways,
		client.MatchingFieldsSelector{
			Selector: fields.OneTermEqualSelector("spec.gatewayClassName",gatewayClass.Name),
		},
	); err != nil {
		// handle
		return
	}

        recs := ([]reconcile.Request, 0, len(gateways.Items))
	for _, gateway := range gateways.Items {
		recs = append(recs, reconcile.Request{
			NamespacedName: types.NamespacedName{
				Namespace: gateway.Namespace,
				Name:      gateway.Name,
			},
		})
	}

[pmalek]

Just tested selectable fields on 1.29 (where they were not supported) and the CRD won't apply:

make crd
kubectl kustomize config/crd | kubectl apply -f -
customresourcedefinition.apiextensions.k8s.io/gatewayclasses.gateway.networking.k8s.io created
customresourcedefinition.apiextensions.k8s.io/grpcroutes.gateway.networking.k8s.io created
customresourcedefinition.apiextensions.k8s.io/httproutes.gateway.networking.k8s.io created
customresourcedefinition.apiextensions.k8s.io/referencegrants.gateway.networking.k8s.io created
Error from server (BadRequest): error when creating "STDIN": CustomResourceDefinition in version "v1" cannot be handled as a CustomResourceDefinition: strict decoding error: unknown field "spec.versions[0].selectableFields"

I assume this warrants that we need to wait until k8s 1.35 is out to introduce this (given the support policy at https://gateway-api.sigs.k8s.io/concepts/versioning/#supported-versions).

[mlavacca]

Thanks for creating this issue, @pmalek!

I think this would be a great improvement, but as you already said, it's something we necessarily need to put off because of our support policy. We'll have to wait for at least Kubernetes 1.35 to leverage this feature.

[howardjohn]

Also tracked in #2862

[pmalek]

/lifecycle frozen