docs: egressgw: document incompatibility with Clustermesh

[julianwiedmann]

The current way of subscribing to node events is not compatible with Clustermesh (see #25794). Reflect this in the documentation.

[julianwiedmann]

@YutaroHayakawa do you know if there is any way of determining whether a node is set up for Clustermesh? So that we could at least give users a warning similar to

cilium/pkg/egressgateway/manager.go

Lines 212 to 215 in 0da3f7e

	if dcfg.EnableL7Proxy {
	log.WithField(logfields.URL, "https://github.com/cilium/cilium/issues/19642").
	Warningf("both egress gateway and L7 proxy (--%s) are enabled. This is currently not fully supported: "+
	"if the same endpoint is selected both by an egress gateway and a L7 policy, endpoint traffic will not go through egress gateway.", option.EnableL7Proxy)

[learnitall]

One nit, but otherwise LGTM!

[YutaroHayakawa]

@YutaroHayakawa do you know if there is any way of determining whether a node is set up for Clustermesh? So that we could at least give users a warning similar to

Umm, I guess we can depend on ClusterMesh object as optional and check if it is nil or not. Like

type EgressGatewayParams struct {
  cell.In

  cm *ClusterMesh `optional:"true"`
}

func NewEgressGateway(params EgressGatewayParams) (EgressGateway, error) {
  if params.cm != nil {
    return nil, fmt.Errorf("Egress Gateway is incompatible with ClusterMesh")
  }
  ...
}

@giorio94 Do you have any other good ideas?

[julianwiedmann]

@YutaroHayakawa do you know if there is any way of determining whether a node is set up for Clustermesh? So that we could at least give users a warning similar to

Umm, I guess we can depend on ClusterMesh object as optional and check if it is nil or not. Like

type EgressGatewayParams struct {
  cell.In

  cm *ClusterMesh `optional:"true"`
}

func NewEgressGateway(params EgressGatewayParams) (EgressGateway, error) {
  if params.cm != nil {
    return nil, fmt.Errorf("Egress Gateway is incompatible with ClusterMesh")
  }
  ...
}

@giorio94 Do you have any other good ideas?

Let's merge the docs change as-is then, and add the check once @giorio94 had a look.

[learnitall]

LGTM, thank you!

[julianwiedmann]

@youngnick I think we don't need the full CI here, but feel free to disagree 👍

[youngnick]

I agree, merging.

[giorio94]

@YutaroHayakawa do you know if there is any way of determining whether a node is set up for Clustermesh? So that we could at least give users a warning similar to

Umm, I guess we can depend on ClusterMesh object as optional and check if it is nil or not. Like

type EgressGatewayParams struct {
  cell.In

  cm *ClusterMesh `optional:"true"`
}

func NewEgressGateway(params EgressGatewayParams) (EgressGateway, error) {
  if params.cm != nil {
    return nil, fmt.Errorf("Egress Gateway is incompatible with ClusterMesh")
  }
  ...
}

@giorio94 Do you have any other good ideas?

Your proposal makes sense to me. The optional tag is not strictly required, as the ClusterMesh object is always provided, although it is nil if not initialized.