Add ability to choose between IP Aliases and Routes

[danawillow]

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Description

(API-side changes in https://container.googleapis.com/$discovery/rest?version=v1beta1, not yet seeing on https://cloud.google.com/kubernetes-engine/docs/reference/rest/v1beta1/projects.locations.clusters)

In addition to useIpAliases, IPAllocationPolicy now supports useRoutes. We got rid of use_ip_aliases in 3.0.0 because it was the only possibility when a user specified an ip_allocation_policy block. Whoever implements this should think about the UX of the change and whether it makes sense to add the two booleans vs an enum, and whether these should be inside or outside the ip_allocation_policy block.

New or Affected Resource(s)

• google_container_cluster

Potential Terraform Configuration

# Propose what you think the configuration to take advantage of this feature should look like.
# We may not use it verbatim, but it's helpful in understanding your intent.

References

• b/150889533 (there's some design ideas in here that are worth thinking about)
• Make VPC-native clusters the default in google_container_cluster #5842

[rileykarson]

fwiw, copy-paste of my thoughts from inside that bug:

I've been trying to think of ways to flip the default to VPC-native in Terraform for a little while, since newer GKE features tend to rely on it. I filed #5842 to capture my thoughts, but I think it'd be better to conform to the API's flip instead.

The API will return an ip_allocation_policy block on every response after this change, including one of use_ip_aliases or use_routes set to true, right? So if I send both set to false, one of them will come back true.

Right now, Terraform assumes that ip_allocation_policy being set means a cluster is VPC-native, and unset means it's routes-based. However, I see that allowRouteOverlap in beta can also apply to both types of clusters. It's probably most clear for our users if we transform the schema clientside a bit to collapse the pair of use_ booleans to an enum ("networking_mode" with "VPC NATIVE" and "ROUTES" as options, probably) at the top level, and to make allow_routes_overlap a top-level field as well.

[ghost]

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