Support optional vm_count for google_compute_resource_policy.group_placement_policy when collocation="COLLOCATED"

[nick-stroud]

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Terraform Version

$ terraform -v
Terraform v1.1.5
on linux_amd64

Affected Resource(s)

• google_compute_resource_policy

Terraform Configuration Files

resource "google_compute_resource_policy" "placement_policy" {
  name   = "my-placement-policy"
  region = "us-central1"
  group_placement_policy {
    collocation = "COLLOCATED"
  }
}

Expected Behavior

The resource policy should be created without a vm_count as described in the official documentation: https://cloud.google.com/compute/docs/instances/define-instance-placement#compact.

Actual Behavior

Terraform fails because vm_count is not specified.

$ terraform apply
╷
│ Error: Missing required argument
│ 
│   with google_compute_resource_policy.placement_policy,
│   on main.tf line 4, in resource "google_compute_resource_policy" "placement_policy":
│    4:   group_placement_policy {
│ 
│ "group_placement_policy.0.availability_domain_count": one of `group_placement_policy.0.availability_domain_count,group_placement_policy.0.vm_count` must be specified
╵
╷
│ Error: Missing required argument
│ 
│   with google_compute_resource_policy.placement_policy,
│   on main.tf line 4, in resource "google_compute_resource_policy" "placement_policy":
│    4:   group_placement_policy {
│ 
│ "group_placement_policy.0.vm_count": one of `group_placement_policy.0.availability_domain_count,group_placement_policy.0.vm_count` must be specified
╵

Important Factoids

• The actual behavior is in line with the provider documentation but is enforcing a limitation that goes against the recommendation of the GCP documentation (See note in this section)
• The equivelent gcloud command succeeds (note lack of --vm-count argument):

gcloud compute resource-policies create group-placement my-placement-policy \
    --collocation COLLOCATED \
    --region us-central1 \
    --project <project-id>

[melinath]

This looks like it's probably a valid enhancement request; I don't think it's a bug, since the provider is behaving as expected (and if it's a bug it's probably a persistent bug, which would be treated as an enhancement request). It looks like gcloud made --vm-count optional in 2020-03. And according to the API user guide it is possible to exclude the field (even though it's not marked optional in the API reference.

Currently vmCount and availabilityDomainCount are marked as at_least_one_of; it might be as simple as changing that to conflicts_with and marking them as optional, but I'm not sure. The logic of which subfields have to be specified and what behavior is expected if they aren't seems to also be influenced by the collocation field.

[github-actions]

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