fix: use dynamic iap block to prevent perpetual no-op diff

[ianb-pomelo]

Problem

When iap_config is not provided in a backend configuration, the static iap block sets oauth2_client_id to null. However, the GCP API returns " " (a single space) for unset oauth2_client_id values, causing Terraform to detect a diff on every plan:

~ iap {
    - oauth2_client_id = " " -> null
  }

This results in a perpetual no-op update to google_compute_backend_service resources that don't use IAP.

Solution

Make the iap block dynamic so it is only rendered when iap_config is explicitly provided. When iap_config is absent, the block is omitted entirely, and Terraform no longer tries to reconcile null vs the " " returned by the API.

Changes

The same change is applied in all 4 locations where the iap block appears:

• main.tf
• modules/serverless_negs/main.tf
• modules/dynamic_backends/main.tf
• autogen/main.tf.tmpl

Before:

iap {
  enabled              = try(each.value["iap_config"], null) == null ? false : ...
  oauth2_client_id     = try(each.value["iap_config"], null) == null ? null : ...
  oauth2_client_secret = try(each.value["iap_config"], null) == null ? null : ...
}

After:

dynamic "iap" {
  for_each = try(each.value["iap_config"], null) != null ? [each.value["iap_config"]] : []
  content {
    enabled              = lookup(iap.value, "enable", false)
    oauth2_client_id     = lookup(iap.value, "oauth2_client_id", null)
    oauth2_client_secret = lookup(iap.value, "oauth2_client_secret", null)
  }
}

Backward Compatibility

This is a non-breaking change:

• Backends with iap_config set behave identically to before
• Backends without iap_config simply stop producing the phantom diff

Related: #451, hashicorp/terraform-provider-google#16585

[ianb-pomelo]

/gcbrun

[github-actions]

This PR is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 7 days

[ianb-pomelo]

Still would like this if possible, it causes a good amount of churn in our state for no reason