Cannot change a MIG from stateful to stateless

[st250475]

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Terraform Version

Terraform v0.14.8

• provider registry.terraform.io/hashicorp/google v3.87.0

Affected Resource(s)

• google_compute_instance_group_manager
• google_compute_region_instance_group_manager

Terraform Configuration Files

The problem arises when the commented-out parts are applied (making the MIG stateful) and then removed (making it stateless).

resource "google_compute_instance_template" "poc" {
  name_prefix  = "poc-"
  machine_type = "e2-medium"
  region       = "europe-west2"

  disk {
    source_image      = "debian-cloud/debian-9"
    auto_delete       = true
    boot              = true
  }

#   disk {
#     device_name       = "stateful"
#     source_image      = "debian-cloud/debian-9"
#     auto_delete       = true
#     boot              = false
#   }

  network_interface {
    subnetwork = "subnet-01"
  }

  lifecycle {
    create_before_destroy = true
  }
}

resource "google_compute_instance_group_manager" "poc" {
  name = "poc"

  base_instance_name = "poc"
  zone               = "europe-west2-a"

  version {
    instance_template  = google_compute_instance_template.poc.id
  }

  target_size  = 1

#   stateful_disk {
#     device_name = "stateful"
#   }

}

Debug Output

This output is what we get when applying the Terraform above (with the stateful configuration removed). What we can see is that the PATCH to the MIG does not touch the Stateful Policy.

https://gist.github.com/srjturner/16505ed454e13d3956a3524031cbc52d

Expected Behavior

The MIG should have had its Stateful Policy removed so that it no longer was stateful, and so that it did not expect the Instance Template to have the (stateful) disk.

Actual Behavior

The MIG does not have its Stateful Policy removed, and it fails to accept the Instance Template because that has no longer got the expected disk.

Steps to Reproduce

1. terraform apply -auto-approve with the commented lines uncommented to make the MIG stateful
2. Comment the lines out (as shown above) to make the MIG stateless
3. terraform apply -auto-approve

Note

If we issue a command line call to remove the SP:

gcloud compute instance-groups managed update poc --remove-stateful-disks stateful

then we can then successfully apply the Terraform without the SP and disk

[nat-henderson]

Okay - I think I can see where this problem comes from, thanks for the report. I think we are probably not sending the empty StatefulPolicy (which is nil if the policy is empty), and we either need to use ForceSendFields, if available, or else return the empty object rather than nil from the expander. I'll try to get an integration test that repros this and see if that fixes it.

[askubis]

I would like to fix this

[github-actions]

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