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google_compute_instance data struct doesn't support splat operator #3638

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slavaaaaaaaaaa opened this issue May 15, 2019 · 3 comments
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bug forward/review In review; remove label to forward service/compute-instances

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@slavaaaaaaaaaa
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slavaaaaaaaaaa commented May 15, 2019

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Terraform Version

Terraform v0.11.13
+ provider.google v2.1.0
+ provider.google-beta v2.2.0
+ provider.template v2.1.2

Affected Resource(s)

  • data.google_compute_instance

Terraform Configuration Files

...
resource "google_compute_region_instance_group_manager" "default" {
  name               = "${var.name}-mig"
  instance_template  = "${google_compute_instance_template.default.self_link}"
  base_instance_name = "${var.name}"
  region             = "${var.region}"
  target_size        = 2

  distribution_policy_zones = ["${data.google_compute_zones.available.names}"]
}

data "google_compute_region_instance_group" "default" {
  self_link = "${google_compute_region_instance_group_manager.default.instance_group}"
}

data "google_compute_instance" "default" {
  count = "${data.google_compute_region_instance_group.default.instances.size}"

  self_link = "${data.google_compute_region_instance_group.default.instances.*.instance[count.index]}"
}
...

Debug Output

* module.pizza.data.google_compute_instance.default[0]: At column 73, line 1: invalid index operation into non-indexable type: TypeString in: ${data.google_compute_region_instance_group.default.instances.*.instance[count.index]}

Panic Output

Expected Behavior

Data structures should be created, each representing an instance in the MIG.

Actual Behavior

The splat (*) operator seems to be ineffective and terraform plan breaks.

Steps to Reproduce

  1. terraform apply

Important Factoids

This bug frustratingly prevents generating DNS records for MIGs. Several other tickets in this repository mention that this way is possible, while stating it's an antipattern. While I agree that it's an antipattern, it is sometimes needed and simply doesn't work.

References

@ghost ghost added the bug label May 15, 2019
@emilymye
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I'm pretty sure this is a limitation of the splat operator, specifically with nested output-only fields (which is exactly what a data source is for) and would be an issue to bring up with Terraform core. However, I believe the official stance will probably be "use 0.12 which has a better splat". :/

We as providers sadly don't have control over how the splat operator works. I'm not sure if you could work around using something like the built-in element or lookup functions. We've typically pointed people to issues like hashicorp/terraform#17048 where people have tried to find alternatives.

@slavaaaaaaaaaa
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@emilymye As terraform 0.12 isn't yet stable, that's not an option for me nor my clients. Thank you nonetheless, I'll continue following the issue you linked.

@ghost
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ghost commented Jun 16, 2019

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. If you feel I made an error 🤖 🙉 , please reach out to my human friends 👉 [email protected]. Thanks!

@ghost ghost locked and limited conversation to collaborators Jun 16, 2019
@github-actions github-actions bot added forward/review In review; remove label to forward service/compute-instances labels Jan 15, 2025
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