Problem using depends_on with splat output from external provider

[thomasinsel]

I don't think this is specific to the external provider, but it's the simplest way to demonstrate the problem.

Terraform Version

Terraform v0.11.13

Terraform Configuration Files

locals {
  fruit = [
    "apple",
    "banana"
  ]
}

resource "null_resource" "foo" { }

data "external" "cat" {
  count = "${length(local.fruit)}"
  depends_on = [ "null_resource.foo" ]
  program = [ "cat" ]
  query = {
    fruit = "${local.fruit[count.index]}"
  }
}

Expected Behavior

fruits = [
    apple,
    banana
]

Actual Behavior

* output.fruits: Resource 'data.external.cat' does not have attribute 'result.fruit' for variable 'data.external.cat.*.result.fruit'

Steps to Reproduce

I'm trying to use an external provider to transform a list. In the real world, this would be converting a list of hostnames to IP addresses. In the example, I just use cat to do nothing. Since I want a list instead of a list of maps, I use output ${data.external.cat.*.result.fruit}.

If I comment out the depends_on line and run terraform init && terraform apply, everything works as expected.

But, with depends_on it gives the error above.

Also, ${data.external.cat.*.result} works fine in either case.

[jbardin]

Hi @thomasinsel,

Sorry about the unexpected behavior here, but the depends_on meta argument is not really useful on a data source (see https://www.terraform.io/docs/configuration/data-sources.html#data-resource-dependencies).

We're tracking this case in #18600, and a proposal for changing the lifecycle of data sources in #17034.

[thomasinsel]

Got it @jbardin, but could you tell me the recommended method to ensure that a custom executable used for an external provider exists?

My plan would have been to make the data provider depend on a local_file resource.

Thanks!

[apparentlymart]

@thomasinsel generating a program with local_exec and then immediately running it with external isn't really a use-case these mechanisms were designed for, but I think you could make it work by using a reference to the other resource rather than using depends_on:

resource "local_file" "example" {
  filename = "${path.module}/example"
  content  = "..."
}

data "external" "example" {
  program = ["${local_file.example.filename}"]
}

local_file currently generates a file with executable permissions set on Unix systems, but that is an implementation detail rather than contract and so may change in a future version, depending on how hashicorp/terraform-provider-local#3 is resolved. If you do the above, be sure to pin your local provider dependency to a specific version to be sure that a future change in that area won't break your configuration.

[ghost]

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