Terraform: coalesce used in output value does not behave as expected

[gurdulu]

Hi there,

_Terraform version: 0.8.8 (at the moment we cannot use 0.9.0 as we rely on the remote option)._

I have created a module that creates an aws_autoscaling_group instance chosen between 2 possible configurations.

resource "aws_autoscaling_group" "a" {
  launch_configuration = "ANY"
  max_size = 1
  min_size = 2
  count = "${var.condition ? 1 : 0}"
}

resource "aws_autoscaling_group" "b" {
  launch_configuration = "ANY"
  max_size = 2
  min_size = 3
  count = "${var.condition ? 0 : 1}"
}

I expected that the value returned in output could be:

output "id" {
  value = "${coalesce(aws_autoscaling_group.a.id, aws_autoscaling_group.b.id)}"
}

but no matter which resource I initialise, the output is always empty.

Thank you in advance.

[apparentlymart]

Hi @gurdulu! Sorry for the problem here.

I believe what's going on here is that your value expression here is returning an error, but because of the long-standing bug #9080 this error is not shown to you.

If you add the following code temporarily you should see the underlying error here:

resource "null_resource" "temp" {
  triggers = {
    output = "${coalesce(aws_autoscaling_group.a.id, aws_autoscaling_group.b.id)}"
  }
}

My guess is that you're also getting tripped up by #11566 here: Terraform evaluates both "sides" of that conditional, even though only one of them will eventually be used. It's an error to interpolate an attribute from a resource block with count = 0, so this fails.

Someone in that other issue found a workaround that works until we get that fixed:

  value = "${coalesce(join("", aws_autoscaling_group.a.*.id), join("", aws_autoscaling_group.b.*.id))}"

This clever trick exploits the fact that this "splat syntax" produces an empty list if count = 0 and so the join will produce an empty string in that case, getting the behavior you were looking for.

Both of these things we'd like to fix, but since they are tracked by existing issues I'm going to close this to consolidate discussion over there. Thanks for taking the time to report this!

(If it turns out that my guess was incorrect about what error you'll get here, do let me know... it's possible that there's a different bug here, masked by the error being hidden.)

[gurdulu]

Hi @apparentlymart

Thank you

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