Display "value of 'count' cannot be computed" when passing list or map with computed value to the module

[gongo]

Hi.

When passing a list or map with computed value to the module, terraform plan displays a message like a title.

$ terraform plan
Error configuring: 1 error(s) occurred:

* aws_sns_topic_subscription.event-test: value of 'count' cannot be computed

Terraform Version

$ terraform -v
Terraform v0.8.1

This error has occured from v0.8.0

Affected Resource(s)

(Maybe) Terraform's core

Terraform Configuration Files

https://gist.github.com/gongo/362975d478a9f4b85b3a213ddcc4d0cf

Debug Output

https://gist.github.com/gongo/362975d478a9f4b85b3a213ddcc4d0cf#gistcomment-1952568

Panic Output

No panic.

Expected Behavior

No error occurs.

Actual Behavior

Error occurs 😱

Steps to Reproduce

1. Input Terraform configuration like above.
2. terraform plan

[gongo]

Workaround 1

Pass count directory.

--- a/main.tf
+++ b/main.tf
@@ -23,4 +23,6 @@ module "sns_subscription_to_lambda" {
     "${aws_sns_topic.test1.arn}",
     "${aws_sns_topic.test2.arn}",
   ]
+
+  topic_arn_count = 2
 }
diff --git a/module/sns_topic_subscription.tf b/module/sns_topic_subscription.tf
index 366d76b..2a5de2c 100644
--- a/module/sns_topic_subscription.tf
+++ b/module/sns_topic_subscription.tf
@@ -2,8 +2,12 @@ variable "topic_arns" {
   type = "list"
 }

+variable "topic_arn_count" {
+  type = "string"
+}
+
 resource "aws_sns_topic_subscription" "event-test" {
-  count = "${length(var.topic_arns)}"
+  count = "${var.topic_arn_count}"

   topic_arn = "${element(var.topic_arns, count.index)}"
   protocol  = "lambda"

Workaround 2

Do not pass computed value

--- a/main.tf
+++ b/main.tf
@@ -20,7 +20,7 @@ module "sns_subscription_to_lambda" {
   source = "./module"

   topic_arns = [
-    "${aws_sns_topic.test1.arn}",
-    "${aws_sns_topic.test2.arn}",
+    "arn:aws:sns:ap-northeast-1:012345678901:event1",
+    "arn:aws:sns:ap-northeast-1:012345678901:event2",
   ]
 }

[mitchellh]

This is the correct behavior not because you're passing in a list, but because you're performing a function call length on a computed value. We're hoping to relax this constraint over time but at the moment that is not allowed and it appears you've found the workarounds that work:

Workaround 1: You didn't use length
Workaround 2: You passed static values (non-computed)

[gongo]

@mitchellh

Thanks for reply! I understood.

We're hoping to relax this constraint over time

I expect it 😄

[pikeas]

Just bit by this. Is there another issue to follow for progress on when this is supported?

Also, this error message is confusing. Could it be updated to value of "count" parameter cannot depend on a computed value (link to docs or this GH issue)?

[fgimian]

Likewise, just got caught out by this today too. Really hoping this is being tracked somewhere 😄

Huge thanks everyone
Fotis

[tata2000]

This was very useful to perform conditional updates in the module.
Very sad to see it gone in the new version.

[pdecat]

Thanks for the workarounds.

Dupe of #1497 I believe.

[iroller]

We're also using computed count for conditionals. Can't see how to properly upgrade to a newer terraform version without braking the functional. Both workarounds make you switch from a dynamic to a static count value.

[johnrengelman]

so, i find myself doing something like the following (especially when testing modules):

// module/foo.tf
variable "ids" {
  type = "list"
}

resource "some_resource" "foo" {
  count = "${length(var.ids)}"
}

// project.tf

module "foo" {
  ids = ["${some_resource.bar.id}", "${some_resource.baz.id}"]
}

In this case, I'm using length but couldn't the length value be computed statically even though each element is computed?

[hauleth]

This is strange, but I have module that creates list of aws_instances and then I pass their IDs as an output value. Then that list is passed to another module. Everything is fine when I define instance like this:

resource "aws_instance" "client" {
  ami                  = "${var.ami}"
  instance_type        = "${var.instance_type}"
  subnet_id            = "${element(var.private_subnets, count.index)}"
  iam_instance_profile = "${aws_iam_instance_profile.client.name}"

  key_name = "${var.key_name}"

  count = 5

  vpc_security_group_ids = [
    "${var.security_group_ids}",
    "${aws_security_group.all_egress.id}",
  ]

  tags {
    App  = "client"
    Name = "client-${count.index}"
  }

  // user_data = "${file("${path.module}/files/client.cloud-config.yml")}"
}

However as soon as I uncomment user_data value everything goes south and I get an error:

* module.cluster-lb-test.aws_elb_attachment.instances: aws_elb_attachment.instances: value of 'count' cannot be computed

[apparentlymart]

Hi @hauleth! Sorry for the confusing behavior there.

What's going on here is that changing user_data is a "forces new resource" change, which means that current the EC2 instance must be deleted and a new one replaced. At that point, Terraform doesn't know the ids of the new instances (since they've not been created yet) and due to the limitations described in this issue therefore count cannot be populated.

Eventually something like #4149 will make this smoother, but for now the workaround is to manually target that EC2 instance for replacement first, thus allowing Terraform to complete the instance replacement before trying to deal with the count attribute:

terraform plan -target=aws_instance.client -out=tfplan
(review the plan; should contain only the aws_instance and its dependencies)
terraform apply tfplan
terraform plan -out=tfplan
(review the plan; should now contain anything else that needs to be updated as a result)
terraform apply tfplan

In principle Terraform should be able to tell how many instances there are even though it doesn't yet have their ids, but currently that doesn't work due to some limitations of the interpolation language. Hopefully we can make that work better in future too, which may avoid the need for a two-step apply process in this specific situation.