for_each causes terraform crash when creating null_resource

[muroj]

Terraform Version

Terraform v0.12.7
+ provider.null v2.1.2

Terraform Configuration Files

variable "subnet_id" {
    description = "IP Subnet specified in CIDR notation (e.g. 10.20.102.0/24) "
}

locals {
  guest_ips = [for i in range(1,255) : cidrhost(var.subnet_id, i)]
}

resource "null_resource" "guest_ips" {
    #for_each = toset(
    #    [for i in range(1,255) : cidrhost("var.subnet_id", i)]
    #)

    for_each = toset(local.guest_ips)

    triggers = {
      all_guest_ips = each.value
    }
}

Calling module:

module "subnet_1" {
    source = "../../../../soltest-terraform-modules/modules/network/ip_range"
    subnet_id = "10.20.102.0/24"
}

Crash Output

https://gist.github.com/muroj/35b5d7ca81cdf96fe81f37e4773f4013

Expected Behavior

A null resource for each IP should have been created.

Actual Behavior

terraform crashes

Steps to Reproduce

1. terraform init
2. terraform plan/apply

References

Similar to #22560

[muroj]

Turns out I can accomplish the same (dynamic resource creation) using count instead of for_each.

For example:

main.tf

 
variable "subnet_id" {
    description = "IP Subnet specified in CIDR notation (e.g. 10.20.102.0/24) "
}
variable "host_range" {
    description = "A list specifying the available IPs"
    type = "list"
}

locals {
  guest_ips = [for i in var.host_range : cidrhost(var.subnet_id, i)]
}

resource "null_resource" "guest_ips" {

    triggers = {
      all_guest_ips = element(local.guest_ips, count.index)
    }

    count = length(local.guest_ips)
}

Here is the calling module:

module "subnet_1" {
    source = "../../../../soltest-terraform-modules/modules/libvirt/network/ip_range"
    subnet_id = "10.20.102.0/24"
    host_range = flatten([range(1,5), range(11,15)])
}

I usually do not own an entire subnet, and must share it amongst other teams (not ideal, of course). This approach allows me to specify exactly which hosts are available in a given subnet.

[jcmaasb]

Hi @muroj

In your case, i think it´s acomplise with this code:

variable "subnet_id" {
  description = "IP Subnet specified in CIDR notation (e.g. 10.20.102.0/24) "
  default     = "10.20.102.0/24"
}

locals {
  guest_ips = { for i in range(1, 255) : i => cidrhost(var.subnet_id, i) }
}

resource "null_resource" "guest_ips" {
  for_each = local.guest_ips

  triggers = {
    all_guest_ips = each.value
  }
}

I have some issues with for_each in resources, and i dont understant how its works.
I'm waiting for the response from the Terraform Team to know how to use lists and maps with "for_each" correctly.

[muroj]

Thanks @jcmaasb, your code snippet works. However, I am not clear on benefits of this approach. Why would I use for_each vs count in this case?

[pselle]

Hi @jcmaasb, I wrote a little more about what for_each is expecting and why over here: #22560 (comment) (the similar, but ultimately different issue). Notably, don't use lists with for_each, which supports maps or sets of strings (explanation in link).

For this issue (thanks @muroj for opening!) it appears it has to do with unknown values. We should not panic here, and will fix that. The rule for using expressions with count (values must be known) should apply to for_each as well, but I'll do more investigation to confirm.

Edit for further clarification: The value is unknown at one point during the plan, but becomes known. Bug found and will fix, it looked like your code should work :)

[jcmaasb]

Hi @pselle
Thanks for your quick reply!!

[ghost]

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