List issue during upgrade from Terraform 0.11 to 0.12

Question

I have a problem trying to rewrite a list to work in Terraform 0.12 that works in Terraform 0.11.

This is my list:

variable "master_authorized_networks_config" {
  type        = list(string)
  description = "The list of CIDR blocks of master authorized networks."

  default = [
    {
      cidr_blocks = [
        { 
          cidr_block   = "XXXXXX/32"
          display_name = "XXXXX"
        },
        { 
          cidr_block   = "XXXXXX/32"
          display_name = "XXXXX"
        },
        { 
          cidr_block   = "XXXXXX/32"
          display_name = "XXXXX"
        },
        { 
          cidr_block   = "XXXXXX/32"
          display_name = "XXXXX"
        },
      ]
    },
  ]

I added it to module: master_authorized_networks_config = var.master_authorized_networks_config

After running terraform apply I get the following error:

Error: Invalid default value for variable

  on ../../modules/xxx/xxx/variables.tf line 71, in variable "master_authorized_networks_config":
  71:   default = [
  72:     {
  73:       cidr_blocks = [
  74:         {
  75:           cidr_block   = "XXXXXX/32"
  76:           display_name = "XXXXX"
  77:         },
  78:         {
  79:           cidr_block   = "XXXXXX/32"
  80:           display_name = "XXXXX"
  81:         },
  82:         {
  83:           cidr_block   = "XXXXXX/32"
  84:           display_name = "XXXXX"
  85:         },
  86:         {
  87:           cidr_block   = "XXXXXX/32"
  88:           display_name = "XXXXX"
  89:         },
 102:       ]
 103:     },
 104:   ]

This default value is not compatible with the variable's type constraint:
element 0: string required.

I can't resolve this problem. Can you help me?

It's not a list of strings. You should set the type to be list(object) or remove the type part altogether and allow Terraform to work out the type from the default. The second option is what I'd do. — ydaetskcoR
list(string) was not an allowed type specification in 0.11. Are you sure you were using this successfully with 0.11? — Matt Schuchard
Pretty sure they've ran 0.12upgrade when they had type = "list" before and ended up with that. It's been a long time since my 0.12 upgrade but I think that was the default behaviour of the tool. — ydaetskcoR

ydaetskcoR ydaetskcoR · Accepted Answer · 2020-03-19T13:19:01

The error is telling you that your default value doesn't match the type constraint that you've told Terraform about with type = list(string). That type constraint says that it must be a list of strings and is the default coming from 0.11 via the 0.12upgrade tool that you may have ran to get that output if you had type = list previously.

You could change your type constraint to be list(object). If you simplified your data structure so that you just had a list of objects with cidr_block and display_name keys instead of a list of objects with key cidr_blocks that has a list of the aforementioned objects then you could even go as far as validating that more precisely:

variable "master_authorized_networks_config" {
  type = list(object{
    cidr_block   = string
    display_name = string
  })

  description = "The list of CIDR blocks of master authorized networks."

  default = [
    { 
      cidr_block   = "XXXXXX/32"
      display_name = "XXXXX"
    },
# ...
  ]
}

Or, much simpler, when providing a default you can choose to let Terraform infer the expected object type automatically by removing the type parameter from the variable. Then if anyone attempts to pass in a variable that doesn't match the type of the default Terraform will tell you at plan or validate time.

List issue during upgrade from Terraform 0.11 to 0.12

1 Answers