Interpolation function CONCAT shouldn't add empty element if a provided list is empty.

[BrunoBonacci]

Problem:

list1 = [a,b,c]
list2 = []
concat(list1, list2) = [a,b,c, ]  # wrong

# but should be
concat(list1, list2) = [a,b,c]  # without empty element at the end

# similarly
concat(list2,list1) = [ ,a,b,c] # wrong empty element 0

Scenario:
Assume I want to have a number of different CIDR as variable and then use there in security groups.

variable "cidr_general_access" {
    default = "1.1.1.1/32,2.2.2.2/32"
}
variable "cidr_service1_access" {
    default = "3.3.3.3/32"
}


output "concat-lists" {
    value = ["${join(" | ", concat( split(",", var.cidr_general_access), split(",", var.cidr_service1_access)))}"]
}

If all CIDR variable contains at least 1 element then everything works fine:

$ terraform apply

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

  concat-lists = 1.1.1.1/32 | 2.2.2.2/32 | 3.3.3.3/32

however if one of the var is empty "" then an empty element is added

variable "cidr_general_access" {
    default = "1.1.1.1/32,2.2.2.2/32"
}
variable "cidr_service1_access" {
    default = ""
}


output "concat-lists" {
    value = ["${join(" | ", concat( split(",", var.cidr_general_access), split(",", var.cidr_service1_access)))}"]
}

Output:

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

  concat-lists = 1.1.1.1/32 | 2.2.2.2/32 |
                                           ^^^^^^^^^^^^^^
                                           empty element

The empty element causes an error during the setup of the security group.

[phinze]

Yeah this is definitely a bug. This is probably related to #3437. We'll get this fixed up.

[mitchellh]

Thanks for the report and the detailed repro cases! For really wonky backwards compat reasons, I'm keen to not change this behavior in light of the fact we just introduced first class lists!

With first class lists, this issue goes away:

variable "cidr_general_access" {
    default = ["1.1.1.1/32", "2.2.2.2/32"]
    type = "list"
}

variable "cidr_service1_access" {
    default = []
    type = "list"
}


output "concat-lists" {
    value = ["${join(" | ", concat(var.cidr_general_access, var.cidr_service1_access))}"]
}

Hurray!

[ghost]

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