data source template_file fails with "read /: is a directory" when template attribute interpolation or literal returns a "/"

[martin-cossy-atwork]

Cannot see this in the documentation, so here I go.

terraform plan fails with read /: is a directory when the value assigned to the template attribute of the template_file data source equals /.

Terraform Version

Terraform v0.7.2

Affected Resource(s)

• template_file

Terraform Configuration Files

variable "ping_path_separator" {
  default = {
    HTTP = "/"
    HTTPS = "/"
    TCP = ""
    SSL = ""
  }
}

data "template_file" "ping_path_separator" { template = "${lookup(var.ping_path_separator, "HTTP")}" }

or simply

data "template_file" "broken" { template = "/" }

Expected Behavior

It should not fail an rendered should return /

Actual Behavior

It fails as described :)

Steps to Reproduce

terraform apply

[levenaux]

I think you're meant to structure it like this:

variable "ping_path_separator" {
  default = {
    HTTP = "/"
    HTTPS = "/"
    TCP = ""
    SSL = ""
  }
}

data "template_file" "ping_path_separator" {
  template = "${ping_path_separator}"

  vars {
    ping_path_separator = "${lookup(var.ping_path_separator, "HTTP")}"
  }
}

the docs are here and here

[martin-cossy-atwork]

@jamielennox1 that works indeed. Thanks. Will use that workaround.

But I still fail to understand why this specific case fails.

I normally do interpolations directly in the template argument because according to the docs it is not forbidden to do so and it keep the descriptive code slim.

[levenaux]

Filename was deprecated for template. This means we can specify both inline templates and reference template files. / is a directory inside UNIX. So, it's going to assume you're trying to load a file instead of inline.

to prove that this is the case:

variable "ping_path_separator" {
  default = {
    HTTP = "Test /"
    HTTPS = "/"
    TCP = ""
    SSL = ""
  }
}

data "template_file" "ping_path_separator" {
  template = "${lookup(var.ping_path_separator, "HTTP")}"
}

This should pass absolutely fine.

It's still a bug but hopefully, that helps you to understand why it's behaving the way it is.

[martin-cossy-atwork]

Yeah. Actually I suspected something like that, but then to reproduce the error by using something like this

data "template_file" "broken" { template = "/test/" }

or

data "template_file" "broken" { template = "/test" }

or

data "template_file" "broken" { template = "test/" }

or this

data "template_file" "broken" { template = "/test/file.txt" }

but none of those failed, only this one

data "template_file" "broken" { template = "/" }

that got me wondering and opened the ticket.

Feel free to close the ticket if this behavior is expected. And thanks again for saving the day :)

[ghost]

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