Cannot find a urldecode function in terraform

Question

I see there is a urlencode in terraform but no urldecode. Is there any reason why it is not there? What would be the workaround to achieve that in terraform?

Thanks, Ram

Answer 1

The functions built in to the Terraform language tend to (with some notable historical exceptions) focus on solving problems that seem to arise very commonly in typical definitions of infrastructure. URL encoding arises in such situations as building URLs for API calls, whereas URL decoding seems to come up less frequently in the typical scope where Terraform is used and so there haven't been sufficient real-world examples of the need to justify making it a built-in.

The Terraform language does include some features that allow for basic manual string tokenization and transforming, but a key missing piece for fully-general URL decoding is that Terraform does not have a function which can take a number and return the corresponding character as defined by a specific character lookup table, such as ASCII or Unicode.

If you know that in practice your inputs will only use URL escaping sequences for specific reserved symbols then you can approximate URL decoding with a lookup table combined with a tokenizing regular expression:

locals {
  input  = "foo%3Fx%3Dtest"
  tokens = regexall("(?:%[0-9a-fA-F]{2}|[^%]+)", local.input)

  replacements = tomap({
    "%3f" = "?"
    "%3d" = "="
    "%25" = "%"
  })

  result = join("", [
    for token in local.tokens : (
      substr(token, 0, 1) == "%" ?
      local.replacements[lower(token)] :
      token
    )
  ])
}

The above works for the limited encoding vocabulary of only ?, =, and % but will fail if there are any encoded characters other than those. You can of course expand this vocabulary to include any additional characters you'd like to include, and you could potentially expand that table to include all 128 ASCII characters if you like.

It would not be possible to decode non-ASCII (i.e. Unicode-only) characters with this simplistic strategy because URL encoding of those involves encoding first as UTF-8 and then encoding the individual bytes that result, and the Terraform language does not include any facilities for working with raw bytes: it works only with unicode strings.