Terragrunt and TFLint above 0.50.0 breaking behavior

[velkovb]

Describe the enhancement

TFLint introduced a new way of handling values for undeclared variables in 0.50.0 - terraform-linters/tflint#1941

This directly affects the Terragrunt hook integration as it does the following:

tflint --config ./.terragrunt-cache/ellCj9USNaAqucT2-U5SmL6rf1A/Gu7P37gZ1L3NBihh7GJE9nHhW38/.tflint.hcl --chdir ./.terragrunt-cache/ellCj9USNaAqucT2-U5SmL6rf1A/Gu7P37gZ1L3NBihh7GJE9nHhW38 --var=test=foo

With Terragrunt release of 0.69.0 which bumped TFLint from 0.47.0 to 0.50.3 this started breaking our projects. We do generate a lot of metadata and automatic variables that are not used by every project. terraform-linters/tflint#2080 (comment)

The way that variables are passed to TFLint doesn't match the way they are passed to OpenTofu/ Terraform. For TFLint it seems to be with --var while for the latter it is TF_VAR_* which has no issues. If we pass OpenTofu/Terraform with --var it will error out as well.

Would be great if we can align both approaches and pass variables from input to TFLint as TF_VAR_* as well. If that cannot be done, passing a .tfvars file should work as well. I would guess the magic needs to happen somewhere here -

terragrunt/tflint/tflint.go

Line 154 in 407f65f

func InputsToTflintVar(inputs map[string]interface{}) ([]string, error) {

Not sure if this is a bug or enhancement

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RFC Not Needed

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[yhakbar]

Hey @velkovb ,

Thanks for creating this issue.

Unfortunately, given that there's a very straightforward workaround (calling tflint with the binary installed in PATH however users like), there isn't much incentive to adjusting how the built-in tflint hook works.

That being said, if this issue is important to you, would you consider contributing the code required to have tflint behave how you like? Even if you don't have a lot of Golang experience, we'd be happy to help you ramp up and collaborate with you on developing a solution.

Our focus is on delivering the features and updates that make Terragrunt ready for 1.0, so I'm jealously guarding the attention of the maintainers for that when I can. I hope you understand.

[velkovb]

@yhakbar Thanks for getting back to me.

Initially, I started using the built-in tflint hook as it worked under the .terragrunt-cache folder where the project is expanded. Back then I didn't find it that straightforward to directly run tflint in that folder. Do you mean that as the workaround or to pass the --terragrunt-external-tflint option? The latter still has the same issue.

Giving it a go crossed my mind. Might try it over the holidays :)

[yhakbar]

Ya, I was discussing the --terragrunt-external-tflint option with an older version of tflint installed so that you have the older behavior.

Please make sure to read these docs before you start contributing so that you have some guidance on how to get started.

The steps I generally recommend for new contributors is to start with the following:

1. Build Terragrunt locally, so that you can play with your own version of Terragrunt.
2. Run the relevant tests (you don't need to run the tests for the entire project, just the stuff that you're interested in changing).
3. Write a test that produces the outcomes you're looking for, and get those tests to fail the way you expect.
4. Write code that fixes the tests so that you have the new behavior you want.

I also heavily encourage you to join the Discord if you haven't. This can be a good place to ask questions about best practices, or to just become an active member of our community.

[velkovb]

Thanks, already in discord :).

My journey started because I actually wanted a newer version of TFLint that includes this fix - terraform-linters/tflint#2124 as we had problems with auto generated providers. So I will try to push forward

[velkovb]

@yhakbar I would guess that this logic is already implemented where the inputs are transformed for OpenTofu/ Terraform somewhere in the codebase. I could find it on my first attempt, could I ask for some directions where it is?

[wakeful]

hey,

Have you already read the main function where the tflint CLI is being constructed?

https://github.com/gruntwork-io/terragrunt/blob/main/tflint/tflint.go#L31