Terragrunt: Expose Dependencies from read_terragrunt_config calls in `PartialDecodeSectionType`

[dmattia]

Hello! I am working on a go binary that generates atlantis config from terragrunt files.

My repo is here: https://github.com/transcend-io/terragrunt-atlantis-config, and the part of my code that tries to find all terragrunt dependencies is here: https://github.com/transcend-io/terragrunt-atlantis-config/blob/master/cmd/generate.go#L88-L123

One thing that I'm noticing is that read_terragrunt_config can now add dependencies from other files, but that those dependencies are not anywhere to be found (as best I can tell), in the output of calls to PartialDecodeSectionType, even though PartialDecodeSectionType does evaluate local values.

Would it be possible to add an option in the config library to get those dependencies?

[yorinasub17]

I understand the use case, but this is not something we want to support in PartialParseConfigFile, as that overcomplicates the function (e.g., we need to track and return all the config that is parsed in the tree) for a use case that is not necessary for terragrunt right now.

It's particularly challenging because it is parsed as part of a function call that is not made at the top level, and so you need to use some kind of global map and look that up later, which is going to be an ugly mess to what is already very difficult to work code.

[yorinasub17]

That said, we would like to help out but we are buried right now. I think this requires some deep thinking and I don't have any cycles at the moment to help out from scratch.

If you have a PR that can do this in a sane way and relatively easy to maintain, happy to review and include that!

[yorinasub17]

Gah! After thinking about this in the shower, I just realized we will most likely need this as apply-all and plan-all is probably broken when read_terragrunt_config is in the mix. Will be taking a deeper look at this next week.

[dmattia]

Sounds great! I did happen to notice earlier today that terragrunt providers error messages weren't ideal when I had an issue with a provider from a dependency I read in in a read_terragrunt_config call. If you think that might be related, I'd gladly try to come up with a simple example that shows the behavior