[WIP] feat: Support version constraints for Terraform Registry modules

[juan-vg]

Description

Fixes #1930 Fixes #2572

TODOs

Read the Gruntwork contribution guidelines.

• [x] Update the docs.
• [x] Run the relevant tests successfully, including pre-commit checks.
• [X] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [X] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added support for version constraints for Terraform Registry modules

Summary by CodeRabbit

• New Features

  • Enhanced module version management with clearer error messages for various failure scenarios.
  • Improved module version retrieval process, ensuring accurate version selection and increased logging transparency during downloads.
  • New configuration file for Terragrunt to retrieve modules from the public Terraform registry.
  • Added a new test for Terraform registry version resolution to ensure correct functionality.

• Documentation

  • Updated documentation for the VERSION argument in the terraform block to clarify acceptable formats.

• Tests

  • Expanded test coverage for the GetTargetVersion function to handle various input scenarios.

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Walkthrough

The changes introduce several new error types in the tf/errors.go file to improve error handling for module version issues. In tf/getter.go, new data structures and functions are added to facilitate the extraction of module version information from a registry's versions endpoint. The Get method is updated to use the new GetTargetVersion function, which constructs the URL for fetching module versions and handles filtering and parsing. Documentation is enhanced to clarify the acceptable formats for the VERSION argument, and new tests are added to validate the functionality.

Changes

File(s)	Change Summary
tf/errors.go	Added four new error types (ModuleVersionsFetchErr, ModuleVersionConstraintErr, ModuleVersionConstraintMalformedErr, ModuleVersionMalformedErr) along with their Error() methods for module version handling.
tf/getter.go	Introduced three new structs (Modules, Module, and ModuleVersion) for JSON deserialization. Added GetTargetVersion, getHighestVersion, and other functions to enhance version resolution logic and logging.
docs/_docs/04_reference/04-config-blocks-and-attributes.md	Updated documentation for the VERSION argument in the terraform block to include details on acceptable formats (specific version number or version constraint).
test/fixtures/tfr/version/terragrunt.hcl	Created a new Terragrunt configuration file that retrieves a module from the public Terraform registry with specified version constraints.
test/integration_registry_test.go	Added a new constant registryFixtureVersion and a test function TestTerraformRegistryVersionResolution to validate version resolution in the Terraform registry context.
tf/getter_test.go	Introduced a new test function TestGetTargetVersion to validate the behavior of tf.GetTargetVersion across various scenarios using a table-driven test structure.

Sequence Diagram(s)

sequenceDiagram
    participant Caller
    participant GTV as "GetTargetVersion()"
    participant HTTP as "HTTP Client"
    participant JSON as "JSON Parser"
    participant Filter as "getHighestVersion()"

    Caller->>GTV: Call with module info and versionQuery
    GTV->>HTTP: Build URL and perform HTTP request
    HTTP-->>GTV: Return JSON response
    GTV->>JSON: Parse JSON into Modules struct
    GTV->>Filter: Filter available versions by constraint
    Filter-->>GTV: Return highest version
    GTV-->>Caller: Return highest version or error

Suggested reviewers

• levkohimins
• denis256

Poem

In code we trust, errors now bloom,
With modules dancing in their room.
New paths, functions, logs to see,
A version's flow set error-free.
Cheers to clean code and joyful spree!
✨🚀

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[juan-vg]

Shame on me I forgot to run the linter... I'm on it!

[yhakbar]

Thanks for addressing those, @juan-vg .

Upon re-review, I realized that the implementation you've added uses a query string parameter for the source URL:

locals {
  base_source = "tfr:///terraform-aws-modules/iam/aws"
  version     = "~>5.0, <5.51.0, !=5.50.0"
}
terraform {
  source = "${local.base_source}?version=${local.version}"
}

Instead of a new version attribute:

terraform {
  source  = "tfr:///terraform-aws-modules/iam/aws"
  version = "~>5.0, <5.51.0, !=5.50.0"
}

I believe a new version attribute would be more in-line with the UI/UX of OpenTofu/Terraform, so users may find it more convenient to follow that pattern as well. What do you think? Would you be willing to try out an implementation that uses a version attribute?

[juan-vg]

@yhakbar

I believe a new version attribute would be more in-line with the UI/UX of OpenTofu/Terraform, so users may find it more convenient to follow that pattern as well. What do you think? Would you be willing to try out an implementation that uses a version attribute?

While I agree the new version attribute completely makes sense (and I'm willing to try that implementation), I'm wondering if it belongs to the scope of this PR since the version attribute is already there via query string. I mean, adding that attribute implies a (probably full) refactor of this code and at the end the target is to add the new attribute and not to support version constraints (even if they're related). From my POV the version attribute should be added as an incremental, later change after supporting version constraints. However, that's my take. WDYT?

[yhakbar]

While I agree the new version attribute completely makes sense (and I'm willing to try that implementation), I'm wondering if it belongs to the scope of this PR since the version attribute is already there via query string. ... However, that's my take. WDYT?

@juan-vg

Normally, I'd be totally onboard with an incremental improvement rather than hold up a PR, but in the case of TG configuration, we want to be more conservative. This introduces configuration for a URL that is not URL encoded, and when we support the attribute, users should switch off using the query string parameter, which would be a breaking change.

If you really don't have time to rework with PR to use an attribute, I'd be open to reviewing it after the changes have been gated behind an experiment flag. That way, we can document the feature as incomplete, and require user opt in to use behavior that might break in the future.

[juan-vg]

@yhakbar

Normally, I'd be totally onboard with an incremental improvement rather than hold up a PR, but in the case of TG configuration, we want to be more conservative. This introduces configuration for a URL that is not URL encoded, and when we support the attribute, users should switch off using the query string parameter, which would be a breaking change.

I understand

If you really don't have time to rework with PR to use an attribute, I'd be open to reviewing it after the changes have been gated behind an experiment flag. That way, we can document the feature as incomplete, and require user opt in to use behavior that might break in the future.

Nah, let's do it! I have some time and much willing, and any guiding or help will be appreciated :)

[stephenbawks]

Bumping this to see if there were any more updates. We would really like to see this adopted.

[OpsTalkJordan]

What is this PR still missing? Would love to see this implemented.