fix(terraform): eval submodules

[nikpivkin]

Description

Related issues

• Close bug(terraform): module output values are not passed into the context of other modules #6274

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[simar7]

I'm curious why this didn't work as intended, is it because we weren't definition.Parser.Load(ctx) for each definition? Like so: https://github.com/aquasecurity/trivy/pull/6411/files#diff-b10704f6636c4e99c08df82aeb21c2283a75a61953d50b6f800289dbfa44979eR187

[nikpivkin]

Modules are loaded and evaluated in alphabetical order. But modules can have different order and cyclic relationships, so we need to evaluate each module several times. Load is just a new function to load modules once, which returns an evaluator.

[simar7]

this is a nice test case! I'm curious what logic prevents the cyclic loop? In this case module1 uses module2 and vice-versa but how do we figure out to resolve them both?

does the logic still hold true if we have 3 modules in a cycle?

[nikpivkin]

Cycling between modules is allowed. Modules are evaluated until the output variables of all modules stop changing or the maximum number of evaluations is reached. In short, if at least one output variable of any module has been updated, we have one unevaluated module and therefore continue the evaluation.

Let's look at an example:

module "module1" {
	source = "./modules/too"
	test_var = module.module2.test_out
}

module "module2" {
	source = "./modules/bar"
	test_var = module.module3.test_out
}

module "module3" {
	source = "./modules/foo"
}

The first evaluation step will completely compute module3, the second module2, and the third module1.

[fwereade]

Hi @nikpivkin, thank you for working on this – I opened a PR against your branch which I hope will be helpful.

[CLAassistant]

All committers have signed the CLA.

[simar7]

lgtm, looks like we need to fix some merge conflicts @nikpivkin

[nikpivkin]

@simar7 Fixed