fix: Resolving symlinks when updating relative paths in template

[calabrla]

Which issue(s) does this change fix?

No issue was created.

Why is this change necessary?

Running sam build against a template that is in a symlinked directory results in incorrect relative paths in the generated template. (See additional context below)

How does it address the issue?

This change resolves symlinks in the original_root and new_root before updating relative paths in the generated template at the end of a build.

What side effects does this change have?

None.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Additional Context

Given the project structure:

.
├── .aws-sam
│   └── build
│       ├── LambdaFunction
│       └── template.yaml
├── build -> /some/other/directory/build
│   └── cdk.out
│       └── Application.template.json
└── lib
    └── app.ts

When the project is built via sam build -t build/cdk.out/Application.template.json, in app_builder.py absolute paths are constructed by resolving all symlinks and then made relative as seen here. Before the template is written, the relative file paths are updated to be relative to a new_root, however neither the new_root nor the original_root are resolved to accommodate symlinks. This results in an incorrect code path and sam local invoke fails to mount the source.

[mildaniel]

Hey @calabrla, thanks for the contribution. Can you please add some tests to validate this change and to make sure we don't break it in the future.

[calabrla]

Hi @mildaniel I added unit tests for the _resolve_relative_to method that was updated and confirmed that the tests fail without my change. Let me know if you need anything else.