GitHub Markdown syntax is not respecting YAML output - specifically lists

[snorlaX-sleeps]

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Overview of the Issue

When running a plan which contains YAML output, the GitHub markdown syntax is incorrectly highlighting YAML lists as Terraform changes.

This causes large red rows to appear in the middle of YAML docs and not actually highlighting a change.
This is most noticeable with the helm_release resource, as it displays both the old and new values in the plan output, where both sets show the same incorrect "changes".

I believe this is due to the fact that YAML uses - to mark a list item, which is the character Terraform uses in plans to indicate an item to be removed

Reproduction Steps

• Have a YAML based object in your plan, e.g a Helm release with a local values file
• Have the Atlantis GitHub markdown syntax enabled: ATLANTIS_ENABLE_DIFF_MARKDOWN_FORMAT

Environment details

• atlantis:latest which has the workspacing fix (not actually a release I believe)
• K8s v1.21
• Using GitHub

Atlantis Flags:

• ATLANTIS_ENABLE_DIFF_MARKDOWN_FORMAT

Additional Context

Example section of the output with incorrect formatting

[ax-nmac]

We are hitting the same issue and its causing confusion for our users. With the adoption of Kubernetes in alot of places we will be seeing more and more yaml in our plans, as I'm sure many others will as well.

[dupuy26]

The problem is not limited to YAML (although it is most common there). I have also seen a case where some Base64 encoded data was in the output, and a + at the beginning of a line (without even whitespace in front of it!) triggered the diff markdown to flag it as an addition...

I feel like there must be a regex somewhere that needs an ^ ? to allow at most one space before the + or - for it to be considered a diff marker...and checking for at least one space after the + or - would be even better.

[pauloconnor]

This is the regex, but the problem is that Terraform output doesn't have the + and - at the start of the line - it's indented