Atlantis can apply un-mergeable MRs

[yertym]

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Overview of the Issue

When using a gitlab project with the merge commit with semi-linear history an MR source branch must be rebased if it's not up to date with the current target branch revision. Gitlab will refuse to merge the branch until the branch is rebased but atlantis will apply the change and then be unable to merge.

The behaviour was changed in #4402 and the linked issue #4390 suggests this was correcting the behaviour back to how it was prior to the needs_rebase merge status being introduced.

It seems to contradict the apply_requirement of mergeable

Reproduction Steps

1. Create a MR with a source branch that is not based on the latest commit of the target branch
2. Comment atlantis apply

Environment details

• Atlantis version: 0.31.0
• Deployment method: helm chart on EKS
• Atlantis flags:

Atlantis server-side config file:

---
    repos:
    - id: "/.*/"
      apply_requirements: [approved, mergeable]
      allowed_overrides: [workflow]
      delete_source_branch_on_merge: true

[dosubot]

Hey @yertym! I'm here to assist you with any bugs, questions, or contributions. Let's dive into this issue together!

I found some relevant threads that might help with the issue you're experiencing:

1. Closed Issue: Atlantis Apply Fails on GitLab v16.10 When the Merge Request Branch Needs Rebasing. This issue was resolved by a pull request that aimed to fix the behavior when a merge request branch needs rebasing ^[1].

2. Closed Issue: apply fails with apply_requirements mergeable on gitlab < 15.6. This was resolved by adding a backwards-compatible mergeability check ^[2].

3. Open Issue: There is a suggestion to manually update the commit status of a "stuck" Atlantis plan job to "success" using the GitLab commit status API as a workaround for merge request status issues ^[3].

The behavior you're observing, where Atlantis applies changes to un-mergeable merge requests, seems to contradict the apply_requirements of mergeable. This might be related to how the needs_rebase status is handled. The pull request #4402 aimed to address issues related to this status, potentially reverting behavior to how it was before the needs_rebase status was introduced ^[1].

If the issue persists, you might want to check if the logic in the PullIsMergeable function aligns with your expectations and whether the merge commit with semi-linear history setting in GitLab is affecting the mergeability checks ^[4].

To continue talking to Dosu, mention @dosu.