fix: Determine need_rebase as not a mergeable status

[lukemassa]

what

In Gitlab, make it so if a merge request is in status needs_rebase, then mark it as not mergeable.

why

Previously, when an MR was determined to be in the need_rebase status, we counted it as "mergeable", see #4402. Since then a number of bugs have been reported (#5826, #4025, #5124), indicating that users expect certain situations with need_rebase to be considered not mergeable.

I dug in a bit, and it looks like it depends on how you have your project configured. If your Merge Method is set to "Merge commit with semi-linear history" or "Fast-forward merge", then gitlab is explicitly saying your MR is not able to be merged like so:

Indeed, if you successfully apply and have automerge set, it will fail to merge, leaving you in a bad state.

The VCS itself pretty clearly indicates that a given MR is "unmergeable" we should follow suit

tests

I added some tests, and also tried this out on a test repo by setting a plan_requirement to mergeable and pushing a stale branch. You'll note that gitlab has noted the MR is not able to be merged, which is reflected in the new error message in the comment.

Note also that, even in newer versions, the API can respond with:

  "merge_status": "can_be_merged",
  "detailed_merge_status": "need_rebase",

merge_status was deprecated in 15.6 https://gitlab.com/gitlab-org/gitlab/-/issues/3169#note_1162532204; our code already detects version and uses detailed_merge_status if the server is new enough. However if it's not new enough we have to fall back to merge_status and we have no choice but to call such an MR "mergeable". This is reflected in the unit tests.

Note also that the unit tests were a bit broken; they included gitlabVersions in the test struct but never used it, always used the variable gitlabServerVersions. I fixed this and it allowed me to test different behavior for different versions (presumably) as intended.

references

closes: #5826, #4025, and #5124
functionally reverts: #4402

[Copilot]

Pull Request Overview

This PR fixes a bug where GitLab merge requests with need_rebase status were incorrectly considered mergeable, when they should be marked as not mergeable for certain GitLab project configurations.

• Updates the gitlabIsMergeable function to exclude need_rebase from mergeable statuses
• Adds comprehensive test cases to handle version differences (pre and post GitLab 15.6)
• Updates documentation to clarify GitLab mergeability requirements

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
server/events/vcs/gitlab_client.go	Removes need_rebase from the list of mergeable statuses
server/events/vcs/gitlab_client_test.go	Fixes test structure and adds new test cases for different GitLab versions
runatlantis.io/docs/command-requirements.md	Updates documentation to clarify GitLab merge requirements

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