Atlantis Apply Fails on GitLab v16.10 When the Merge Request Branch Needs Rebasing

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Overview of the Issue

In Gitlab if:

• The Gitlab version is 16.10.0 or above
• The repo has the merge commit with semi-linear history or fast-forward merge methods enabled
• The merge request branch is behind the target branch by one or more commits

Then the atlantis apply command on the MR will fail with the following error:

Apply Failed: Pull request must be mergeable before running apply.

Logs

Logs

[DBUG] Getting GitLab merge request 31
[DBUG] GET /projects/sheather/test/merge_requests/31 returned: 200
[DBUG] Updating GitLab commit status for 'atlantis/apply' to 'pending'
[DBUG] Getting GitLab merge request 31
[DBUG] GET /projects/sheather/test/merge_requests/31 returned: 200
[DBUG] Head pipeline found for merge request 31, source 'merge_request_event'. refTarget 'refs/merge-requests/31/head'
[DBUG] POST /projects/sheather/test/statuses/91a7ea6756cee6e9e62ab72665556c363bbf9af7 returned: 201
[DBUG] Checking if GitLab merge request 31 is approved
[DBUG] GET /projects/sheather/test/merge_requests/31/approvals returned: 200
[DBUG] Checking if GitLab merge request 31 is mergeable
[DBUG] GET /projects/sheather/test/merge_requests/31 returned: 200
[DBUG] GET /projects/2 returned: 200
[DBUG] GET /projects/2/commits/91a7ea6756cee6e9e62ab72665556c363bbf9af7/statuses returned: 200
[DBUG] Checking if GitLab supports detailed merge status
[DBUG] Getting GitLab version
[DBUG] GET /version returned: 200
[DBUG] building config based on server-side config
[DBUG] setting apply_requirements: [mergeable] from repos[1], id: /.*/
[DBUG] setting workflow: "default" from default server config
[DBUG] setting allowed_overrides: [] from default server config
[DBUG] setting custom_policy_check: false from default server config
[DBUG] setting repo_locking: true from default server config
[DBUG] setting policy_check: false from default server config
[DBUG] setting plan_requirements: [] from default server config
[DBUG] setting import_requirements: [] from default server config
[DBUG] setting allow_custom_workflows: false from default server config
[DBUG] setting delete_source_branch_on_merge: false from default server config
[DBUG] Building project command context for apply
[DBUG] Found required_version setting of ">=1.3.9"
[DBUG] Listing Terraform versions available at: https://releases.hashicorp.com/terraform
[INFO] Detected module requires version: 1.7.5
[DBUG] Updating GitLab commit status for 'atlantis/apply: live/aws/123456789012/develop/eu-west-2/stack11/default' to 'pending'
[DBUG] Getting GitLab merge request 31
[DBUG] GET /projects/sheather/test/merge_requests/31 returned: 200
[DBUG] Head pipeline found for merge request 31, source 'merge_request_event'. refTarget 'refs/merge-requests/31/head'
[DBUG] POST /projects/sheather/test/statuses/91a7ea6756cee6e9e62ab72665556c363bbf9af7 returned: 201
[DBUG] Updating GitLab commit status for 'atlantis/apply: live/aws/123456789012/develop/eu-west-2/stack11/default' to 'failed'
[DBUG] Getting GitLab merge request 31
[DBUG] GET /projects/sheather/test/merge_requests/31 returned: 200
[DBUG] Head pipeline found for merge request 31, source 'merge_request_event'. refTarget 'refs/merge-requests/31/head'
[DBUG] POST /projects/sheather/test/statuses/91a7ea6756cee6e9e62ab72665556c363bbf9af7 returned: 201
[EROR] Failure running apply operation: Pull request must be mergeable before running apply.
[DBUG] Hiding previous plan comments for command: 'Apply', directory: ''
[DBUG] Hiding previous command comments on GitLab merge request 31
[DBUG] /projects/sheather/test/merge_requests/31/notes
[DBUG] GET /projects/sheather/test/merge_requests/31/notes returned: 200
[DBUG] Updating merge request note: Repo: 'sheather/test', MR: '31', comment ID: '749'
[DBUG] PUT /projects/sheather/test/merge_requests/31/notes/749 returned: 200

Environment details

• Atlantis version: 0.27.2

Additional Context

Gitlab have introduced the following new detailed merge statuses in MR !144729 which was released in v16.10.0:

• conflict
• need_rebase.

Reference: GitLab MR Merge Status

In previous versions of GitLab, merge requests in this state return mergeable in the detailed merge status.

The need_rebase status needs adding to the list of acceptable merge statuses that Atlantis checks for in the GitLab client PullIsMergeable function here:

atlantis/server/events/vcs/gitlab_client.go

Lines 358 to 361 in 890df73

	if ((supportsDetailedMergeStatus &&
	(mr.DetailedMergeStatus == "mergeable" ||
	mr.DetailedMergeStatus == "ci_still_running" ||
	mr.DetailedMergeStatus == "ci_must_pass")) ||