[FF] prevent_reopen_merge_request_without_branch -- Prevent reopening an MR when a branch is missing

Summary

Roll out the feature currently behind the prevent_reopen_merge_request_without_branch feature flag.

• DRI: @patrickbajao
• Team Slack channel: #g_code_review

Note

Process and guidance live in the docs — this issue is just the commands and a place to track the rollout. "Rolling out" means incrementally enabling the flag on GitLab.com to validate stability — it is not the same as releasing the feature, which happens when the flag is removed. Feature flag controls · Feature flag lifecycle

What could go wrong?

Small blast radius: the flag only changes behaviour when reopening a closed merge request whose source or target branch no longer exists. When enabled, such a reopen is rejected (the merge request stays closed) instead of recreating an empty diff. There is no data-loss risk — the change prevents the existing data loss (an empty diff overwriting the stored one). Watch merge request reopen error rates and ~"group::code review" dashboards on https://dashboards.gitlab.net.

Rollout

Run all production /chatops in #production and cross-post the results to #g_code_review. Background: incremental rollout process, feature actors.

Non-production

/chatops gitlab run feature set prevent_reopen_merge_request_without_branch 50 --actors --dev --pre --staging --staging-ref
/chatops gitlab run feature set prevent_reopen_merge_request_without_branch true --dev --pre --staging --staging-ref

Production — percentage rollout (wait ≥15 min between steps, watch dashboards):

/chatops gitlab run feature set prevent_reopen_merge_request_without_branch <percentage> --actors

Or target specific actors instead:

/chatops gitlab run feature set --project=gitlab-org/gitlab,gitlab-org/gitlab-foss prevent_reopen_merge_request_without_branch true
/chatops gitlab run feature set --group=gitlab-org,gitlab-com prevent_reopen_merge_request_without_branch true
/chatops gitlab run feature set --user=patrickbajao prevent_reopen_merge_request_without_branch true

Before global rollout

Confirm the relevant gotchas before going to 100% — see enabling a feature for GitLab.com:

• Docs + version history updated
• Breaking changes announced, if any
• Change management issue opened, if required
• External API consumers handled with a fail-open mechanism, if applicable

Cleanup

Remove the flag once deemed stable — see cleaning up. Track it here, or open a follow-up Feature Flag Cleanup issue. Remove the flag and its YAML definition from the codebase, then:

/chatops gitlab run release check <merge-request-url> <milestone>
/chatops gitlab run feature delete prevent_reopen_merge_request_without_branch --dev --pre --staging --staging-ref --production

Rollback

/chatops gitlab run feature set prevent_reopen_merge_request_without_branch false                                         # production
/chatops gitlab run feature set prevent_reopen_merge_request_without_branch false --dev --pre --staging --staging-ref     # non-production
/chatops gitlab run feature delete prevent_reopen_merge_request_without_branch --dev --pre --staging --staging-ref --production  # remove entirely