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[Feature flag] Enable display_merge_conflicts_in_diff

Summary

This issue is to rollout the feature on production, that is currently behind the display_merge_conflicts_in_diff feature flag.

Owners

  • Team: groupcode review
  • Most appropriate slack channel to reach out to: #g_create_code-review
  • Best individuals to reach out to: @garyh, @iamphill, @patrickbajao
  • PM: @phikai

Stakeholders

Expectations

What are we expecting to happen?

When MR has conflicts, they'll be displayed on the diff under the "Changes" tab of the MR page.

When is the feature viable?

N/A

What might happen if this goes wrong?

  • The conflicts displayed on the MR diff is wrong or the HEAD diff wasn't generated.
  • MergeRequestMergeabilityCheckWorker execution times/error rate is going up

If these happen, disable the feature flag.

What can we monitor to detect problems with this?

What can we check for monitoring production after rollouts?

https://dashboards.gitlab.net/d/web-rails-controller/web-rails-controller?orgId=1&var-PROMETHEUS_DS=Global&var-environment=gprd&var-stage=main&var-controller=Projects::MergeRequests::DiffsController&var-action=diffs_batch.json&from=now-1h&to=now

Rollout Steps

Rollout on non-production environments

  • Ensure that the feature MRs have been deployed to non-production environments.
    • /chatops run auto_deploy status <merge-commit-of-your-feature>
  • Enable the feature globally on non-production environments.
    • /chatops run feature set display_merge_conflicts_in_diff true --dev
    • /chatops run feature set display_merge_conflicts_in_diff true --staging
  • Verify that the feature works as expected. Posting the QA result in this issue is preferable.

Specific rollout on production

  • Ensure that the feature MRs have been deployed to both production and canary.
    • /chatops run auto_deploy status <merge-commit-of-your-feature>
  • If you're using project-actor, you must enable the feature on these entries:
    • /chatops run feature set --project=gitlab-org/gitlab,gitlab-org/gitlab-foss,gitlab-com/www-gitlab-com display_merge_conflicts_in_diff true
  • Verify that the feature works on the specific entries. Posting the QA result in this issue is preferable.

Preparation before global rollout

  • Check if the feature flag change needs to be accompanied with a change management issue. Cross link the issue here if it does.
  • Ensure that you or a representative in development can be available for at least 2 hours after feature flag updates in production. If a different developer will be covering, or an exception is needed, please inform the oncall SRE by using the @sre-oncall Slack alias.
  • Ensure that documentation has been updated (More info).
  • Announce on the feature issue an estimated time this will be enabled on GitLab.com.
  • Ensure that any breaking changes have been announced following the release post process to ensure GitLab customers are aware.
  • Notify #support_gitlab-com and your team channel (more guidance when this is necessary in the dev docs).

Global rollout on production

For visibility, all /chatops commands that target production should be executed in the #production slack channel and cross-posted (with the command results) to the responsible team's slack channel (#g_create_code-review).

  • Incrementally roll out the feature.
    • If the feature flag in code has an actor, perform actor-based rollout.
      • /chatops run feature set display_merge_conflicts_in_diff 25 --actors
      • /chatops run feature set display_merge_conflicts_in_diff 50 --actors
      • /chatops run feature set display_merge_conflicts_in_diff 75 --actors
      • /chatops run feature set display_merge_conflicts_in_diff 100 --actors
    • Enable the feature globally on production environment.
      • /chatops run feature set display_merge_conflicts_in_diff true
  • Announce on the feature issue that the feature has been globally enabled.
  • Wait for at least one day for the verification term.

(Optional) Release the feature with the feature flag

If you're still unsure whether the feature is deemed stable but want to release it in the current milestone, you can change the default state of the feature flag to be enabled. To do so, follow these steps:

  • Create a merge request with the following changes. Ask for review and merge it.
  • Ensure that the default-enabling MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
    • /chatops run auto_deploy status <merge-commit-of-default-enabling-mr>
  • Close the feature issue to indicate the feature will be released in the current milestone.
  • Set the next milestone to this rollout issue for scheduling the flag removal.
  • (Optional) You can create a separate issue for scheduling the steps below to Release the feature.
    • Set the title to "[Feature flag] Cleanup display_merge_conflicts_in_diff".
    • Execute the /copy_metadata <this-rollout-issue-link> quick action to copy the labels from this rollout issue.
    • Link this rollout issue as a related issue.
    • Close this rollout issue.

WARNING: This approach has the downside that it makes it difficult for us to clean up the flag. For example, on-premise users could disable the feature on their GitLab instance. But when you remove the flag at some point, they suddenly see the feature as enabled and they can't roll it back to the previous behavior. To avoid this potential breaking change, use this approach only for urgent matters.

Release the feature

After the feature has been deemed stable, the clean up should be done as soon as possible to permanently enable the feature and reduce complexity in the codebase.

You can either create a follow-up issue for Feature Flag Cleanup or use the checklist below in this same issue.

  • Create a merge request to remove display_merge_conflicts_in_diff feature flag. Ask for review and merge it.
    • Remove all references to the feature flag from the codebase.
    • Remove the YAML definitions for the feature from the repository.
    • Create a changelog entry.
  • Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
    • /chatops run auto_deploy status <merge-commit-of-cleanup-mr>
  • Close the feature issue to indicate the feature will be released in the current milestone.
  • Clean up the feature flag from all environments by running these chatops command in #production channel:
    • /chatops run feature delete display_merge_conflicts_in_diff --dev --staging --staging-ref --production
  • Close this rollout issue.

Rollback Steps

  • This feature can be disabled by running the following Chatops command:
/chatops run feature set display_merge_conflicts_in_diff false

Testing

Integration test or E2E test to be added to view conflict on Changes tab. SET: @jay_mccure

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Edited by Patrick Bajao