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[Feature flag] Rollout of `merge_commit_diff_modes`

Feature Issue

MR

Summary

This issue is to rollout diffing merge request commits against all parents on production, that is currently behind the merge_request_diff_modes feature flag.

LFS locks are broken when a merge commit is pushed that includes an LFS file locked by another user. Gitaly recently introduced a new flag to allow us to diff merge commits against all parents so that we only return a list of new changes in the merge commits.

In practice this means a merge commit will only return a change if it had resolved merge conflicts that weren't just picked from ours or theirs or the user manually edited/added/deleted a file and added the change to the merge commit.

Owners

Expectations

What are we expecting to happen?

Pushing changes to gitlab when push rules are in place should work as expected. Pushing merge commits with locked lfs files should now work correctly.

When is the feature viable?

When we turn on the FF and observe no errors being produced

What might happen if this goes wrong?

If we turn on the FF and we see errors we can turn off the FF and reassess the issue

What can we monitor to detect problems with this?

Consider mentioning checks for 5xx errors or other anomalies like an increase in redirects (302 HTTP response status)

https://dashboards.gitlab.net/d/stage-groups-detail-source_code/stage-groups-source-code-group-error-budget-detail?orgId=1

Errors related to the Gitlab::Checks::DiffCheck class

What can we check for monitoring production after rollouts?

Consider adding links to check for Sentry errors, Production logs for 5xx, 302s, etc.

https://new-sentry.gitlab.net/organizations/gitlab/projects/gitlabcom/?issuesType=new&project=3 https://dashboards.gitlab.net/d/stage-groups-detail-source_code/stage-groups-source-code-group-error-budget-detail?orgId=1

Rollout Steps

Note: Please make sure to run the chatops commands in the slack channel that gets impacted by the command.

Rollout on non-production environments

  • Verify the MR with the feature flag is merged to master.
  • Verify that the feature MRs have been deployed to non-production environments with:
    • /chatops run auto_deploy status 353e82ea
  • Enable the feature globally on non-production environments.
    • /chatops run feature set merge_commit_diff_modes true --dev --staging --staging-ref
    • If the feature flag causes QA end-to-end tests to fail:
      • Disable the feature flag on staging to avoid blocking deployments.
  • Verify that the feature works as expected. Posting the QA result in this issue is preferable. The best environment to validate the feature in is staging-canary as this is the first environment deployed to. Note you will need to make sure you are configured to use canary as outlined here when accessing the staging environment in order to make sure you are testing appropriately.

For assistance with QA end-to-end test failures, please reach out via the #quality Slack channel. Note that QA test failures on staging-ref don't block deployments.

Preparation before global rollout

  • Set a milestone to the rollout issue to signal for enabling and removing the feature flag when it is stable.
  • 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).
  • Leave a comment on the feature issue announcing estimated time when this feature flag 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).
  • Ensure that the feature flag rollout plan is reviewed by another developer familiar with the domain.

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_TEAM_NAME).

  • Incrementally roll out the feature.
    • Between every step wait for at least 15 minutes and monitor the appropriate graphs on https://dashboards.gitlab.net.
    • If the feature flag in code has an actor, perform actor-based rollout.
      • /chatops run feature set merge_commit_diff_modes <rollout-percentage> --actors
    • If the feature flag in code does NOT have an actor, perform time-based rollout (random rollout).
      • /chatops run feature set merge_commit_diff_modes <rollout-percentage> --random
    • Enable the feature globally on production environment.
      • /chatops run feature set merge_commit_diff_modes true
  • Observe appropriate graphs on https://dashboards.gitlab.net and verify that services are not affected.
  • Leave a comment on the feature issue announcing that the feature has been globally enabled.
  • Wait for at least one day for the verification term.

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 merge_commit_diff_modes 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 included in the release package. If the merge request was deployed before the monthly release was tagged, the feature can be officially announced in a release blog post.
    • /chatops run release check <merge-request-url> <milestone>
  • 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 merge_commit_diff_modes --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 merge_commit_diff_modes false
Edited by Joe Woodward