[Feature flag] Rollout of skip_pipelines_for_mirror_first_sync_non_default_branches

Summary

This issue tracks the rollout of the skip_pipelines_for_mirror_first_sync_non_default_branches feature flag.

When a pull mirror performs its first sync, it imports all branches and tags from the upstream repository. Previously, this triggered a pipeline for every branch and tag, causing a "pipeline explosion" that could overwhelm CI resources.

This feature flag gates the fix that ensures only the default branch triggers a pipeline on first sync, while other branches and tags are created without triggering pipelines.

Owners

  • Team: Source Code
  • Most appropriate slack channel to reach out to: #g_source_code
  • Best individual to reach out to: @kerrizor

Expectations

What are we expecting to happen?

When enabled, pull mirror first sync will:

  1. Create the default branch normally (pipeline runs)
  2. Create all other branches with skip_ci (no pipelines)
  3. Create all tags with create_pipelines: false (no pipelines)

Subsequent delta syncs continue to work normally.

What might happen if this goes wrong?

  • Worst case: Default branch pipeline doesn't run on first sync (user would need to manually trigger)
  • The fix includes proper first-sync detection, so delta syncs should be unaffected

What can we monitor to detect problems?

  • Monitor for user reports of missing pipelines after mirror setup
  • Check Sidekiq error rates for RepositoryUpdateMirrorWorker

Rollout Steps

Rollout on GitLab.com 🚀

  • Enable on GitLab.com for a small percentage of projects 
    Feature.enable_percentage_of_actors(:skip_pipelines_for_mirror_first_sync_non_default_branches, 5)
  • Monitor for errors/issues
  • Increase to 50% 
    Feature.enable_percentage_of_actors(:skip_pipelines_for_mirror_first_sync_non_default_branches, 50)
  • Monitor for errors/issues
  • Enable for 100% 
    Feature.enable(:skip_pipelines_for_mirror_first_sync_non_default_branches)

Rollout on Self-managed 🚢

  • Ensure the feature flag is removed in a future release (flag type is gitlab_com_derisk with 2-month max lifespan)

Rollback Steps

If something goes wrong:

Feature.disable(:skip_pipelines_for_mirror_first_sync_non_default_branches)

Related Links