Add merge_request_refs_cleanup feature flag

We are experiencing performance issues with the merge request
refs cleanup functionality on production and some customers
instances. This became more visible when we started backfilling
`MergeRequest::CleanupSchedule` records for old merge requests.

Disabling the worker and scheduler for now behind a feature flag
so we can test the feature once we improve it's scheduling logic
before enabling it on other instances.

app/workers/merge_request_cleanup_refs_worker.rb

@@ -7,6 +7,8 @@ class MergeRequestCleanupRefsWorker
   idempotent!
 
   def perform(merge_request_id)
+    return unless Feature.enabled?(:merge_request_refs_cleanup, default_enabled: false)
+
     merge_request = MergeRequest.find_by_id(merge_request_id)
 
     unless merge_request

app/workers/schedule_merge_request_cleanup_refs_worker.rb

@@ -16,6 +16,7 @@ class ScheduleMergeRequestCleanupRefsWorker
 
   def perform
     return if Gitlab::Database.read_only?
+    return unless Feature.enabled?(:merge_request_refs_cleanup, default_enabled: false)
 
     ids = MergeRequest::CleanupSchedule.scheduled_merge_request_ids(LIMIT).map { |id| [id] }
 

config/feature_flags/development/merge_request_refs_cleanup.yml

+---
+name: merge_request_refs_cleanup
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51558
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/296874
+milestone: '13.8'
+type: development
+group: group::code review
+default_enabled: false

spec/workers/merge_request_cleanup_refs_worker_spec.rb

@@ -17,6 +17,18 @@ RSpec.describe MergeRequestCleanupRefsWorker do
           subject
         end
       end
+
+      context 'when merge_request_refs_cleanup flag is disabled' do
+        before do
+          stub_feature_flags(merge_request_refs_cleanup: false)
+        end
+
+        it 'does not clean up the merge request' do
+          expect(MergeRequests::CleanupRefsService).not_to receive(:new)
+
+          perform_multiple(1)
+        end
+      end
     end
 
     context 'when merge request does not exist' do

spec/workers/schedule_merge_request_cleanup_refs_worker_spec.rb

@@ -20,6 +20,18 @@ RSpec.describe ScheduleMergeRequestCleanupRefsWorker do
       worker.perform
     end
 
+    context 'when merge_request_refs_cleanup flag is disabled' do
+      before do
+        stub_feature_flags(merge_request_refs_cleanup: false)
+      end
+
+      it 'does not schedule any merge request clean ups' do
+        expect(MergeRequestCleanupRefsWorker).not_to receive(:bulk_perform_in)
+
+        worker.perform
+      end
+    end
+
     include_examples 'an idempotent worker' do
       it 'schedules MergeRequestCleanupRefsWorker to be performed by batch' do
         expect(MergeRequestCleanupRefsWorker)