Introuce linear root method in UpdateAllMirrorsWorker

In this commit we're introducing the new linear root method
in the UpdateAllMirrorsWorker behind a feature flag.

config/feature_flags/development/linear_mirrors_worker_roots.yml

+---
+name: linear_mirrors_worker_roots
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/76735
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/348415
+milestone: '14.7'
+type: development
+group: group::authentication and authorization
+default_enabled: false

ee/app/workers/update_all_mirrors_worker.rb

@@ -117,12 +117,6 @@ class UpdateAllMirrorsWorker # rubocop:disable Scalability/IdempotentWorker
     relation = relation.where('import_state.next_execution_timestamp > ?', offset_at) if offset_at
 
     if check_mirror_plans_in_query?
-      root_namespaces_sql = Gitlab::ObjectHierarchy
-        .new(Namespace.where('id = projects.namespace_id'))
-        .roots
-        .select(:id)
-        .to_sql
-
       root_namespaces_join = "INNER JOIN namespaces AS root_namespaces ON root_namespaces.id = (#{root_namespaces_sql})"
 
       relation = relation
@@ -147,4 +141,19 @@ class UpdateAllMirrorsWorker # rubocop:disable Scalability/IdempotentWorker
   def check_mirror_plans_in_query?
     ::Gitlab::CurrentSettings.should_check_namespace_plan?
   end
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def root_namespaces_sql
+    namespace = Namespace.where('id = projects.namespace_id')
+
+    if Feature.enabled?(:linear_mirrors_worker_roots, default_enabled: :yaml)
+      namespace.roots.as_ids
+    else
+      Gitlab::ObjectHierarchy
+        .new(namespace)
+        .roots
+        .select(:id)
+    end.to_sql
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
 end

ee/spec/workers/update_all_mirrors_worker_spec.rb

@@ -187,28 +187,40 @@ RSpec.describe UpdateAllMirrorsWorker do
       let(:unlicensed_projects) { [unlicensed_project1, unlicensed_project2, unlicensed_project3, unlicensed_project4] }
 
       context 'when using SQL to filter projects' do
-        before do
-          allow(subject).to receive(:check_mirror_plans_in_query?).and_return(true)
-        end
+        shared_examples 'examples checking namespace plans' do
+          before do
+            allow(subject).to receive(:check_mirror_plans_in_query?).and_return(true)
+          end
 
-        context 'when capacity is in excess' do
-          it 'schedules all available mirrors' do
-            schedule_mirrors!(capacity: 4)
+          context 'when capacity is in excess' do
+            it 'schedules all available mirrors' do
+              schedule_mirrors!(capacity: 4)
 
-            expect_import_scheduled(licensed_project1, licensed_project2, public_project)
-            expect_import_not_scheduled(*unlicensed_projects)
+              expect_import_scheduled(licensed_project1, licensed_project2, public_project)
+              expect_import_not_scheduled(*unlicensed_projects)
+            end
+          end
+
+          context 'when capacity is exactly sufficient' do
+            it 'does not include unlicensed non-public projects in batches' do
+              # We expect that all three eligible projects will be
+              # included in the first batch because the query will only
+              # return eligible projects.
+              expect(subject).to receive(:pull_mirrors_batch).with(hash_including(batch_size: 6)).and_call_original.once
+
+              schedule_mirrors!(capacity: 3)
+            end
           end
         end
 
-        context 'when capacity is exactly sufficient' do
-          it 'does not include unlicensed non-public projects in batches' do
-            # We expect that all three eligible projects will be
-            # included in the first batch because the query will only
-            # return eligible projects.
-            expect(subject).to receive(:pull_mirrors_batch).with(hash_including(batch_size: 6)).and_call_original.once
+        it_behaves_like 'examples checking namespace plans'
 
-            schedule_mirrors!(capacity: 3)
+        context 'when feature flag ' do
+          before do
+            stub_feature_flags(linear_mirrors_worker_roots: false)
           end
+
+          it_behaves_like 'examples checking namespace plans'
         end
       end