Use ApplicationSetting ancestors linear scopes

In this commit we're replacing the recursive ancestors scope in
the `ApplicationSetting` model, for their linear version.

Changelog: performance
EE: true

config/feature_flags/development/linear_application_setting_ancestor_scopes.yml

+---
+name: linear_application_setting_ancestor_scopes
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70579
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/341346
+milestone: '14.4'
+type: development
+group: group::access
+default_enabled: false

ee/app/models/ee/application_setting.rb

@@ -384,11 +384,14 @@ module EE
     private
 
     def elasticsearch_limited_project_exists?(project)
-      indexed_namespaces = ::Gitlab::ObjectHierarchy
-        .new(::Namespace.where(id: project.namespace_id))
-        .base_and_ancestors
-        .joins(:elasticsearch_indexed_namespace)
-
+      project_namespaces = ::Namespace.where(id: project.namespace_id)
+      indexed_namespaces = if ::Feature.enabled?(:linear_application_setting_ancestor_scopes, default_enabled: :yaml)
+                             project_namespaces.self_and_ancestors
+                           else
+                             ::Gitlab::ObjectHierarchy.new(project_namespaces).base_and_ancestors
+                           end
+
+      indexed_namespaces = indexed_namespaces.joins(:elasticsearch_indexed_namespace)
       indexed_namespaces = ::Project.where('EXISTS (?)', indexed_namespaces)
       indexed_projects = ::Project.where('EXISTS (?)', ElasticsearchIndexedProject.where(project_id: project.id))
 

ee/spec/models/application_setting_spec.rb

@@ -427,31 +427,43 @@ RSpec.describe ApplicationSetting do
         end
 
         describe '#elasticsearch_indexes_project?' do
-          context 'when project is in a subgroup' do
-            let(:root_group) { create(:group) }
-            let(:subgroup) { create(:group, parent: root_group) }
-            let(:project) { create(:project, group: subgroup) }
-
-            before do
-              create(:elasticsearch_indexed_namespace, namespace: root_group)
+          shared_examples 'whether the project is indexed' do
+            context 'when project is in a subgroup' do
+              let(:root_group) { create(:group) }
+              let(:subgroup) { create(:group, parent: root_group) }
+              let(:project) { create(:project, group: subgroup) }
+
+              before do
+                create(:elasticsearch_indexed_namespace, namespace: root_group)
+              end
+
+              it 'allows project to be indexed' do
+                expect(setting.elasticsearch_indexes_project?(project)).to be(true)
+              end
             end
 
-            it 'allows project to be indexed' do
-              expect(setting.elasticsearch_indexes_project?(project)).to be(true)
+            context 'when project is in a namespace' do
+              let(:namespace) { create(:namespace) }
+              let(:project) { create(:project, namespace: namespace) }
+
+              before do
+                create(:elasticsearch_indexed_namespace, namespace: namespace)
+              end
+
+              it 'allows project to be indexed' do
+                expect(setting.elasticsearch_indexes_project?(project)).to be(true)
+              end
             end
           end
 
-          context 'when project is in a namespace' do
-            let(:namespace) { create(:namespace) }
-            let(:project) { create(:project, namespace: namespace) }
+          it_behaves_like 'whether the project is indexed'
 
+          context 'when feature flag :linear_application_setting_ancestor_scopes is disabled' do
             before do
-              create(:elasticsearch_indexed_namespace, namespace: namespace)
+              stub_feature_flags(linear_application_setting_ancestor_scopes: false)
             end
 
-            it 'allows project to be indexed' do
-              expect(setting.elasticsearch_indexes_project?(project)).to be(true)
-            end
+            it_behaves_like 'whether the project is indexed'
           end
         end
       end