Introduce linear ancestors upto in GroupDescendantsFinder

app/finders/group_descendants_finder.rb

@@ -110,8 +110,13 @@ class GroupDescendantsFinder
   # rubocop: disable CodeReuse/ActiveRecord
   def ancestors_of_groups(base_for_ancestors)
     group_ids = base_for_ancestors.except(:select, :sort).select(:id)
-    Gitlab::ObjectHierarchy.new(Group.where(id: group_ids))
-      .base_and_ancestors(upto: parent_group.id)
+    groups = Group.where(id: group_ids)
+
+    if Feature.enabled?(:linear_group_descendants_finder_upto, current_user, default_enabled: :yaml)
+      groups.self_and_ancestors(upto: parent_group.id)
+    else
+      Gitlab::ObjectHierarchy.new(groups).base_and_ancestors(upto: parent_group.id)
+    end
   end
   # rubocop: enable CodeReuse/ActiveRecord
 

config/feature_flags/development/linear_group_descendants_finder_upto.yml

+---
+name: linear_group_descendants_finder_upto
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/78991
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/350972
+milestone: '14.8'
+type: development
+group: group::authentication and authorization
+default_enabled: false

spec/finders/group_descendants_finder_spec.rb

@@ -165,8 +165,8 @@ RSpec.describe GroupDescendantsFinder do
   end
 
   context 'with nested groups' do
-    let!(:project) { create(:project, namespace: group) }
-    let!(:subgroup) { create(:group, :private, parent: group) }
+    let_it_be(:project) { create(:project, namespace: group) }
+    let_it_be_with_reload(:subgroup) { create(:group, :private, parent: group) }
 
     describe '#execute' do
       it 'contains projects and subgroups' do
@@ -208,6 +208,7 @@ RSpec.describe GroupDescendantsFinder do
       context 'with a filter' do
         let(:params) { { filter: 'test' } }
 
+        shared_examples 'filter examples' do
           it 'contains only matching projects and subgroups' do
             matching_project = create(:project, namespace: group, name: 'Testproject')
             matching_subgroup = create(:group, name: 'testgroup', parent: group)
@@ -261,6 +262,17 @@ RSpec.describe GroupDescendantsFinder do
             end
           end
         end
+
+        it_behaves_like 'filter examples'
+
+        context 'when feature flag :linear_group_descendants_finder_upto is disabled' do
+          before do
+            stub_feature_flags(linear_group_descendants_finder_upto: false)
+          end
+
+          it_behaves_like 'filter examples'
+        end
+      end
     end
   end
 end