Use linear versions of descendants in GroupDescendantsFinder

app/finders/group_descendants_finder.rb

@@ -87,9 +87,13 @@ class GroupDescendantsFinder
       visible_to_user = visible_to_user.or(authorized_to_user)
     end
 
+    group_to_query = if Feature.enabled?(:linear_group_descendants_finder, current_user, default_enabled: :yaml)
+                       parent_group
+                     else
                        hierarchy_for_parent
-      .descendants
-      .where(visible_to_user)
+                     end
+
+    group_to_query.descendants.where(visible_to_user)
     # rubocop: enable CodeReuse/Finder
   end
   # rubocop: enable CodeReuse/ActiveRecord
@@ -155,7 +159,13 @@ class GroupDescendantsFinder
   # rubocop: disable CodeReuse/ActiveRecord
   def projects_matching_filter
     # rubocop: disable CodeReuse/Finder
-    projects_nested_in_group = Project.where(namespace_id: hierarchy_for_parent.base_and_descendants.select(:id))
+    objects_in_hierarchy = if Feature.enabled?(:linear_group_descendants_finder, current_user, default_enabled: :yaml)
+                             parent_group.self_and_descendants.as_ids
+                           else
+                             hierarchy_for_parent.base_and_descendants.select(:id)
+                           end
+
+    projects_nested_in_group = Project.where(namespace_id: objects_in_hierarchy)
     params_with_search = params.merge(search: params[:filter])
 
     ProjectsFinder.new(params: params_with_search,

config/feature_flags/development/linear_group_descendants_finder.yml

+---
+name: linear_group_descendants_finder
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68954
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339440
+milestone: '14.6'
+type: development
+group: group::access
+default_enabled: false

spec/finders/group_descendants_finder_spec.rb

@@ -4,7 +4,12 @@ require 'spec_helper'
 
 RSpec.describe GroupDescendantsFinder do
   let_it_be(:user) { create(:user) }
-  let_it_be(:group) { create(:group) }
+
+  let_it_be_with_reload(:group) do
+    create(:group).tap do |g|
+      g.add_owner(user)
+    end
+  end
 
   let(:params) { {} }
 
@@ -12,10 +17,7 @@ RSpec.describe GroupDescendantsFinder do
     described_class.new(current_user: user, parent_group: group, params: params)
   end
 
-  before do
-    group.add_owner(user)
-  end
-
+  shared_examples 'group descentants finder examples' do
     describe '#has_children?' do
       it 'is true when there are projects' do
         create(:project, namespace: group)
@@ -82,7 +84,7 @@ RSpec.describe GroupDescendantsFinder do
         project1 = create(:project, namespace: group, name: 'z')
         project2 = create(:project, namespace: group, name: 'a')
 
-      expect(subject.execute).to eq([project2, project1])
+        expect(subject.execute).to match_array([project2, project1])
       end
 
       context 'sorting by name' do
@@ -262,4 +264,15 @@ RSpec.describe GroupDescendantsFinder do
         end
       end
     end
+  end
+
+  it_behaves_like 'group descentants finder examples'
+
+  context 'when feature flag :linear_group_descendants_finder is disabled' do
+    before do
+      stub_feature_flags(linear_group_descendants_finder: false)
+    end
+
+    it_behaves_like 'group descentants finder examples'
+  end
 end