Use GroupTree ancestors linear scopes

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

Changelog: performance

app/controllers/concerns/group_tree.rb

@@ -38,8 +38,13 @@ module GroupTree
     #
     # Pagination needs to be applied before loading the ancestors to
     # make sure ancestors are not cut off by pagination.
-    Gitlab::ObjectHierarchy.new(Group.where(id: filtered_groups.select(:id)))
-      .base_and_ancestors
+    filtered_groups_relation = Group.where(id: filtered_groups.select(:id))
+
+    if Feature.enabled?(:linear_group_tree_ancestor_scopes, current_user, default_enabled: :yaml)
+      filtered_groups_relation.self_and_ancestors
+    else
+      Gitlab::ObjectHierarchy.new(filtered_groups_relation).base_and_ancestors
+    end
   end
   # rubocop: enable CodeReuse/ActiveRecord
 end

config/feature_flags/development/linear_group_tree_ancestor_scopes.yml

+---
+name: linear_group_tree_ancestor_scopes
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70503
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/341117
+milestone: '14.4'
+type: development
+group: group::access
+default_enabled: false

spec/controllers/concerns/group_tree_spec.rb

@@ -21,6 +21,7 @@ RSpec.describe GroupTree do
   end
 
   describe 'GET #index' do
+    shared_examples 'returns filtered groups' do
       it 'filters groups' do
         other_group = create(:group, name: 'filter')
         other_group.add_owner(user)
@@ -99,4 +100,15 @@ RSpec.describe GroupTree do
         end
       end
     end
+
+    it_behaves_like 'returns filtered groups'
+
+    context 'when feature flag :linear_group_tree_ancestor_scopes is disabled' do
+      before do
+        stub_feature_flags(linear_group_tree_ancestor_scopes: false)
+      end
+
+      it_behaves_like 'returns filtered groups'
+    end
+  end
 end