Linear traversal query for Namespace#root_ancestor

Linear queries have better performance than recursive CTEs.

Changelog: performance

app/models/namespaces/traversal/linear.rb

@@ -64,6 +64,18 @@ module Namespaces
         traversal_ids.present?
       end
 
+      def root_ancestor
+        return super if parent.nil?
+        return super unless persisted?
+
+        return super if traversal_ids.blank?
+        return super unless Feature.enabled?(:use_traversal_ids_for_root_ancestor, default_enabled: :yaml)
+
+        strong_memoize(:root_ancestor) do
+          Namespace.find_by(id: traversal_ids.first)
+        end
+      end
+
       def self_and_descendants
         return super unless use_traversal_ids?
 
@@ -100,7 +112,8 @@ module Namespaces
         # Clear any previously memoized root_ancestor as our ancestors have changed.
         clear_memoization(:root_ancestor)
 
-        Namespace::TraversalHierarchy.for_namespace(root_ancestor).sync_traversal_ids!
+        # We cannot rely on Namespaces::Traversal::Linear#root_ancestor because it might be stale
+        Namespace::TraversalHierarchy.for_namespace(recursive_root_ancestor).sync_traversal_ids!
       end
 
       # Lock the root of the hierarchy we just left, and lock the root of the hierarchy

app/models/namespaces/traversal/recursive.rb

@@ -16,6 +16,7 @@ module Namespaces
           parent.root_ancestor
         end
       end
+      alias_method :recursive_root_ancestor, :root_ancestor
 
       # Returns all ancestors, self, and descendants of the current namespace.
       def self_and_hierarchy

app/services/groups/transfer_service.rb

@@ -108,10 +108,13 @@ module Groups
 
       @group.parent = @new_parent_group
       @group.clear_memoization(:self_and_ancestors_ids)
+      @group.clear_memoization(:root_ancestor) if different_root_ancestor?
 
       inherit_group_shared_runners_settings
 
       @group.save!
+      # #reload is called to make sure traversal_ids are reloaded
+      @group.reload # rubocop:disable Cop/ActiveRecordAssociationReload
     end
 
     # rubocop: disable CodeReuse/ActiveRecord

config/feature_flags/development/use_traversal_ids_for_root_ancestor.yml

+---
+name: use_traversal_ids_for_root_ancestor
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61163
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/331907
+milestone: '14.0'
+type: development
+group: group::access
+default_enabled: false

spec/support/shared_examples/namespaces/traversal_examples.rb

@@ -17,6 +17,28 @@ RSpec.shared_examples 'namespace traversal' do
     end
   end
 
+  describe '#root_ancestor' do
+    let_it_be(:group) { create(:group) }
+    let_it_be(:nested_group) { create(:group, parent: group) }
+    let_it_be(:deep_nested_group) { create(:group, parent: nested_group) }
+
+    it 'returns the correct root ancestor' do
+      expect(group.root_ancestor).to eq(group)
+      expect(nested_group.root_ancestor).to eq(group)
+      expect(deep_nested_group.root_ancestor).to eq(group)
+    end
+
+    describe '#recursive_root_ancestor' do
+      let(:groups) { [group, nested_group, deep_nested_group] }
+
+      it "is equivalent to #recursive_root_ancestor" do
+        groups.each do |group|
+          expect(group.root_ancestor).to eq(group.recursive_root_ancestor)
+        end
+      end
+    end
+  end
+
   describe '#self_and_hierarchy' do
     let!(:group) { create(:group, path: 'git_lab') }
     let!(:nested_group) { create(:group, parent: group) }