Use linear version of User#manageable_groups

Changelog: performance

app/models/user.rb

@@ -1550,7 +1550,11 @@ class User < ApplicationRecord
   end
 
   def manageable_groups(include_groups_with_developer_maintainer_access: false)
-    owned_and_maintainer_group_hierarchy = Gitlab::ObjectHierarchy.new(owned_or_maintainers_groups).base_and_descendants
+    owned_and_maintainer_group_hierarchy = if Feature.enabled?(:linear_user_manageable_groups, self, default_enabled: :yaml)
+                                             owned_or_maintainers_groups.self_and_descendants
+                                           else
+                                             Gitlab::ObjectHierarchy.new(owned_or_maintainers_groups).base_and_descendants
+                                           end
 
     if include_groups_with_developer_maintainer_access
       union_sql = ::Gitlab::SQL::Union.new(

config/feature_flags/development/linear_user_manageable_groups.yml

+---
+name: linear_user_manageable_groups
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68845
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339434
+milestone: '14.3'
+type: development
+group: group::access
+default_enabled: false

spec/models/user_spec.rb

@@ -1755,14 +1755,26 @@ RSpec.describe User do
       end
 
       describe '#manageable_groups' do
-        it 'includes all the namespaces the user can manage' do
-          expect(user.manageable_groups).to contain_exactly(group, subgroup)
+        shared_examples 'manageable groups examples' do
+          it 'includes all the namespaces the user can manage' do
+            expect(user.manageable_groups).to contain_exactly(group, subgroup)
+          end
+
+          it 'does not include duplicates if a membership was added for the subgroup' do
+            subgroup.add_owner(user)
+
+            expect(user.manageable_groups).to contain_exactly(group, subgroup)
+          end
         end
 
-        it 'does not include duplicates if a membership was added for the subgroup' do
-          subgroup.add_owner(user)
+        it_behaves_like 'manageable groups examples'
+
+        context 'when feature flag :linear_user_manageable_groups is disabled' do
+          before do
+            stub_feature_flags(linear_user_manageable_groups: false)
+          end
 
-          expect(user.manageable_groups).to contain_exactly(group, subgroup)
+          it_behaves_like 'manageable groups examples'
         end
       end