User ParticipantService linear ancestor scopes

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

Changelog: performance

app/services/projects/participants_service.rb

@@ -36,14 +36,17 @@ module Projects
     private
 
     def project_members_through_invited_groups
-      groups_with_ancestors_ids = Gitlab::ObjectHierarchy
+      groups_with_ancestors = if ::Feature.enabled?(:linear_participants_service_ancestor_scopes, current_user, default_enabled: :yaml)
+                                visible_groups.self_and_ancestors
+                              else
+                                Gitlab::ObjectHierarchy
                                   .new(visible_groups)
                                   .base_and_ancestors
-        .pluck_primary_key
+                              end
 
       GroupMember
         .active_without_invites_and_requests
-        .with_source_id(groups_with_ancestors_ids)
+        .with_source_id(groups_with_ancestors.pluck_primary_key)
     end
 
     def visible_groups

config/feature_flags/development/linear_participants_service_ancestor_scopes.yml

+---
+name: linear_participants_service_ancestor_scopes
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70684
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/341348
+milestone: '14.4'
+type: development
+group: group::access
+default_enabled: false

spec/services/projects/participants_service_spec.rb

@@ -104,6 +104,7 @@ RSpec.describe Projects::ParticipantsService do
   describe '#project_members' do
     subject(:usernames) { service.project_members.map { |member| member[:username] } }
 
+    shared_examples 'return project members' do
       context 'when there is a project in group namespace' do
         let_it_be(:public_group) { create(:group, :public) }
         let_it_be(:public_project) { create(:project, :public, namespace: public_group)}
@@ -204,4 +205,15 @@ RSpec.describe Projects::ParticipantsService do
         end
       end
     end
+
+    it_behaves_like 'return project members'
+
+    context 'when feature flag :linear_participants_service_ancestor_scopes is disabled' do
+      before do
+        stub_feature_flags(linear_participants_service_ancestor_scopes: false)
+      end
+
+      it_behaves_like 'return project members'
+    end
+  end
 end