Namespace#self_and_hierarchy linear and recursive scopes

Provide a linear version of:

  Gitlab::ObjectHierarchy.new(some_groups).all_objects

We also create a recursive + linear scope which is a more friendly
developer interface.

app/models/namespaces/traversal/linear_scopes.rb

@@ -75,6 +75,12 @@ module Namespaces
           end
         end
 
+        def self_and_hierarchy
+          return super unless use_traversal_ids_for_self_and_hierarchy_scopes?
+
+          unscoped.from_union([all.self_and_ancestors, all.self_and_descendants(include_self: false)])
+        end
+
         def order_by_depth(hierarchy_order)
           return all unless hierarchy_order
 
@@ -114,6 +120,11 @@ module Namespaces
           use_traversal_ids?
         end
 
+        def use_traversal_ids_for_self_and_hierarchy_scopes?
+          Feature.enabled?(:use_traversal_ids_for_self_and_hierarchy_scopes, default_enabled: :yaml) &&
+            use_traversal_ids?
+        end
+
         def self_and_descendants_with_comparison_operators(include_self: true)
           base = all.select(
             :traversal_ids,

app/models/namespaces/traversal/recursive_scopes.rb

@@ -53,6 +53,11 @@ module Namespaces
           self_and_descendants(include_self: include_self).as_ids
         end
         alias_method :recursive_self_and_descendant_ids, :self_and_descendant_ids
+
+        def self_and_hierarchy
+          Gitlab::ObjectHierarchy.new(all).all_objects
+        end
+        alias_method :recursive_self_and_hierarchy, :self_and_hierarchy
       end
     end
   end

config/feature_flags/development/use_traversal_ids_for_self_and_hierarchy_scopes.yml

+---
+name: use_traversal_ids_for_self_and_hierarchy_scopes
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80045
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/352120
+milestone: '14.8'
+type: development
+group: group::workspace
+default_enabled: false

spec/support/shared_examples/namespaces/traversal_scope_examples.rb

@@ -299,4 +299,51 @@ RSpec.shared_examples 'namespace traversal scopes' do
       include_examples '.self_and_descendant_ids'
     end
   end
+
+  shared_examples '.self_and_hierarchy' do
+    let(:base_scope) { Group.where(id: base_groups) }
+
+    subject { base_scope.self_and_hierarchy }
+
+    context 'with ancestors only' do
+      let(:base_groups) { [group_1, group_2] }
+
+      it { is_expected.to match_array(groups) }
+    end
+
+    context 'with descendants only' do
+      let(:base_groups) { [deep_nested_group_1, deep_nested_group_2] }
+
+      it { is_expected.to match_array(groups) }
+    end
+
+    context 'nodes with both ancestors and descendants' do
+      let(:base_groups) { [nested_group_1, nested_group_2] }
+
+      it { is_expected.to match_array(groups) }
+    end
+
+    context 'with duplicate base groups' do
+      let(:base_groups) { [nested_group_1, nested_group_1] }
+
+      it { is_expected.to contain_exactly(group_1, nested_group_1, deep_nested_group_1) }
+    end
+  end
+
+  describe '.self_and_hierarchy' do
+    it_behaves_like '.self_and_hierarchy'
+
+    context "use_traversal_ids_for_self_and_hierarchy_scopes feature flag is false" do
+      before do
+        stub_feature_flags(use_traversal_ids_for_self_and_hierarchy_scopes: false)
+      end
+
+      it_behaves_like '.self_and_hierarchy'
+
+      it 'make recursive queries' do
+        base_groups = Group.where(id: nested_group_1)
+        expect { base_groups.self_and_hierarchy.load }.to make_queries_matching(/WITH RECURSIVE/)
+      end
+    end
+  end
 end