Replace Group ancestors scope with linear version

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

Changelog: performance

app/models/group.rb

@@ -192,9 +192,15 @@ def preset_root_ancestor_for(groups)
     # Returns the ids of the passed group models where the `emails_disabled`
     # column is set to true anywhere in the ancestor hierarchy.
     def ids_with_disabled_email(groups)
-      innner_query = Gitlab::ObjectHierarchy
-        .new(Group.where('id = namespaces_with_emails_disabled.id'))
-        .base_and_ancestors
+      inner_groups = Group.where('id = namespaces_with_emails_disabled.id')
+
+      inner_ancestors = if Feature.enabled?(:linear_group_ancestor_scopes, default_enabled: :yaml)
+                          inner_groups.self_and_ancestors
+                        else
+                          Gitlab::ObjectHierarchy.new(inner_groups).base_and_ancestors
+                        end
+
+      inner_query = inner_ancestors
         .where(emails_disabled: true)
         .select('1')
         .limit(1)
@@ -202,7 +208,7 @@ def ids_with_disabled_email(groups)
       group_ids = Namespace
         .from('(SELECT * FROM namespaces) as namespaces_with_emails_disabled')
         .where(namespaces_with_emails_disabled: { id: groups })
-        .where('EXISTS (?)', innner_query)
+        .where('EXISTS (?)', inner_query)
         .pluck(:id)
 
       Set.new(group_ids)

config/feature_flags/development/linear_group_ancestor_scopes.yml

+---
+name: linear_group_ancestor_scopes
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70495
+rollout_issue_url: 
+milestone: '14.4'
+type: development
+group: group::access
+default_enabled: false

spec/models/group_spec.rb

@@ -2607,17 +2607,29 @@ def setup_group_members(group)
   end
 
   describe '.ids_with_disabled_email' do
-    let!(:parent_1) { create(:group, emails_disabled: true) }
-    let!(:child_1) { create(:group, parent: parent_1) }
+    let_it_be(:parent_1) { create(:group, emails_disabled: true) }
+    let_it_be(:child_1) { create(:group, parent: parent_1) }
 
-    let!(:parent_2) { create(:group, emails_disabled: false) }
-    let!(:child_2) { create(:group, parent: parent_2) }
+    let_it_be(:parent_2) { create(:group, emails_disabled: false) }
+    let_it_be(:child_2) { create(:group, parent: parent_2) }
 
-    let!(:other_group) { create(:group, emails_disabled: false) }
+    let_it_be(:other_group) { create(:group, emails_disabled: false) }
 
-    subject(:group_ids_where_email_is_disabled) { described_class.ids_with_disabled_email([child_1, child_2, other_group]) }
+    shared_examples 'returns namespaces with disabled email' do
+      subject(:group_ids_where_email_is_disabled) { described_class.ids_with_disabled_email([child_1, child_2, other_group]) }
 
-    it { is_expected.to eq(Set.new([child_1.id])) }
+      it { is_expected.to eq(Set.new([child_1.id])) }
+    end
+
+    it_behaves_like 'returns namespaces with disabled email'
+
+    context 'when feature flag :linear_group_ancestor_scopes is disabled' do
+      before do
+        stub_feature_flags(linear_group_ancestor_scopes: false)
+      end
+
+      it_behaves_like 'returns namespaces with disabled email'
+    end
   end
 
   describe '.timelogs' do