Use the for no key update lock when upadting traversal ids

Updating traversal_ids for a namespace does require an update
lock however we can use a weaker lock to avoid deadlocks
in cases where multiple projects or namespaces are created
concurently at the root namespace level due to KEY SHARE and
UPDATE locks blocking each other. NO KEY UPDATE lock
and KEY SHARE lock do not block each other.

app/models/namespace/traversal_hierarchy.rb

@@ -31,15 +31,21 @@ class Namespace
       # ActiveRecord. https://github.com/rails/rails/issues/13496
       # Ideally it would be:
       #   `incorrect_traversal_ids.update_all('traversal_ids = cte.traversal_ids')`
-      sql = """
-            UPDATE namespaces
-            SET traversal_ids = cte.traversal_ids
-            FROM (#{recursive_traversal_ids}) as cte
-            WHERE namespaces.id = cte.id
-              AND namespaces.traversal_ids::bigint[] <> cte.traversal_ids
-            """
+      sql = <<-SQL
+        UPDATE namespaces
+        SET traversal_ids = cte.traversal_ids
+        FROM (#{recursive_traversal_ids}) as cte
+        WHERE namespaces.id = cte.id
+          AND namespaces.traversal_ids::bigint[] <> cte.traversal_ids
+      SQL
+
       Namespace.transaction do
-        @root.lock!
+        if Feature.enabled?(:for_no_key_update_lock, default_enabled: :yaml)
+          @root.lock!("FOR NO KEY UPDATE")
+        else
+          @root.lock!
+        end
+
         Namespace.connection.exec_query(sql)
       end
     rescue ActiveRecord::Deadlocked

config/feature_flags/development/for_no_key_update_lock.yml

+---
+name: for_no_key_update_lock
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81239
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/353619
+milestone: '14.9'
+type: development
+group: group::workspaces
+default_enabled: false

spec/models/group_spec.rb

@@ -385,23 +385,43 @@ RSpec.describe Group do
         end
       end
 
-      before do
-        subject
-        reload_models(old_parent, new_parent, group)
-      end
-
       context 'within the same hierarchy' do
         let!(:root) { create(:group).reload }
         let!(:old_parent) { create(:group, parent: root) }
         let!(:new_parent) { create(:group, parent: root) }
 
-        it 'updates traversal_ids' do
-          expect(group.traversal_ids).to eq [root.id, new_parent.id, group.id]
+        context 'with FOR UPDATE lock' do
+          before do
+            stub_feature_flags(for_no_key_update_lock: false)
+            subject
+            reload_models(old_parent, new_parent, group)
+          end
+
+          it 'updates traversal_ids' do
+            expect(group.traversal_ids).to eq [root.id, new_parent.id, group.id]
+          end
+
+          it_behaves_like 'hierarchy with traversal_ids'
+          it_behaves_like 'locked row', 'FOR UPDATE' do
+            let(:row) { root }
+          end
         end
 
-        it_behaves_like 'hierarchy with traversal_ids'
-        it_behaves_like 'locked row' do
-          let(:row) { root }
+        context 'with FOR NO KEY UPDATE lock' do
+          before do
+            stub_feature_flags(for_no_key_update_lock: true)
+            subject
+            reload_models(old_parent, new_parent, group)
+          end
+
+          it 'updates traversal_ids' do
+            expect(group.traversal_ids).to eq [root.id, new_parent.id, group.id]
+          end
+
+          it_behaves_like 'hierarchy with traversal_ids'
+          it_behaves_like 'locked row', 'FOR NO KEY UPDATE' do
+            let(:row) { root }
+          end
         end
       end
 
@@ -410,6 +430,11 @@ RSpec.describe Group do
         let!(:new_parent) { create(:group) }
         let!(:group) { create(:group, parent: old_parent) }
 
+        before do
+          subject
+          reload_models(old_parent, new_parent, group)
+        end
+
         it 'updates traversal_ids' do
           expect(group.traversal_ids).to eq [new_parent.id, group.id]
         end
@@ -435,6 +460,11 @@ RSpec.describe Group do
         let!(:old_parent) { nil }
         let!(:new_parent) { create(:group) }
 
+        before do
+          subject
+          reload_models(old_parent, new_parent, group)
+        end
+
         it 'updates traversal_ids' do
           expect(group.traversal_ids).to eq [new_parent.id, group.id]
         end
@@ -452,6 +482,11 @@ RSpec.describe Group do
         let!(:old_parent) { create(:group) }
         let!(:new_parent) { nil }
 
+        before do
+          subject
+          reload_models(old_parent, new_parent, group)
+        end
+
         it 'updates traversal_ids' do
           expect(group.traversal_ids).to eq [group.id]
         end

spec/models/namespace/traversal_hierarchy_spec.rb

@@ -68,11 +68,24 @@ RSpec.describe Namespace::TraversalHierarchy, type: :model do
       end
     end
 
-    it_behaves_like 'locked row' do
+    it_behaves_like 'locked row', 'FOR UPDATE' do
       let(:recorded_queries) { ActiveRecord::QueryRecorder.new }
       let(:row) { root }
 
       before do
+        stub_feature_flags(for_no_key_update_lock: false)
+
+        recorded_queries.record { subject }
+      end
+    end
+
+    it_behaves_like 'locked row', 'FOR NO KEY UPDATE' do
+      let(:recorded_queries) { ActiveRecord::QueryRecorder.new }
+      let(:row) { root }
+
+      before do
+        stub_feature_flags(for_no_key_update_lock: true)
+
         recorded_queries.record { subject }
       end
     end

spec/support/shared_examples/row_lock_shared_examples.rb

@@ -4,10 +4,10 @@
 # Ensure a transaction also occurred.
 # Be careful! This form of spec is not foolproof, but better than nothing.
 
-RSpec.shared_examples 'locked row' do
+RSpec.shared_examples 'locked row' do |lock_type|
   it "has locked row" do
     table_name = row.class.table_name
-    ids_regex = /SELECT.*FROM.*#{table_name}.*"#{table_name}"."id" = #{row.id}.+FOR UPDATE/m
+    ids_regex = /SELECT.*FROM.*#{table_name}.*"#{table_name}"."id" = #{row.id}.+#{lock_type}/m
 
     expect(recorded_queries.log).to include a_string_matching 'SAVEPOINT'
     expect(recorded_queries.log).to include a_string_matching ids_regex