Add store_mentions_without_subtransaction FF

Adds a feature flag to the optimization done for store_mentions!

app/models/concerns/cache_markdown_field.rb

@@ -160,6 +160,39 @@ module CacheMarkdownField
     # We can only store mentions if the mentionable is a database object
     return unless self.is_a?(ApplicationRecord)
 
+    return store_mentions_without_subtransaction! if Feature.enabled?(:store_mentions_without_subtransaction, default_enabled: :yaml)
+
+    refs = all_references(self.author)
+
+    references = {}
+    references[:mentioned_users_ids] = refs.mentioned_user_ids.presence
+    references[:mentioned_groups_ids] = refs.mentioned_group_ids.presence
+    references[:mentioned_projects_ids] = refs.mentioned_project_ids.presence
+
+    # One retry is enough as next time `model_user_mention` should return the existing mention record,
+    # that threw the `ActiveRecord::RecordNotUnique` exception in first place.
+    self.class.safe_ensure_unique(retries: 1) do
+      user_mention = model_user_mention
+
+      # this may happen due to notes polymorphism, so noteable_id may point to a record
+      # that no longer exists as we cannot have FK on noteable_id
+      break if user_mention.blank?
+
+      user_mention.mentioned_users_ids = references[:mentioned_users_ids]
+      user_mention.mentioned_groups_ids = references[:mentioned_groups_ids]
+      user_mention.mentioned_projects_ids = references[:mentioned_projects_ids]
+
+      if user_mention.has_mentions?
+        user_mention.save!
+      else
+        user_mention.destroy!
+      end
+    end
+
+    true
+  end
+
+  def store_mentions_without_subtransaction!
     identifier = user_mention_identifier
 
     # this may happen due to notes polymorphism, so noteable_id may point to a record

app/models/concerns/mentionable.rb

@@ -217,6 +217,17 @@ module Mentionable
   def user_mention_association
     association(:user_mentions).reflection
   end
+
+  # User mention that is parsed from model description rather then its related notes.
+  # Models that have a description attribute like Issue, MergeRequest, Epic, Snippet may have such a user mention.
+  # Other mentionable models like Commit, DesignManagement::Design, will never have such record as those do not have
+  # a description attribute.
+  #
+  # Using this method followed by a call to *save* may result in *ActiveRecord::RecordNotUnique* exception
+  # in a multi-threaded environment. Make sure to use it within a *safe_ensure_unique* block.
+  def model_user_mention
+    user_mentions.where(note_id: nil).first_or_initialize
+  end
 end
 
 Mentionable.prepend_mod_with('Mentionable')

app/models/note.rb

@@ -603,6 +603,14 @@ class Note < ApplicationRecord
 
   private
 
+  # Using this method followed by a call to *save* may result in *ActiveRecord::RecordNotUnique* exception
+  # in a multi-threaded environment. Make sure to use it within a *safe_ensure_unique* block.
+  def model_user_mention
+    return if user_mentions.is_a?(ActiveRecord::NullRelation)
+
+    user_mentions.first_or_initialize
+  end
+
   def system_note_viewable_by?(user)
     return true unless system_note_metadata
 

config/feature_flags/development/store_mentions_without_subtransaction.yml

+---
+name: store_mentions_without_subtransaction
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68433
+rollout_issue_url: 
+milestone: '14.3'
+type: development
+group: group::project management
+default_enabled: false

spec/support/shared_examples/models/mentionable_shared_examples.rb

@@ -207,7 +207,7 @@ RSpec.shared_examples 'an editable mentionable' do
 end
 
 RSpec.shared_examples 'mentions in description' do |mentionable_type|
-  describe 'when storing user mentions' do
+  shared_examples 'when storing user mentions' do
     before do
       mentionable.store_mentions!
     end
@@ -238,10 +238,26 @@ RSpec.shared_examples 'mentions in description' do |mentionable_type|
       end
     end
   end
+
+  context 'when store_mentions_without_subtransaction is enabled' do
+    before do
+      stub_feature_flags(store_mentions_without_subtransaction: true)
+    end
+
+    it_behaves_like 'when storing user mentions'
+  end
+
+  context 'when store_mentions_without_subtransaction is disabled' do
+    before do
+      stub_feature_flags(store_mentions_without_subtransaction: false)
+    end
+
+    it_behaves_like 'when storing user mentions'
+  end
 end
 
 RSpec.shared_examples 'mentions in notes' do |mentionable_type|
-  context 'when mentionable notes contain mentions' do
+  shared_examples 'when mentionable notes contain mentions' do
     let(:user) { create(:user) }
     let(:user2) { create(:user) }
     let(:group) { create(:group) }
@@ -261,6 +277,22 @@ RSpec.shared_examples 'mentions in notes' do |mentionable_type|
       expect(mentionable.referenced_groups(user)).to eq [group]
     end
   end
+
+  context 'when store_mentions_without_subtransaction is enabled' do
+    before do
+      stub_feature_flags(store_mentions_without_subtransaction: true)
+    end
+
+    it_behaves_like 'when mentionable notes contain mentions'
+  end
+
+  context 'when store_mentions_without_subtransaction is disabled' do
+    before do
+      stub_feature_flags(store_mentions_without_subtransaction: false)
+    end
+
+    it_behaves_like 'when mentionable notes contain mentions'
+  end
 end
 
 RSpec.shared_examples 'load mentions from DB' do |mentionable_type|