Add feature flag

app/models/concerns/cross_database_modification.rb

@@ -9,15 +9,27 @@ module CrossDatabaseModification
 
   class_methods do
     def transaction(**options, &block)
-      super(**options) do
-        if connection.current_transaction.respond_to?(:add_gitlab_schema) && gitlab_schema
-          connection.current_transaction.add_gitlab_schema(gitlab_schema)
-        end
+      if track_gitlab_schema_in_current_transaction?
+        super(**options) do
+          if connection.current_transaction.respond_to?(:add_gitlab_schema) && gitlab_schema
+            connection.current_transaction.add_gitlab_schema(gitlab_schema)
+          end
 
-        yield
+          yield
+        end
+      else
+        super(**options, &block)
       end
     end
 
+    def track_gitlab_schema_in_current_transaction?
+      return false unless Feature::FlipperFeature.table_exists?
+
+      Feature.enabled?(:track_gitlab_schema_in_current_transaction, default_enabled: :yaml)
+    rescue ActiveRecord::NoDatabaseError, PG::ConnectionBad
+      false
+    end
+
     def gitlab_schema
       case self.name
       when 'ActiveRecord::Base', 'ApplicationRecord'

config/feature_flags/development/track_gitlab_schema_in_current_transaction.yml

+---
+name: track_gitlab_schema_in_current_transaction
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/76717
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/349944
+milestone: '14.7'
+type: development
+group: group::sharding
+default_enabled: false

spec/models/concerns/cross_database_modification_spec.rb

@@ -4,6 +4,56 @@ require 'spec_helper'
 
 RSpec.describe CrossDatabaseModification do
   describe '.transaction' do
+    def assert_no_gitlab_schema_comment(log)
+      include_arg = [/gitlab_schema/] * log.size
+
+      expect(log).not_to include(*include_arg)
+    end
+
+    context 'feature flag disabled' do
+      before do
+        stub_feature_flags(track_gitlab_schema_in_current_transaction: false)
+      end
+
+      it 'does not add gitlab_schema comment' do
+        recorder = ActiveRecord::QueryRecorder.new do
+          ApplicationRecord.transaction do
+            Project.first
+          end
+        end
+
+        expect(recorder.log).to include(
+          /SAVEPOINT/,
+          /SELECT.*FROM "projects"/,
+          /RELEASE SAVEPOINT/
+        )
+
+        assert_no_gitlab_schema_comment(recorder.log)
+      end
+    end
+
+    context 'feature flag is not yet setup' do
+      before do
+        allow(Feature::FlipperFeature).to receive(:table_exists?).and_raise(ActiveRecord::NoDatabaseError)
+      end
+
+      it 'does not add gitlab_schema comment' do
+        recorder = ActiveRecord::QueryRecorder.new do
+          ApplicationRecord.transaction do
+            Project.first
+          end
+        end
+
+        expect(recorder.log).to include(
+          /SAVEPOINT/,
+          /SELECT.*FROM "projects"/,
+          /RELEASE SAVEPOINT/
+        )
+
+        assert_no_gitlab_schema_comment(recorder.log)
+      end
+    end
+
     it 'adds gitlab_schema to the current transaction', :aggregate_failures do
       recorder = ActiveRecord::QueryRecorder.new do
         ApplicationRecord.transaction do