Merge branch 'fix_ignore_columns_race_condition' into 'master'

Fix the (potential) race-condition issue on `IgnorableColumns`

See merge request gitlab-org/gitlab!49163

app/models/concerns/ignorable_columns.rb

@@ -31,15 +31,13 @@ module IgnorableColumns
     alias_method :ignore_column, :ignore_columns
 
     def ignored_columns_details
-      unless defined?(@ignored_columns_details)
-        IGNORE_COLUMN_MUTEX.synchronize do
-          @ignored_columns_details ||= superclass.try(:ignored_columns_details)&.dup || {}
-        end
-      end
+      return @ignored_columns_details if defined?(@ignored_columns_details)
 
-      @ignored_columns_details
+      IGNORE_COLUMN_MONITOR.synchronize do
+        @ignored_columns_details ||= superclass.try(:ignored_columns_details)&.dup || {}
+      end
     end
 
-    IGNORE_COLUMN_MUTEX = Mutex.new
+    IGNORE_COLUMN_MONITOR = Monitor.new
   end
 end

spec/models/concerns/ignorable_columns_spec.rb

@@ -59,6 +59,14 @@ RSpec.describe IgnorableColumns do
       it_behaves_like 'storing removal information'
     end
 
+    context 'when called on a subclass without setting the ignored columns' do
+      let(:subclass) { Class.new(record_class) }
+
+      it 'does not raise Deadlock error' do
+        expect { subclass.ignored_columns_details }.not_to raise_error
+      end
+    end
+
     it 'defaults to empty Hash' do
       expect(subject.ignored_columns_details).to eq({})
     end