Fix editing issues and MRs with NULL lock_version

config/initializers/config_initializers_active_record_locking.rb

+# frozen_string_literal: true
+# rubocop:disable Lint/RescueException
+module ActiveRecord
+  module Locking
+    module Optimistic
+      private
+
+      def _update_row(attribute_names, attempted_action = "update")
+        return super unless locking_enabled?
+
+        begin
+          locking_column = self.class.locking_column
+          previous_lock_value = read_attribute_before_type_cast(locking_column)
+          attribute_names << locking_column
+
+          self[locking_column] += 1
+
+          # Patched because when `lock_version` is read as `0`, it may actually be `NULL` in the DB.
+          possible_previous_lock_value = previous_lock_value == 0 ? [nil, 0] : previous_lock_value
+
+          affected_rows = self.class.unscoped._update_record(
+            arel_attributes_with_values(attribute_names),
+            self.class.primary_key => id_in_database,
+            locking_column => possible_previous_lock_value
+          )
+
+          if affected_rows != 1
+            raise ActiveRecord::StaleObjectError.new(self, attempted_action)
+          end
+
+          affected_rows
+
+        # If something went wrong, revert the locking_column value.
+        rescue Exception
+          self[locking_column] = previous_lock_value.to_i
+          raise
+        end
+      end
+    end
+  end
+end

spec/models/issue_spec.rb

@@ -55,6 +55,18 @@ describe Issue do
     end
   end
+  describe 'locking' do
+    it 'works when an issue has a NULL lock_version' do
+      issue = create(:issue)
+
+      described_class.where(id: issue.id).update_all('lock_version = NULL')
+
+      issue.update!(lock_version: 0, title: 'locking test')
+
+      expect(issue.reload.title).to eq('locking test')
+    end
+  end
+
   describe '#order_by_position_and_priority' do
     let(:project) { create :project }
     let(:p1) { create(:label, title: 'P1', project: project, priority: 1) }

spec/models/merge_request_spec.rb

@@ -31,6 +31,18 @@ describe MergeRequest do
     end
   end
+  describe 'locking' do
+    it 'works when a merge request has a NULL lock_version' do
+      merge_request = create(:merge_request)
+
+      described_class.where(id: merge_request.id).update_all('lock_version = NULL')
+
+      merge_request.update!(lock_version: 0, title: 'locking test')
+
+      expect(merge_request.reload.title).to eq('locking test')
+    end
+  end
+
   describe '#squash_in_progress?' do
     let(:repo_path) do
       Gitlab::GitalyClient::StorageSettings.allow_disk_access do