class ActiveRecord::Migration::CheckPending

def call(env)
  @mutex.synchronize do
    @watcher ||= build_watcher do
      @needs_check = true
      ActiveRecord::Migration.check_pending!(connection)
      @needs_check = false
    end
    if @needs_check
      @watcher.execute
    else
      @watcher.execute_if_updated
    end
  end
  @app.call(env)
end