def default_parallel_manager
      @default_parallel_manager ||= begin
        adapter = @builder.adapter.klass if @builder.adapter

        if support_parallel?(adapter)
          adapter.setup_parallel_manager
        elsif block_given?
          yield
        end
      end
    end
def default_parallel_manager
      @default_parallel_manager ||= begin
        adapter = @builder.adapter.klass if @builder.adapter

        if support_parallel?(adapter)
          adapter.setup_parallel_manager
        elsif block_given?
          yield
        end
      end
    end
Check if the adapter is parallel-capable.

 @yield if the adapter isn't parallel-capable, or if no adapter is set yet.

 @return [Object, nil] a parallel manager or nil if yielded
 @api private