DRb::DRbServer#shutdown

# File tmp/rubies/ruby-3.0.5/lib/drb/drb.rb, line 1539
def shutdown
  current = Thread.current
  if @protocol.respond_to? :shutdown
    @protocol.shutdown
  else
    [@thread, *@grp.list].each { |thread|
      thread.kill unless thread == current # xxx: Thread#kill
    }
  end
  @thread.join unless @thread == current
end