挂起EventMachine中启动的MySQL2连接

我正在从mysql2 gem文档中运行以下代码:

require 'mysql2/em'

EM.run do
  client1 = Mysql2::EM::Client.new
  defer1 = client1.query "SELECT sleep(3) as first_query"
  defer1.callback do |result|
    puts "Result: #{result.to_a.inspect}"
  end

  client2 = Mysql2::EM::Client.new
  defer2 = client2.query "SELECT sleep(1) second_query"
  defer2.callback do |result|
    puts "Result: #{result.to_a.inspect}"
  end
end

运行良好,打印结果

Result: [{"second_query"=>0}]
Result: [{"first_query"=>0}]

但是,脚本只是挂起,再也不会返回到命令行。知道发生了什么事吗?