test.rb

require 'thread'

mutex = Mutex.new

10000.times do |i|
  r,w = IO.pipe

  t = Thread.new {
    begin
      loop do
        c = r.getc
        break if c == '!'
      end
      # you need this, so you don't shut down the socket prior to print being fully handled
      mutex.synchronize do
        r.close
        w.close
      end
    rescue => e
      p e
    end
  }

  mutex.synchronize do
    w.print '!'
  end

  t.join
end