check_then_set_with_sheep_lock.rb

class Sheep
  def initialize
    @shorn = false
    
    # Here the sheep owns the mutex. But now the
    # shearing logic is muddied up by synchronization
    # logic. This doesn't seem like the right place for
    # this.
    @mutex = Mutex.new
  end

  def shorn?
    @shorn
  end
  private :shorn?

  def shear!
    @mutex.synchronize do
      return if shorn?

      puts "shearing..."
      @shorn = true
    end
  end
end

sheep = Sheep.new

# You could create the mutex here and let
# the threads fight over it before shearing
# the sheep, but it's just not necessary.
5.times.map do
  Thread.new do
    sheep.shear!
  end
end.each(&:join)