check_then_set_with_sheep_lock.rb

# http://www.rubyinside.com/does-the-gil-make-your-ruby-code-thread-safe-6051.html

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)
#---------------------------------------
# Better way to do sychonization and avoid race conditions. Use a queue:

class Sheep
  def initialize
    @shorn = false
  end

  def shorn?
    @shorn
  end

  def shear!
    return if shorn?
    puts "shearing..."
    @shorn = true
  end
end

sheep = Sheep.new
sheep_queue = Queue.new
sheep_queue << sheep

5.times.map do
  Thread.new do
    begin
      sheep = sheep_queue.pop(true)
      sheep.shear!
    rescue ThreadError
    end
  end
end.each(&:join)