rspec foo.rb is looping in while

foo.rb

require 'rubygems'

class Foo
  attr_accessor :mutex
  attr_accessor :started
  attr_accessor :running

  def initialize
    @mutex = Mutex.new
    @started = ConditionVariable.new
  end

  def start
    @mutex.synchronize do
      while !@running
        puts @running
        @started.wait(@mutex)
      end
    end
  end

end

describe "Foo" do
  before do
    @a = Foo.new
  end

  it "must start" do
    expect(@a.mutex).to receive(:synchronize).and_yield

    once = true
    @a.stub(:running) {
      if once
        once = false
        puts "Once"
        false
      else
        puts "Twice"
        once = true
        true
      end
    }

    expect(@a.started).to receive(:wait)

    @a.start
  end
end