class BlockingMatcher
  def matches?(block)
    started = false
    blocking = true

    thread = Thread.new do
      started = true
      block.call

      blocking = false
    end

    while !started and status = thread.status and status != "sleep"
      Thread.pass
    end
    thread.kill
    thread.join

    blocking
  end

  def failure_message
    ['Expected the given Proc', 'to block the caller']
  end

  def negative_failure_message
    ['Expected the given Proc', 'to not block the caller']
  end
end

class Object
  def block_caller(timeout = 0.1)
    BlockingMatcher.new
  end
end