From: Paul Brannan <pbrannan@...>
Date: 2004-08-28T13:06:31+09:00
Subject: Re: Request For Comments: exception safe ConditionVariable#wait

On Sat, Aug 28, 2004 at 06:08:39AM +0900, Bill Kelly wrote:
> I'm sorry, I'm not sure I'm following.  Does the nested
> timeout here violate any safeguards inside the revised
> ConditionVariable#wait implementation?  I realize the
> rescue Timeout::Error there is not sufficient to 
> squelch timeout exceptions from any nested/outer timeout
> block.  Your outer timeout would need to rescue its own
> exceptions as well. . . . Is that what you were referring
> to?  Or am I not understanding your example?

Your revised ConditionVariable#wait:

  def wait(mutex)
    unlocked = false
    begin
      mutex.exclusive_unlock do
        unlocked = true
        @waiters.push(Thread.current)
        Thread.stop
      end
    rescue # Timeout::Error
      @waiters.delete Thread.current  # Q: is Array#delete an atomic operation?
      raise
    ensure
      mutex.lock if unlocked
    end
  end

What happens if you get a timeout first inside the exclusive_unlock
block, then inside the rescue (before the delete occurs)?

(I believe that this is a pretty rare race condition, but is still
possible).

Paul