From: Brian Candler <b.candler@...>
Date: 2009-12-16T18:29:29+09:00
Subject: Re: Using threads to show progress

Piyush Ranjan wrote:
> In the enumerate thread keep pushing id to queue whenever there is a new
> object being worked on.
> 
>  dictated_exams.each do |exam|
>        queue.push exam.id
>  end

That's a really good suggestion: the consumer will block when it tries 
to pop from the queue, so you don't need to spin.

However in that case you can't also poll Thread#alive?, so I suggest you 
wrap the producer thread to make it send a termination message, even if 
it terminates abnormally with an exception.

    require 'thread'
    dictated_exams = ["foo","bar"]
    queue = Queue.new
    enumerate = Thread.new do
      begin
        dictated_exams.each do |exam|
          queue.push exam
          raise "Oops" if exam == "baz"
        end
      ensure
        queue.push nil
      end
    end
    while current = queue.pop
      puts current
    end
    enumerate.join   # just to propagate exception
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