From: Tim Becker <a2800276@...>
Date: 2007-05-12T22:39:29+09:00
Subject: Queue, with timeout. (Thread.notify...)

Hi,

I'm trying to build a message queue that can receive messages, blocks
until messages arrive and has the ability to only block for a limited
amount of time. Unfortunately, the build-in Queue doesn't support
timeout behaviour, so I went about implementing a queue myself. I
missed java's Object.wait method, i.e. using a plain object to notify
the blocking thread to resume .

I ended up using `thread.join(timeout)` which blocks the current
thread until the thread that `join` was called on finishes (or until
timeout is reached). My queue doesn't need to join any particular
thread, though, it just needs to be notified when a new message is
added to the queue. So I needed to create a `dummy` thread solely to
provide notification. What I came up with is this:

require 'thread'

class MessageQueue
  def initialize
    @lock = Mutex.new
    @t = Thread.new{Thread.stop}  # this thread exists solely for notification
    @queue = []
  end

  def enqueue msg
    @lock.synchronize {
      @queue.push msg
      t=@t
      @t=Thread.new{Thread.stop}
      t.kill# notifiy waiting thread
    }
  end

  def dequeue timeout=nil
    msg = nil
    stop = Time.now+timeout if timeout
    while msg == nil
      @lock.synchronize {msg = @queue.shift}
      break if timeout && Time.new >= stop
      @t.join(timeout) unless msg # wait for new message
    end #while
    msg
  end

end

While this works fine, it seems like a bit of a kludge because I have
to create/misuses threads solely to provided notification to the
blocking thread.

Any thought? Am I missing an easier/build-in method to block a thread
only for a limited about of time?

Thanks,
   -tim