From: Paul Brannan <pbrannan@...>
Date: 2004-08-30T23:07:08+09:00
Subject: Re: Blocking Problem

On Sun, Aug 29, 2004 at 07:28:59AM +0900, Zachary P. Landau wrote:
> The problem is, I can't avoid blocking.  There is a GTK application, and
> then a separate thread that tries to read barcodes and whenever it finds
> one, it tells the GTK app.
> 
> So my code looks like this:
> 
> Thread.new do
>     File.open(CUECAT_DEV, File::RDONLY | File::NONBLOCK) do |cuecat|
> 	loop do
> 	    id, barcode_type, barcode = cuecat.gets.split(/,/)
> 	    # do stuff with the barcode
> 	end
>     end
> end

Some notes:

1. IIRC, #gets always has the potential to block, even if you are in
   non-blocking mode (as for why it doesn't relinquish control back to
   the GUI thread, I don't know; that's a totally different question)
2. Gtk has a method for dealing with long-running IO operations;
   essentially what you want to do is register it with Gtk using
   Gtk.input_add(io, Gdk::INPUT_READ) { ... } (I'm still using Gtk 1.2;
   this may be a little different in 2.x?)
3. This will give you a callback whenever data are available, but this
   doesn't mean an entire line is available or that multiple lines are
   not available.  So for that I use:
      data = io.sysread(512)
      @data << data
      while line = @data.slice!(/.*\n/) do
        line.strip!
        handle_line(line)
      end

To put it together, instead of starting a new thread, you can instead
use:

  cuecat = File.open(CUECAT_DEV, File::RDONLY)
  Gtk.input_add(io, Gdk::INPUT_READ) do
    data = io.sysread(512)
    @data << data
    while line = @data.slice!(/.*\n/) do
      id, barcode_type, barcode = cuecat.gets.split(/,/)
      # do stuff with the barcode
    end
  end

I think this will make your life much simpler (especially since you
don't have all those thread-safety issues to worry about).

Paul