From: Daniel Berger <djberg96@...>
Date: 2008-08-27T23:16:25+09:00
Subject: Re: open3 and background processes



On Aug 27, 2:22 am, Brian Candler <B.Cand...@pobox.com> wrote:
> > I
> > thought there was a way to peek ahead on an IO object to see if any
> > data is available on the handle before attempting to read it
>
> You're probably thinking of Kernel.select. To poll if data is available for
> read on IO object foo, and read as much data is available, up to 1024 bytes
> max:
>
>   if select([foo], nil, nil, 0)
>     res = foo.readpartial(1024)
>   end
>
> (select also returns true if the file is closed, in which case read will
> return nil)
>
> Beware of race conditions in your code - i.e. the child might not generate
> output on stderr until some time later, so you may need to continue to poll
> periodically.

This works, though I had to add a sleep call to give it time to pick
up an error:

require 'open3'
require 'timeout'

program = File.join(Dir.pwd, "miniserver.rb")

cmd = "ruby #{program} &"

Open3.popen3(cmd) do |stdin, stdout, stderr|
   sleep 0.1

   if select([stderr], nil, nil, 0)
      p stderr.readlines
   end
end

puts "Done"

This approach actually makes me less comfortable than my original
approach, however, because now I'll have to worry that I didn't sleep
long enough and an error will go unnoticed.

Regards,

Dan