From: Brian Candler <B.Candler@...>
Date: 2008-08-27T17:22:59+09:00
Subject: Re: open3 and background processes

> 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.