popen IO lockup with multiple threads

• Testcase included:

  yes

Running the attached program locks up in first iteration in JRuby but allows thousands of iterations in MRI.

This change works around the issue in JRuby:

— testt.rb 2008-08-21 10:18:07.000000000 +0200
+++ testt.rb~ 2008-08-21 09:03:24.000000000 +0200
@@ -2,7 +2,7 @@
analyzer = IO.popen( 'cat', IO::RDWR )
res = []

• t = Thread.new( (analyzer),res){|fd,ary|
  + t = Thread.new( (IO::for_fd analyzer.fileno),res){|fd,ary|
  while l = fd.gets do
  STDERR.putc 8
  ary.push l