IO.for_fd behaves strangely in jruby

inotify.rb

require "rubygems"
require "ffi"

class Inotify
  module CFunctions
    extend FFI::Library
    ffi_lib FFI::Library::LIBC
    attach_function :inotify_init, [], :int
  end

  include CFunctions

  def initialize
    @fd = inotify_init
    puts "inotify_init returned: #{@fd.inspect}"

    @io = IO.for_fd(@fd)
    puts "inotify_init (through IO.for_fd.fileno) returned: #{@io.fileno.inspect}"
  end
end

inotify = Inotify.new

results

% rvm 1.8.7,1.9.2,jruby-1.5.6,jruby-1.6.0.RC2 inotify.rb

# 1.8.7
inotify_init returned: 3
inotify_init (through IO.for_fd.fileno) returned: 3

# 1.9.2
inotify_init returned: 3
inotify_init (through IO.for_fd.fileno) returned: 3

# jruby 1.5.6, crashes in IO.for_fd(8)
inotify_init returned: 8
inotify.rb:17:in `initialize': Bad file descriptor - Bad file descriptor (Errno::EBADF)
        from inotify.rb:17:in `for_fd'
        from inotify.rb:17:in `initialize'
        from inotify.rb:22:in `new'
        from inotify.rb:22

# jruby 1.6.0.RC2, IO.for_fd(5).fileno == 2? Huh?
inotify_init returned: 5
inotify_init (through IO.for_fd.fileno) returned: 2