ruby sync io bug on fbsd + osx, 1.8

sync_io_bug.rb

# This bug seems to occur on 1.8.6..1.9 on OSX and FreeBSD. It's known not to occur on Linux.

require 'rubygems'
require 'minitest/unit'
include MiniTest::Assertions

require 'tempfile'

path = nil
begin
  Tempfile.open('testing.sync.pos') do |f| # Has been tested without tempfile.
    path = f.path
  end

  open(path, 'a+') do |f| # Has been tested with w+, etc.
    assert_equal 0, f.pos
    f.write('a')
    assert_equal 1, f.pos
    f.write('b')
    assert_equal 2, f.pos
    f.sync = true
    assert_equal 2, f.pos
    f.write('c')
    assert_equal 3, f.pos # Bug returns 2 here
  end
ensure
  File.delete(path) if File.exists?(path)
end