rails--rails/railties/lib/commands/servers/base.rb

def tail(log_file)
  cursor = File.size(log_file)
  last_checked = Time.now
  tail_thread = Thread.new do
    File.open(log_file, 'r') do |f|
      loop do
        f.seek cursor
        if f.mtime > last_checked
          last_checked = f.mtime
          contents = f.read
          cursor += contents.length
          print contents
        end
        sleep 1
      end
    end
  end
  tail_thread
end