class Fluent::Plugin::TailInput::TailWatcher

def on_rotate(stat)
  if @io_handler.nil?
    if stat
      # first time
      fsize = stat.size
      inode = stat.ino
      last_inode = @pe.read_inode
      if inode == last_inode
        # rotated file has the same inode number with the last file.
        # assuming following situation:
        #   a) file was once renamed and backed, or
        #   b) symlink or hardlink to the same file is recreated
        # in either case of a and b, seek to the saved position
        #   c) file was once renamed, truncated and then backed
        # in this case, consider it truncated
        @pe.update(inode, 0) if fsize < @pe.read_pos
      elsif last_inode != 0
        # this is FilePositionEntry and fluentd once started.
        # read data from the head of the rotated file.
        # logs never duplicate because this file is a rotated new file.
        @pe.update(inode, 0)
      else
        # this is MemoryPositionEntry or this is the first time fluentd started.
        # seek to the end of the any files.
        # logs may duplicate without this seek because it's not sure the file is
        # existent file or rotated new file.
        pos = @read_from_head ? 0 : fsize
        @pe.update(inode, pos)
      end
      @io_handler = IOHandler.new(self, &method(:wrap_receive_lines))
    else
      @io_handler = NullIOHandler.new
    end
  else
    watcher_needs_update = false
    if stat
      inode = stat.ino
      if inode == @pe.read_inode # truncated
        @pe.update_pos(0)
        @io_handler.close
      elsif !@io_handler.opened? # There is no previous file. Reuse TailWatcher
        @pe.update(inode, 0)
      else # file is rotated and new file found
        watcher_needs_update = true
        # Handle the old log file before renewing TailWatcher [fluentd#1055]
        @io_handler.on_notify
      end
    else # file is rotated and new file not found
      # Clear RotateHandler to avoid duplicated file watch in same path.
      @rotate_handler = nil
      watcher_needs_update = true
    end
    log_msg = "detected rotation of #{@path}"
    log_msg << "; waiting #{@rotate_wait} seconds" if watcher_needs_update # wait rotate_time if previous file exists
    @log.info log_msg
    if watcher_needs_update
      @update_watcher.call(@path, swap_state(@pe))
    else
      @io_handler = IOHandler.new(self, &method(:wrap_receive_lines))
    end
  end
end