Fix in_tail shutdown race condition problem

lib/fluent/plugin/in_tail.rb

@@ -158,7 +158,7 @@ def stop_watchers(paths, immediate = false, unwatched = false)
  if tw
  tw.unwatched = unwatched
  if immediate
- close_watcher(tw)
+ close_watcher(tw, false)
  else
  close_watcher_after_rotate_wait(tw)
  end
@@ -173,8 +173,12 @@ def update_watcher(path, pe)
  close_watcher_after_rotate_wait(rotated_tw) if rotated_tw
  end
 
- def close_watcher(tw)
- tw.close
+ # TailWatcher#close is called by another thread at shutdown phase.
+ # It causes 'can't modify string; temporarily locked' error in IOHandler
+ # so adding close_io argument to avoid this problem.
+ # At shutdown, IOHandler's io will be released automatically after detached the event loop
+ def close_watcher(tw, close_io = true)
+ tw.close(close_io)
  flush_buffer(tw)
  if tw.unwatched && @pf
  @pf[tw.path].update_pos(PositionFile::UNWATCHED_POSITION)
@@ -314,8 +318,8 @@ def detach
  @stat_trigger.detach if @stat_trigger.attached?
  end
 
- def close
- if @io_handler
+ def close(close_io = true)
+ if close_io && @io_handler
  @io_handler.on_notify
  @io_handler.close
  end