in_tail: add warning for silent stop on !follow_inodes case

[daipom]

Which issue(s) this PR fixes:
This PR just adds a warning log for the following issue's !follow_inodes case.

• Fluentd in_tail "unreadable" file causes "following tail of <file>" to stop and no logs pushed #3614

I believe this problem has been resolved with the next PR.

• in_tail: Fix a stall bug on !follow_inode case #4327

This PR adds a warning log just in case in order to prevent silent collection stops like that.

What this PR does / why we need it:
For refresh_watcher, if an exisiting TailWatcher already follows a target path with a different inode, it means that the TailWatcher following the rotated file still exists.

In this case, refresh_watcher can't start the new TailWatcher for the new current file.
So, we should output a warning log in order to prevent silent collection stops, such as #4327.

The warning log example:

Could not follow a file (inode: 101584481) because an existing watcher for that filepath
follows a different inode: 101584480 (e.g. keeps watching a already rotated file).
If you keep getting this message, please restart Fluentd. filepath="/test/fluentd/input4/test.log"

A similar warning may work for follow_inodes too. Just limiting the case to suppress the impact to existing logic.

Docs Changes:
Not needed. (This adds a warning log only.)

Release Note:
Same as the title.

[daipom]

If reverting #4327 and reproducing #3614, Fluentd outputs the following logs.

2023-11-10 08:20:59 +0900 [info]: #0 fluent/log.rb:362:info: detected rotation of /test/fluentd/input4/test.log; waiting 5 seconds
2023-11-10 08:20:59 +0900 [info]: #0 fluent/log.rb:362:info: following tail of /test/fluentd/input4/test.log
2023-11-10 08:20:59.276009558 +0900 test: {"message":"aaa"}
2023-11-10 08:20:59.480580739 +0900 test: {"message":"foo"}
2023-11-10 08:21:00.484049606 +0900 test: {"message":"foo"}
2023-11-10 08:21:01.487598040 +0900 test: {"message":"foo"}
2023-11-10 08:21:02.491227841 +0900 test: {"message":"foo"}
2023-11-10 08:21:03.494664049 +0900 test: {"message":"foo"}
2023-11-10 08:21:03 +0900 [debug]: #0 fluent/log.rb:341:debug: tailing paths: target = /test/fluentd/input4/test.log | existing = /test/fluentd/input4/test.log
2023-11-10 08:21:04.498259386 +0900 test: {"message":"foo"}
2023-11-10 08:21:05.501824361 +0900 test: {"message":"foo"}
2023-11-10 08:21:06.505372942 +0900 test: {"message":"foo"}
2023-11-10 08:21:07 +0900 [info]: #0 fluent/log.rb:362:info: detected rotation of /test/fluentd/input4/test.log; waiting 5 seconds
2023-11-10 08:21:07 +0900 [warn]: #0 fluent/log.rb:383:warn: Skip update_watcher because watcher has been already updated by other inotify event path="/test/fluentd/input4/test.log" inode=101584455 inode_in_pos_file=0
2023-11-10 08:21:13 +0900 [debug]: #0 fluent/log.rb:341:debug: tailing paths: target = /test/fluentd/input4/test.log | existing = /test/fluentd/input4/test.log
2023-11-10 08:21:13 +0900 [warn]: #0 fluent/log.rb:383:warn: Could not follow a file because an existing watcher follows the different file (inode). If you keep getting this message, please restart Fluentd. filepath="/test/fluentd/input4/test.log" inode=101584480 existing_inode=101584455
2023-11-10 08:21:23 +0900 [debug]: #0 fluent/log.rb:341:debug: tailing paths: target = /test/fluentd/input4/test.log | existing = /test/fluentd/input4/test.log
2023-11-10 08:21:23 +0900 [warn]: #0 fluent/log.rb:383:warn: Could not follow a file because an existing watcher follows the different file (inode). If you keep getting this message, please restart Fluentd. filepath="/test/fluentd/input4/test.log" inode=101584480 existing_inode=101584455
2023-11-10 08:21:33 +0900 [debug]: #0 fluent/log.rb:341:debug: tailing paths: target = /test/fluentd/input4/test.log | existing = /test/fluentd/input4/test.log
2023-11-10 08:21:33 +0900 [warn]: #0 fluent/log.rb:383:warn: Could not follow a file because an existing watcher follows the different file (inode). If you keep getting this message, please restart Fluentd. filepath="/test/fluentd/input4/test.log" inode=101584480 existing_inode=101584455

conf

<system>
  log_level debug
</system>

<source>
  @type tail
  tag test
  path /test/fluentd/input4/test.log
  pos_file /test/fluentd/pos/pos
  read_from_head true
  refresh_interval 10
  <parse>
    @type none
  </parse>
</source>

<match test.**>
  @type stdout
</match>

[ashie]

I got it, it seems reasonable.

[daipom]

The fixed message example (old: #4339 (comment))

2023-11-10 14:56:48 +0900 [info]: #0 fluent/log.rb:362:info: detected rotation of /test/fluentd/input4/test.log; waiting 5 seconds
2023-11-10 14:56:48 +0900 [info]: #0 fluent/log.rb:362:info: following tail of /test/fluentd/input4/test.log
2023-11-10 14:56:48.355641918 +0900 test: {"message":"aaa"}
2023-11-10 14:56:48.359470882 +0900 test: {"message":"foo"}
2023-11-10 14:56:49.361542995 +0900 test: {"message":"foo"}
2023-11-10 14:56:50.365331647 +0900 test: {"message":"foo"}
2023-11-10 14:56:51.369635278 +0900 test: {"message":"foo"}
2023-11-10 14:56:52.373733110 +0900 test: {"message":"foo"}
2023-11-10 14:56:53.377578050 +0900 test: {"message":"foo"}
2023-11-10 14:56:54.381483406 +0900 test: {"message":"foo"}
2023-11-10 14:56:55.384613635 +0900 test: {"message":"foo"}
2023-11-10 14:56:56 +0900 [debug]: #0 fluent/log.rb:341:debug: tailing paths: target = /test/fluentd/input4/test.log | existing = /test/fluentd/input4/test.log
2023-11-10 14:56:56.385753030 +0900 test: {"message":"foo"}
2023-11-10 14:56:57.387629180 +0900 test: {"message":"foo"}
2023-11-10 14:56:58.393369764 +0900 test: {"message":"foo"}
2023-11-10 14:56:59 +0900 [info]: #0 fluent/log.rb:362:info: detected rotation of /test/fluentd/input4/test.log; waiting 5 seconds
2023-11-10 14:56:59 +0900 [warn]: #0 fluent/log.rb:383:warn: Skip update_watcher because watcher has been already updated by other inotify event path="/test/fluentd/input4/test.log" inode=101584480 inode_in_pos_file=0
2023-11-10 14:57:06 +0900 [debug]: #0 fluent/log.rb:341:debug: tailing paths: target = /test/fluentd/input4/test.log | existing = /test/fluentd/input4/test.log
2023-11-10 14:57:06 +0900 [warn]: #0 fluent/log.rb:383:warn: Could not follow a file (inode: 101584481) because an existing watcher for that filepath follows a different inode: 101584480 (e.g. keeps watching a already rotated file). If you keep getting this message, please restart Fluentd. filepath="/test/fluentd/input4/test.log"
2023-11-10 14:57:16 +0900 [debug]: #0 fluent/log.rb:341:debug: tailing paths: target = /test/fluentd/input4/test.log | existing = /test/fluentd/input4/test.log
2023-11-10 14:57:16 +0900 [warn]: #0 fluent/log.rb:383:warn: Could not follow a file (inode: 101584481) because an existing watcher for that filepath follows a different inode: 101584480 (e.g. keeps watching a already rotated file). If you keep getting this message, please restart Fluentd. filepath="/test/fluentd/input4/test.log"

[kenhys]

LGTM.

[daipom]

@kenhys @ashie Thanks!