Fluentd fails to rotate supervisor log on Windows

[fujimotos]

Describe the bug

I notice another rotate bug while implementing a fix for #3921.

Suppose we launch Fluentd as follows:

PS> fluentd -c fluent.conf -o log/test.log --log-rotate-size=2k

This is perfectly normal settings, but it ends up causing an internal error.
Here is the actual error message:

PS> bundle exec fluentd -c fluentd.conf -o test.log  --log-rotate-size=2k
...
log shifting failed. Permission denied @ rb_file_s_rename - (log/test-supervisor-0.log, log/test-supervisor-0.log.0)
log writing failed. closed stream

To Reproduce

1. Run the following command on Windows:

   PS> fluentd -c fluent.conf -o log/test.log --log-rotate-size=2k

2. Observe fluentd outputs error messages to console on rotation.

Expected behavior

No error

Your Environment

- Fluentd version: v1.15.2 (latest HEAD)
- TD Agent version: -
- Operating system: Windows Server 2019
- Kernel version: -

Your Configuration

<source>
  @type dummy
  tag test.log
</source>
<match>
  @type stdout
</match>

Your Error Log

See above.

Additional context

No response

[fujimotos]

I don't fully trace the root cause yet, but my strong suspicion is that
the supervisor actually has two separate logger instances (on the same file):

• https://github.com/fluent/fluentd/blob/master/lib/fluent/supervisor.rb#L445-L453
• https://github.com/fluent/fluentd/blob/master/lib/fluent/supervisor.rb#L771-L773

Since Windows locks files with an open handler by default
and there is no coordination between two loggers, they are
conflicting with each other.