"[warn]: section <buffer> is not used in ..." from s3 plugin

[abicky]

Describe the bug

S3 output plugin supports <buffer> section, but we receive "[warn]: section is not used in ..." warning like below:

2019-08-16 23:17:26 +0900 [warn]: section <buffer> is not used in <match debug.**>

To Reproduce

We can reproduce the behavior by executing fluentd using the configuration file describged later and fluent-plugin-s3 1.1.11.

Expected behavior

I expect to receive unused parameter names in <buffer> section like below:

2019-08-16 23:20:40 +0900 [warn]: parameter 'unused_key' in <buffer>
  unused_key 1
</buffer> is not used.

Your Environment

• Fluentd or td-agent version:

  % bundle exec fluentd --version
  fluentd 1.6.3
  % git rev-parse HEAD
  975ca8b4ec60274f7361c7e96b0402b4f93a1dba
  

• Operating system:

  % sw_vers
  ProductName:    Mac OS X
  ProductVersion: 10.14.4
  BuildVersion:   18E226
  

• Kernel version:

  % uname -r
  18.5.0
  

Your Configuration

<match debug.**>
  @type s3
  s3_bucket dummy
  <buffer>
    unused_key 1
  </buffer>
</match>

Your Error Log

% bundle exec fluentd -c fluent.conf
2019-08-16 23:29:08 +0900 [info]: parsing config file is succeeded path="fluent.conf"
2019-08-16 23:29:08 +0900 [info]: using configuration file: <ROOT>
  <match debug.**>
    @type s3
    s3_bucket "dummy"
    <buffer>
      unused_key 1
    </buffer>
  </match>
</ROOT>
2019-08-16 23:29:08 +0900 [info]: starting fluentd-1.6.3 pid=62109 ruby="2.6.2"
2019-08-16 23:29:08 +0900 [info]: spawn command to main:  cmdline=["/Users/arabiki/.anyenv/envs/rbenv/versions/2.6.2/bin/ruby", "-Eascii-8bit:ascii-8bit", "-rbundler/setup", "/Users/arabiki/.anyenv/envs/rbenv/versions/2.6.2/lib/ruby/gems/2.6.0/bin/fluentd", "-c", "fluent.conf", "--under-supervisor"]
2019-08-16 23:29:09 +0900 [info]: gem 'fluentd' version '1.6.3'
2019-08-16 23:29:09 +0900 [info]: gem 'fluent-plugin-s3' version '1.1.11'
2019-08-16 23:29:09 +0900 [info]: adding match pattern="debug.**" type="s3"
2019-08-16 23:29:09 +0900 [warn]: section <buffer> is not used in <match debug.**>
2019-08-16 23:29:09 +0900 [info]: #0 starting fluentd worker pid=62138 ppid=62109 worker=0
(snip)

Additional context

The behavior seems caused by the following steps:

1. Fluent::Plugin::S3Output::Compressor#configure is called in Fluent::Plugin::S3Output#configure
2. unused_in is set in fluent/config/section.rb#L212-L217