-p overwrites the default plugin path

[Bonko]

Describe the bug

When updating from fluent/fluentd-kubernetes-daemonset:v1.14.6-debian-logzio-amd64-1.0 to fluent/fluentd-kubernetes-daemonset:v1.16.2-debian-logzio-amd64-1.0 our setup stopped working.

During investigation it turned out that plugins in the default plugin directory (/etc/fluent/plugin/) are not processed anymore when using the -p parameter.

To Reproduce

Add custom pluins into /etc/fluent/plugin/ directory and ensure that they are used by the config:

root@27cf7d7bb6eb:/home/fluent# ls -1  /etc/fluent/plugin/
filter_empty_field.rb
filter_field_size.rb
filter_field_type.rb
filter_message.rb

When starting fluentd with the default options from the Dockerfile command (fluentd -c /test/fluentd.conf -p /fluentd/plugins/ --gemfile /fluentd/Gemfile) I am getting this error:

2023-08-16 10:20:01 +0000 [error]: config error file="/test/fluentd.conf" error_class=Fluent::NotFoundPluginError error="Unknown filter plugin 'message'. Run 'gem search -rd fluent-plugin' to find plugins"

I can get rid of this error by either not specifying -p at all:

fluentd -c /test/fluentd.conf  --gemfile /fluentd/Gemfile

or by adding the default plugin explicitly:

fluentd -c /test/fluentd.conf -p /fluentd/plugins/ -p /etc/fluent/plugin  --gemfile /fluentd/Gemfile

Expected behavior

The -p parameter should add plugin directories as documented without unsetting the default directory.

Your Environment

- Fluentd version: `1.16.2`
- Operating system: `Debian 11`
- Kernel version: `5.15.49-linuxkit-pr`

Your Configuration

<label @FLUENT_LOG>
  <match fluent.**>
    @type null
  </match>
</label>

<source>
  @type tail
  @id in_tail_container_logs
  path /var/log/containers/*.log
  tag "kubernetes.*"
  pos_file /var/log/containers/fluentd-containers.log.pos
  read_from_head true
  <parse>
    @type "json"
    time_format %Y-%m-%dT%H:%M:%S.%NZ
  </parse>
</source>

@include /fluentd/etc/conf.d/*.conf
[..]

/fluentd/etc/conf.d/01-pre-parsing-filters.conf:

<filter kubernetes.**>
  @type message
  message_fields log, MESSAGE
</filter>

### Your Error Log

```shell
2023-08-16 10:27:01 +0000 [info]: init supervisor logger path=nil rotate_age=nil rotate_size=nil
2023-08-16 10:27:01 +0000 [warn]: '@' is the system reserved prefix. It works in the nested configuration for now but it will be rejected: @timestamp
2023-08-16 10:27:01 +0000 [info]: parsing config file is succeeded path="/test/fluentd.conf"
2023-08-16 10:27:02 +0000 [info]: gem 'fluentd' version '1.16.2'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-concat' version '2.5.0'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-detect-exceptions' version '0.0.15'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-grok-parser' version '2.6.2'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-json-in-json-2' version '1.0.2'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-kubernetes_metadata_filter' version '3.2.0'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-logzio' version '0.2.1'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-multi-format-parser' version '1.0.0'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-parser-cri' version '0.1.1'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-prometheus' version '2.1.0'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-record-modifier' version '2.1.1'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-rewrite-tag-filter' version '2.4.0'
2023-08-16 10:27:02 +0000 [info]: gem 'fluent-plugin-systemd' version '1.0.5'
2023-08-16 10:27:02 +0000 [error]: config error file="/test/fluentd.conf" error_class=Fluent::NotFoundPluginError error="Unknown filter plugin 'message'. Run 'gem search -rd fluent-plugin' to find plugins"

### Additional context

_No response_

[kenhys]

Thank you for feedback.

Currently, it seems that actually overrides default plugin directory (reset) and it seems intended behavior.

https://github.com/fluent/fluentd/blob/master/lib/fluent/supervisor.rb#L512

As you mentioned, specify both of your specific directory and default plugin directory works.

[Bonko]

Thanks for your reply @kenhys

While I get that this is intended behaviour now, the problem is that this is a behavioural change that was not documented. I think it would have make sense to highlight this as a breaking change in the release notes. The documentation/help section also still states that -p adds a directory, omitting that this overwrites the default path:

-p, --plugin DIR                 add plugin directory

[kenhys]

Hmm, breaking change is not desirable, it seems that expecting not overriding default path is reasonable. :thinking:

[kenhys]

with further investigation, it seems that regression since v1.16.0.

Fix problem that some system configs are not reflected https://github.com/fluent/fluentd/pull/4064 https://github.com/fluent/fluentd/pull/4065 https://github.com/fluent/fluentd/pull/4086 https://github.com/fluent/fluentd/pull/4090 https://github.com/fluent/fluentd/pull/4096

It maybe side effect of above fix.

At least v1.15.3 seems work. (it does not reset default plugin path)

[daipom]

I'm sorry. This change breaks the specification.

https://github.com/fluent/fluentd/pull/4064/files#diff-fe3c9c8a2fe3872b84e0de1d14ecd669258b0e57609c470b601bdfabedb91224R49

This should be fixed.

[mrudrego]

Hi @daipom , i was looking at this issue to see what could be the fix.

There are 2 hash that is used, default_opts is used for storing the default configuration, while cmd_opts is used to store user provided command line options. Later these 2 hashes are merged using the merge function . As a result of this merge function, the plugin_dirs key value available in default_opts is overwritten by the value in cmd_opts. If the old behaviour has to be continued, then values of the key(plugin_dirs) in the 2 hashes should have been concatenated instead of overwriting.

The fix which i could think of is using concat function to join the content of plugin_dirs key array values after the 2 hashes are merged at line https://github.com/fluent/fluentd/blob/master/lib/fluent/command/fluentd.rb#L251C29-L251C29

opts[:plugin_dirs].concat(default_opts[:plugin_dirs])

Please let me know if this looks ok or is there a better option.

Thanks,

[mrudrego]

@daipom, can you please let me know if the approach looks ok or is there any other suggestion for the fix. Based on this i can raise a PR.

Thanks,

[daipom]

@mrudrego Thanks! Sorry for my late response. Currently, I'm busy on checking the issues related to #3614, so my response may be slow for a while. Sorry.

The fix for this issue would be difficult. The implementation around this is very complicated. :cry: It is mainly because we have 3 different option values: default option; command line option; system config. In addition, some options are used in the command script fluentd.rb, and some options are used in the Fluentd processes supervisor.rb.

Actually, it's not opts that is passed to Supervisor, but cmd_opts.

https://github.com/fluent/fluentd/blob/52e46f04b3cb7bddc809841e1e54f3cf8925681c/lib/fluent/command/fluentd.rb#L350-L359

It is merged here. :cry:

https://github.com/fluent/fluentd/blob/52e46f04b3cb7bddc809841e1e54f3cf8925681c/lib/fluent/supervisor.rb#L510-L541

So, I think this fix is very difficult. It would require knowledge of the entire Fluentd startup logic.

I don't think this fix will be done in time for the next release v1.16.3. We are planning to release v1.16.3 very soon for fixing #3614. After v1.16.3 is released, I will think about how we can fix this.

Of course, if you have any good ideas, please let us know! Thanks!

[daipom]

Fixed by #4605. It will be released on v1.16.6 and v1.18.0 (or possibly v1.17.2 (undecided))