YAML Config File Syntax: Single int value for an array option causes a config error

[daipom]

Describe the bug

In YAML syntax, a single int value for an array option causes a config error.

Found in the following fix.

• #5126

Note: This PR doesn't address the issue where setting an Int directly to an Array option causes ConfigError: retryable_response_codes: 503 (The C case in https://github.com/fluent/fluentd/pull/5126#issuecomment-3424253314) It could be a different issue.

The current behavior is inconsistent and somewhat unnatural.

Unnatural current behavior:

• An error occurs only when a single int value is specified for an array option.
• It can be avoided by setting it as a String.
• There’s no such issue with options that are initially String arrays.
• There’s no such issue with the normal config syntax.

To Reproduce

NG(Causes config error file="..." error_class=Fluent::ConfigError error="array required but got 503"):

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: 503

OK as YAML syntax:

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: "503"

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: 503,

OK as normal syntax:

<match test>
  @type http
  endpoint "http://localhost:9880/tag"
  retryable_response_codes 503
</match>

OK as YAML syntax since v1.19.1 (after #5126):

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: [503]

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes:
        - 503

Please also refer to the C case in https://github.com/fluent/fluentd/pull/5126#issuecomment-3424253314.

Expected behavior

Fluentd can parse a single int value for an array option, such as the following:

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: 503

Your Environment

- Fluentd version: v1.19.1
- Package version: None
- Operating system: Ubuntu 22.04.5 LTS
- Kernel version: 6.8.0-85-generic

Your Configuration

See `To reproduce`.

Your Error Log

2025-11-11 14:27:25 +0900 [info]: init supervisor logger path=nil rotate_age=nil rotate_size=nil
2025-11-11 14:27:25 +0900 [info]: parsing config file is succeeded path="/test/fluentd/config/yaml/tmp.yaml"
2025-11-11 14:27:25 +0900 [info]: inaccessible include directory was specified path="/etc/fluent/conf.d"
2025-11-11 14:27:25 +0900 [info]: gem 'fluentd' version '1.19.1'
2025-11-11 14:27:25 +0900 [error]: config error in:
<match test>
  @type http
  endpoint "http://localhost:9880/tag"
  retryable_response_codes 503
</match>

2025-11-11 14:27:25 +0900 [error]: config error file="/test/fluentd/config/yaml/tmp.yaml" error_class=Fluent::ConfigError error="array required but got 503"

Additional context

No response