[8.x] setting: enforce non-nullable string (restore 8.15.x behavior) (backport #17522)

[mergify]

Release notes

• Fixes an issue introduced in 8.16.4, 8.17.2, and 8.18.0 where null values were incorrectly accepted for non-nullable settings.

What does this PR do?

Ensures that SettingString is correctly non-nullable and SettingNullableString is nullable.

Why is it important/What is the impact to the user?

Improves error messages and reliability of behaviour when Logstash is misconfigured.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

How to test this PR locally

With Logstash 8.15.5, set a non-nullable string setting to null in the logstash.yml and run Logstash, observing that it refuses to start and gives guidance about the wrong setting

(cd logstash-8.15.5 && echo 'keystore.file: null' > config/logstash.yml && bin/logstash -e 'input { generator { count => 1 } }')

Your settings are invalid. Reason: Setting "keystore.file" must be a String. Received:  (NilClass)
[2025-04-08T21:51:30,611][FATAL][org.logstash.Logstash    ] Logstash stopped processing because of an error: (SystemExit) exit

Repeat with Logstash 8.17.4 and observe that runtime errors that are less clear occur later

(cd logstash-8.17.4 && echo 'keystore.file: null' > config/logstash.yml && bin/logstash -e 'input { generator { count => 1 } }')

[2025-04-08T22:46:34,161][ERROR][logstash.agent           ] Failed to execute action {:action=>LogStash::PipelineAction::Create/pipeline_id:main, :exception=>"TypeError", :message=>"nil is not a string", :backtrace=>["org/logstash/execution/AbstractPipelineExt.java:293:in `initialize'", "org/logstash/execution/AbstractPipelineExt.java:227:in `initialize'", "/Users/rye/src/elastic/scratch/logstash-8.17.4/logstash-core/lib/logstash/java_pipeline.rb:47:in `initialize'", "org/jruby/RubyClass.java:949:in `new'", "/Users/rye/src/elastic/scratch/logstash-8.17.4/logstash-core/lib/logstash/pipeline_action/create.rb:50:in `execute'", "/Users/rye/src/elastic/scratch/logstash-8.17.4/logstash-core/lib/logstash/agent.rb:420:in `block in converge_state'"]}
[2025-04-08T22:46:34,188][INFO ][logstash.agent           ] Successfully started Logstash API endpoint {:port=>9600, :ssl_enabled=>false}
[2025-04-08T22:46:34,192][INFO ][logstash.runner          ] Logstash shut down.
[2025-04-08T22:46:34,194][FATAL][org.logstash.Logstash    ] Logstash stopped processing because of an error: (SystemExit) exit

Repeat with this patch, and observe the restored behaviour:

echo 'keystore.file: null' > config/logstash.yml && bin/logstash -e 'input { generator { count => 1 } }'

Your settings are invalid. Reason: Setting "keystore.file" must be a String. Received:  (NilClass)
[2025-04-08T22:49:04,092][FATAL][org.logstash.Logstash    ] Logstash stopped processing because of an error: (SystemExit) exit

Related issues

Caused-By: #16576

---

This is an automatic backport of pull request #17522 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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[elasticmachine]

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cc @yaauie

[yaauie]

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