kafka: use rd_kafka_conf_destroy in flb_kafka_conf_create error path

[immanuwell]

flb_kafka_conf_create leaks memory when it hits the error path (e.g. missing brokers config). code calls flb_free(kafka_cfg) on the rd_kafka_conf_t* from rd_kafka_conf_new(), which skips rd_kafka_anyconf_destroy() and leaves all internal strings (default props, client.id, group.id) hanging in memory.

one-line fix: swap flb_free for rd_kafka_conf_destroy. every other caller in the codebase already does this (in_kafka.c:459, out_kafka/kafka_config.c:255,320).

How to repro: configure a kafka output without brokers and run under valgrind:

[OUTPUT]
    name  kafka
    topic test

valgrind will show leaks from rd_kafka_conf_set allocations that never get freed on teardown.

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change (above)
• [N/A] Debug log output from testing the change
• Attached Valgrind output that shows no leaks or memory corruption was found

Documentation

• [N/A] Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

Summary by CodeRabbit

• Bug Fixes
  • Corrected error handling in Kafka initialization to ensure proper cleanup of configuration resources when startup fails.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ba955e44-192f-4298-95d9-8f79e69a218c

📥 Commits

Reviewing files that changed from the base of the PR and between 19cc8f9 and 7a87555.

📒 Files selected for processing (1)

• src/flb_kafka.c

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📝 Walkthrough

Walkthrough

A single-line fix in the error cleanup path of flb_kafka_conf_create() that correctly uses rd_kafka_conf_destroy() to release the rdkafka configuration object instead of flb_free(), aligning destruction with the rdkafka allocation lifecycle.

Changes

rdkafka Config Cleanup

Layer / File(s)	Summary
Error cleanup in conf_create src/flb_kafka.c	flb_kafka_conf_create() error handling now destroys rd_kafka_conf_t using rd_kafka_conf_destroy() instead of flb_free().

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• fluent/fluent-bit#11359: Both PRs fix incorrect rdkafka rd_kafka_conf_t cleanup on rd_kafka_new()/initialization failure by switching to rd_kafka_conf_destroy(...) instead of freeing the config object directly.

Suggested labels

backport to v4.2.x

Suggested reviewers

• edsiper
• cosmo0920
• patrick-stephens
• fujimotos

Poem

🐰 A config so fine, now freed the right way,
With rdkafka's own destroy to save the day,
No more confusion when init goes south,
Cleanup flows true from source to mouth! 🎉

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main change: replacing flb_free with rd_kafka_conf_destroy in the error path of flb_kafka_conf_create, which is exactly what the changeset does.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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