out_s3: Add a NULL check for plugging SEGV on dry_run

[cosmo0920]

Fixes #11104.

---

Enter [N/A] in the box, if an item is not applicable to your change.

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

• Example configuration file for the change

service:
    flush_interval: 1
    daemon: Off
    log_level: debug

pipeline:
    inputs:
        -   name: tcp
            listen: 0.0.0.0
            port: 24224
            format: json
            tag: input.json_tcp
            buffer_size: 128 #kb

    outputs:
        -   name: s3
            match: 'common_output'
            region: eu-west-1
            bucket: TEST
            store_dir_limit_size: 200M
            compression: gzip
            total_file_size: 50M
            s3_key_format: "/%Y/%m/%d/%H_%M_%S_$UUID.json"

• Debug log output from testing the change

% bin/fluent-bit -c out_s3_segv.yaml --dry-run
Fluent Bit v4.2.0
* Copyright (C) 2015-2025 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _             ___   __  
|  ___| |                | |   | ___ (_) |           /   | /  | 
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| | `| | 
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| |  | | 
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |__| |_
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/


[2025/11/06 19:44:26.316110000] [ info] Configuration:
[2025/11/06 19:44:26.316116000] [ info]  flush time     | 1.000000 seconds
[2025/11/06 19:44:26.316120000] [ info]  grace          | 5 seconds
[2025/11/06 19:44:26.316121000] [ info]  daemon         | 0
[2025/11/06 19:44:26.316123000] [ info] ___________
[2025/11/06 19:44:26.316124000] [ info]  inputs:
[2025/11/06 19:44:26.316126000] [ info]      tcp
[2025/11/06 19:44:26.316128000] [ info] ___________
[2025/11/06 19:44:26.316129000] [ info]  filters:
[2025/11/06 19:44:26.316131000] [ info] ___________
[2025/11/06 19:44:26.316132000] [ info]  outputs:
[2025/11/06 19:44:26.316134000] [ info]      s3.0
[2025/11/06 19:44:26.316135000] [ info] ___________
[2025/11/06 19:44:26.316137000] [ info]  collectors:
configuration test is successful

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

---

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
  • Enhanced the S3 output plugin's worker termination process with additional defensive checks to prevent crashes and ensure stable plugin shutdown.
  • Improved logging accuracy during worker lifecycle management to clearly distinguish between initialization and termination phases, enabling better operational visibility and troubleshooting.

[coderabbitai]

Walkthrough

A NULL context guard is added to the S3 worker exit handler to prevent an invalid memory access during shutdown. The change intercepts early returns when the context is NULL, resolving a segmentation fault that occurs during --dry-run with S3 output enabled.

Changes

Cohort / File(s)	Summary
S3 Worker Exit Safety plugins/out_s3/s3.c	Added NULL pointer guard in cb_s3_worker_exit to return early if context is NULL; updated log message from "initializing worker" to "terminating worker"; maintains existing cleanup behavior for non-NULL context.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

• Straightforward NULL check guard addition preventing invalid memory access
• Single file, localized change with clear intent tied to the shutdown path
• Log message update is cosmetic and contextually appropriate

Suggested reviewers

• edsiper
• koleini
• fujimotos

Poem

A worker exits, but context falls through,
A NULL check catches what chaos could brew,
The rabbit hops safely, no segfault in sight,
One guard does the trick—dry-run's now alright! 🐰✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The PR addresses the core requirement from #11104 by adding a NULL guard in cb_s3_worker_exit to prevent invalid memory access, but does not implement the requested test coverage for dry-run scenarios.	Add a test case that verifies --dry-run with s3 output enabled does not segfault, as requested in the linked issue's expected behavior section.

✅ 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 accurately describes the main change: adding a NULL check in the s3 plugin to prevent a segmentation fault during dry-run operations.
Out of Scope Changes check	✅ Passed	The changes remain focused on the reported issue: adding NULL check and updating log message in cb_s3_worker_exit as a targeted fix for the segfault.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch cosmo0920-plug-segv-without-null-check

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[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

plugins/out_s3/s3.c (1)

1115-1119: LGTM! Critical NULL check fixes the dry-run segfault.

The NULL guard directly addresses issue #11104 by preventing the invalid memory access to ctx->ins at line 1119 that caused the segfault during --dry-run. Returning early is appropriate for this cleanup function.

Optional refinement: The TLS cleanup code at lines 1121-1125 doesn't depend on ctx, so theoretically it could still execute even when ctx is NULL:

 static int cb_s3_worker_exit(void *data, struct flb_config *config)
 {
     struct worker_info *info;
     struct flb_s3 *ctx = data;
 
-    if (!ctx) {
-        return 0;
-    }
-
-    flb_plg_info(ctx->ins, "terminating worker");
+    if (ctx) {
+        flb_plg_info(ctx->ins, "terminating worker");
+    }
 
     info = FLB_TLS_GET(s3_worker_info);
     if (info != NULL) {
         flb_free(info);
         FLB_TLS_SET(s3_worker_info, NULL);
     }
 
     return 0;
 }

However, the current implementation is perfectly fine—if ctx is NULL during dry-run, the worker was likely never fully initialized, so skipping TLS cleanup is reasonable.

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 02c9e9f and 896b8fb.

📒 Files selected for processing (1)

• plugins/out_s3/s3.c (1 hunks)

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