S3 MultiparUploads accepts case insensitive 'ETag' headers

[jcborras]

Prevents case sensitive ETag header names.
Fixes #11201

Testing
YAML config includes an S3 output stage to an S3 compatible service, and an HTTP input stage which we will use for testing:

  inputs:
  - name: http
    listen: "0.0.0.0"
    port: 9880
    tag_key: log_tag_key  # the value of this attribute defines the event tag
  outputs:
  - name: s3
    log_level: trace
    match: '*'
    bucket: logs
    endpoint: REDACTED
    region: US
    json_date_format: iso8601
    json_date_key: ts
    part_size: 1M
    preserve_data_ordering: true
    profile: jcb-tester
    s3_key_format: /fluent-bit-logs/$TAG/%Y/%m/%d/%H
    use_put_object: false

To generate a large file to upload use the following script:

 for i in $(seq 128) ; do
   JSON="{\"log_tag_key\":\"log_tag\",\"msg\":\"This is my message\", \"ts\": \"$(date --utc --iso-8601=seconds)\", \"data\": \"$(dd if=/dev/random bs=1024 count=16 2> /dev/null | base64 --wrap=0)\" }" ;
   curl  --request POST --header "content-type: application/json" --data "$JSON" http://localhost:9880 ;
  done

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• Debug log output from testing the change

[2025/11/25 17:26:14.114997595] [ info] [output:s3:s3.0] UploadPart http status=200
[2025/11/25 17:26:14.115815938] [ info] [output:s3:s3.0] Successfully uploaded part #2 for /fluent-bit-logs/log_tag/2025/11/25/15, UploadId=AQAAAZq7nyPECHRyrbgGQ4S40hGAHSFeUEDbras, ETag=541c2b1cff8aafd05c8bc47144e001b8
[2025/11/25 17:26:14.117334902] [debug] [output:s3:s3.0] Successfully persisted upload data, UploadId=AQAAAZq7nyPECHRyrbgGQ4S40hGAHSFeUEDbras

Valgrind report:

==551332== 
==551332== HEAP SUMMARY:
==551332==     in use at exit: 0 bytes in 0 blocks
==551332==   total heap usage: 457,215 allocs, 457,215 frees, 695,242,903 bytes allocated
==551332== 
==551332== All heap blocks were freed -- no leaks are possible
==551332== 
==551332== For lists of detected and suppressed errors, rerun with: -s
==551332== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Backporting

Not required.

Summary by CodeRabbit

• Bug Fixes
  • Fixed S3 multipart upload header detection to be case-insensitive, improving compatibility with responses that use varying header casing and enhancing overall reliability of multipart upload operations.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Walkthrough

The get_etag function in the S3 multipart upload handler now performs case-insensitive searching for the ETag header using strcasestr() instead of strstr(), ensuring compatibility with S3-compatible services that return headers in lowercase, as permitted by RFC 9110.

Changes

Cohort / File(s)	Summary
ETag header search case-insensitivity plugins/out_s3/s3_multipart.c	Changed header lookup from strstr() to strcasestr() in get_etag() function to enable case-insensitive matching of ETag header name

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

• Single-function modification with straightforward logic change
• Consider verifying strcasestr() availability on target platforms (GNU extension, may require compatibility layer on non-glibc systems)

Poem

🐰 A header that whispered in lowercase before,

Now heard loud and clear, case matters no more!

RFC's blessing, Postel's law reigns,

ETag searches through compatible domains! 🎉

Pre-merge checks and finishing touches

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately reflects the main change: making the ETag header search case-insensitive in S3 multipart uploads, addressing RFC 9100 compliance.
Linked Issues check	✅ Passed	The code change replaces strstr with strcasestr for case-insensitive ETag header matching, directly addressing the RFC 9100 violation and multipart upload failures described in issue #11201.
Out of Scope Changes check	✅ Passed	The change is narrowly scoped to the get_etag function in s3_multipart.c, replacing case-sensitive with case-insensitive header search, remaining within the bounds of issue #11201 requirements.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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