This fixes log_key extraction in the S3 output to require an exact key match.

Previously, flb_pack_msgpack_extract_log_key() compared the configured log_key against each record key using the record key length, but did not verify that both keys had the same length. This allowed shorter prefix keys to match longer configured keys.

For example, with log_key log_processed, a record containing both log and log_processed could upload the value of log if it appeared first in the record.

This change checks that the configured log_key length matches the record key length before comparing the key bytes.

For example, with this config:

log_key log_processed

and this record:

{"log":"wrong","log_processed":"correct"}

the record key log could match the configured key log_processed if log appeared first in the record, because only the first 3 characters are compared:

resulting output:

wrong

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        1
    Log_Level    debug

[INPUT]
    Name    dummy
    Tag     test.s3
    Dummy   {"log":"wrong","log_processed":"correct"}
    Samples 1

[OUTPUT]
    Name                         s3
    Match                        test.s3
    bucket                       fluent-bit-test
    region                       us-east-1
    endpoint                     http://127.0.0.1:9000
    total_file_size            1M
    upload_timeout         5s
    store_dir                    /tmp/flb-s3-store
    s3_key_format          /logs/$TAG/%Y/%m/%d/%H/%M/%S-$UUID
    log_key                      log_processed

Verified uploaded object content locally with MinIO.
Before this change, the output uploadis:
wrong
After this change, with the same config, the uploaded object content is:
correct

• Debug log output from testing the change

[2026/06/27 07:22:30.686] [debug] [upstream] KA connection #45 to 127.0.0.1:9000 is now available
[2026/06/27 07:22:30.686] [debug] [output:s3:s3.0] PutObject http status=200
[2026/06/27 07:22:30.686] [ info] [output:s3:s3.0] Successfully uploaded object /logs/test.s3/2026/06/27/01/52/23-qEfrOjVi

• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found
  This change only tightens the existing log_key comparison logic and does not add new allocations or frees.
  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

• [N/A] 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
  • Improved log key matching for structured records so only exact key names are treated as matches.
  • Prevented partial prefix matches from being accepted when a shorter field name appears in the data.