compat: fix basename buffer to be thread-local on Windows

[slab-msft]

Add __declspec(thread) to the static buffer in the Windows basename func to prevent data races in multi-threaded contexts, where concurrent calls could corrupt a shared static buffer.

__declspec(thread)

Issue: Windows basename() race condition causing false file rotation.

basename() in flb_compat.h returns a pointer to a global static char buf[_MAX_PATH].

When two tail inputs run their
worker threads concurrently call basename() via [tail_file.h]:
flb_tail_file_is_rotated() -> flb_tail_target_file_name_cmp()

Thread A sets base_b = basename(name_b) (pointer to shared buf), then Thread B
overwrites that buf before Thread A's _stricmp(base_a, base_b) reads it.
The corrupted comparison returns non-zero -> file falsely detected as rotated.

Symptom: repeated log entries on Windows:
[debug] inode=XXXXXXXXXX rotated: C:\logs\svc\svc_2.log => C:\logs\svc\svc_2.log
followed by the file being purged from DB and re-read from offset 0.

Example Config to reproduce:

[INPUT]
    Name      tail
    Alias     in.tail.app
    Tag       svc.*
    Path      C:\logs\app\*.log
    DB        C:\fluent-bit\db\app.db
    Threaded  On

[INPUT]
    Name      tail
    Alias     in.tail.svc
    Tag       svc.*
    Path      C:\logs\svc\*.log
    DB        C:\fluent-bit\db\svc.db
    Threaded  On

Logs:

[2026/03/13 09:30:43.857854900] [debug] [input:tail:in.tail.svc] inode=1407374883875495 with offset=0 appended as C:\logs\svc\svc.log
[2026/03/13 09:30:44.357575600] [debug] [input:tail:in.tail.svc] inode=1407374883875495 file=C:\logs\svc\svc.log promote to TAIL_EVENT
[2026/03/13 09:39:44.336436200] [debug] [input:tail:in.tail.svc] inode=1407374883875495 rotated: C:\logs\svc\svc.log => C:\logs\svc\svc.log
[2026/03/13 09:39:44.336504700] [debug] [input:tail:in.tail.svc] inode=1407374883875495 rotated C:\logs\svc\svc.log -> C:\logs\svc\svc.log
[2026/03/13 09:39:44.336823100] [ info] [input:tail:in.tail.svc] inode=1407374883875495 handle rotation(): C:\logs\svc\svc.log => C:\logs\svc\svc.log
[2026/03/13 09:39:49.338439700] [debug] [input:tail:in.tail.svc] inode=1407374883875495 purge rotated file C:\logs\svc\svc.log (offset=120909 / size = 120909)
[2026/03/13 09:39:49.338471200] [debug] [input:tail:in.tail.svc] inode=1407374883875495 removing file name C:\logs\svc\svc.log
[2026/03/13 09:39:49.338552700] [debug] [input:tail:in.tail.svc] db: file deleted from database: C:\logs\svc\svc.log
[2026/03/13 09:39:59.322467100] [debug] [input:tail:in.tail.svc] inode=1407374883875495 with offset=0 appended as C:\logs\svc\svc.log
[2026/03/13 09:39:59.326828400] [debug] [input:tail:in.tail.svc] inode=1407374883875495 file=C:\logs\svc\svc.log promote to TAIL_EVENT
[2026/03/13 09:43:04.305067800] [debug] [input:tail:in.tail.svc] inode=1407374883875495 rotated: C:\logs\svc\svc.log => C:\logs\svc\svc.log
[2026/03/13 09:43:04.305146400] [debug] [input:tail:in.tail.svc] inode=1407374883875495 rotated C:\logs\svc\svc.log -> C:\logs\svc\svc.log
[2026/03/13 09:43:04.305202400] [ info] [input:tail:in.tail.svc] inode=1407374883875495 handle rotation(): C:\logs\svc\svc.log => C:\logs\svc\svc.log
[2026/03/13 09:43:09.318865100] [debug] [input:tail:in.tail.svc] inode=1407374883875495 purge rotated file C:\logs\svc\svc.log (offset=120909 / size = 120909)
[2026/03/13 09:43:09.318889600] [debug] [input:tail:in.tail.svc] inode=1407374883875495 removing file name C:\logs\svc\svc.log
[2026/03/13 09:43:09.318964600] [debug] [input:tail:in.tail.svc] db: file deleted from database: C:\logs\svc\svc.log
[2026/03/13 09:43:14.306752900] [debug] [input:tail:in.tail.svc] inode=1407374883875495 with offset=0 appended as C:\logs\svc\svc.log
[2026/03/13 09:43:14.310211200] [debug] [input:tail:in.tail.svc] inode=1407374883875495 file=C:\logs\svc\svc.log promote to TAIL_EVENT

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• Example configuration file for the change
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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

• Refactor
  • Improved thread-safety in an internal compatibility layer to enhance stability in multithreaded scenarios.
  • Reduces risk of concurrency-related errors and intermittent crashes, improving overall reliability without changing public behavior or APIs.

[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: ec116b2d-9e99-4099-a3b6-1f0d2be68063

📥 Commits

Reviewing files that changed from the base of the PR and between 6fff0e0 and 3904047.

📒 Files selected for processing (1)

• include/fluent-bit/flb_compat.h

---

📝 Walkthrough

Walkthrough

A thread-local storage modifier (__declspec(thread)) was added to the static buffer declaration inside the basename function in include/fluent-bit/flb_compat.h; no other logic or public signatures were changed.

Changes

Cohort / File(s)	Summary
Thread-local Buffer Declaration include/fluent-bit/flb_compat.h	Changed static char buf[_MAX_PATH]; to static __declspec(thread) char buf[_MAX_PATH]; in the inline basename implementation to make the buffer thread-local.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested labels

backport to v4.2.x

Suggested reviewers

• edsiper
• niedbalski
• patrick-stephens

Poem

🐰 I nudged a buffer, soft and small,
Made it thread-local — safe for all.
One tiny change, no logic to fight,
A hopeful hop into threaded light. ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 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 (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and clearly describes the main change: adding thread-local storage to the basename buffer on Windows to fix a race condition.

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[cosmo0920]

This patch looks fine.