build: cmake: Use safer unistd.h stub for Windows

[cosmo0920]

For WIN64, we should include BaseTsd.h and use typedef for ssize_t definition.

Some building environments define ssize_t as a macro and we should include a proper header for pulling the original SSIZE_T definition.

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Testing
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• Example configuration file for the change
• Debug log output from testing the change

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

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• Run local packaging test showing all targets (including any new ones) build.
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Documentation

• 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

Release Notes

• Chores
  • Updated Windows build configuration for improved compatibility on 64-bit systems.

[coderabbitai]

📝 Walkthrough

Walkthrough

A modification to the Windows/MSVC compatibility wrapper in cmake/libbacktrace.cmake that changes how ssize_t is defined on 64-bit Windows builds. Instead of directly typedefing __int64, the implementation now includes BaseTsd.h and aliases SSIZE_T after undefining any existing ssize_t.

Changes

Cohort / File(s)	Summary
Windows/MSVC ssize_t compatibility wrapper cmake/libbacktrace.cmake	Updated unistd.h compatibility layer to define ssize_t by including BaseTsd.h and aliasing SSIZE_T instead of direct __int64 typedef, with preceding undef to avoid conflicts.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested labels

backport to v4.0.x

Suggested reviewers

• edsiper
• koleini
• fujimotos

Poem

🐰 A rabbit hops through Windows trails,
Where types were tangled, now prevails,
BaseTsd.h brings proper grace,
ssize_t finds its rightful place! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: updating the Windows unistd.h stub in the CMake build configuration to use a safer approach for handling ssize_t definition.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ 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-use-safer-unistd.h-stub-for-windows

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[cosmo0920]

Windows checks are green! 💚 https://github.com/fluent/fluent-bit/actions/runs/21848718161/job/63051825737