Modify vsnprintf condition checks for size

[vast0906]

Fixes #11876

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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

• Bug Fixes
  • Improved internal string buffer handling and error detection to prevent rare edge-case overflows and off-by-one issues, enhancing stability and reliability.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 92dda547-3ef3-41fe-89df-baae429a98fe

📥 Commits

Reviewing files that changed from the base of the PR and between 972ff20 and d774f5e.

📒 Files selected for processing (1)

• src/flb_sds.c

🚧 Files skipped from review as they are similar to previous changes (1)

• src/flb_sds.c

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📝 Walkthrough

Walkthrough

Two small fixes in SDS formatting: flb_sds_printf tightens the vsnprintf insatiable check at available-space boundaries; flb_sds_snprintf returns on negative vsnprintf and adjusts buffer-growth math (ret >= size, grow by ret - size + 1, set size = ret + 1).

Changes

SDS Buffer Boundary Handling

Layer / File(s)	Summary
SDS format function boundary fixes src/flb_sds.c	flb_sds_printf uses >= to detect an insatiable vsnprintf when output reaches available space. flb_sds_snprintf returns -1 for ret < 0, grows the SDS when (size_t)ret >= size by ret - size + 1 (for NUL), and sets size = ret + 1.

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs:

• fluent/fluent-bit#10949: Tightens flb_sds_printf/flb_sds_snprintf sizing and off-by-one growth logic, related to SDS formatting used in ES header construction.

Suggested labels:
backport to v4.1.x, backport to v4.2.x

Suggested reviewers:

• edsiper
• cosmo0920
• braydonk

I hopped through buffers late at night,
Counting bytes by soft moonlight,
Fixed the gap where NUL once slipped,
Now strings land whole — not half-equipped,
A quiet hop, a tidy byte! 🐇

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ 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 'Modify vsnprintf condition checks for size' accurately describes the main changes in flb_sds.c, which adjust the condition checks and buffer growth logic for vsnprintf handling.
Linked Issues check	✅ Passed	The code changes directly address the core issue #11876 by fixing vsnprintf error handling and off-by-one buffer sizing bugs that caused malformed bulk bodies with embedded NUL bytes when ES output uses Id_Key plus Write_Operation.
Out of Scope Changes check	✅ Passed	All changes in src/flb_sds.c are scoped to fixing vsnprintf condition checks and error handling, which are directly related to the buffer serialization issues described in linked issue #11876.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/flb_sds.c`:
- Around line 405-414: The vsnprintf retry path in flb_sds_snprintf can
mis-handle negative returns and leaks the va_list on error; after calling
vsnprintf (in the retry_snprintf block) first check if ret < 0 and if so call
va_end(va) and return -1, then perform the resize check using a size-safe
comparison (e.g. if ((size_t)ret >= size) or cast ret to size_t) before
computing the grow amount; also ensure any early returns (like tmp == NULL) call
va_end(va) before returning; update references to flb_sds_increase and the
va_list handling in flb_sds_snprintf accordingly.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

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Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ff636f22-8d35-4a1b-a723-ebcd18438202

📥 Commits

Reviewing files that changed from the base of the PR and between 19cc8f9 and 972ff20.

📒 Files selected for processing (1)

• src/flb_sds.c

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Actionable comments posted: 0