input_chunk: input_log: in_storage_backlog: address underflowing / overflowing chunk limit size

[cosmo0920]

Implemented general filesystem chunk-accounting guards.

• fs_chunks_size now uses saturating subtraction: underflow logs a warning and resets the counter to zero instead of wrapping near SIZE_MAX.
• Additions are overflow-checked and saturate at SIZE_MAX.
• Chunk-size adjustments remain signed, so negative trim deltas correctly subtract rather than becoming huge unsigned additions.
• Per-record routing paths use the centralized guard.
• fs_backlog_chunks_size has equivalent guards and subtracts the size recorded when the backlog entry was created.
• Added regression coverage for subtraction underflow, negative adjustments, and addition overflow in input_chunk_routes.c.

Verification:

• cmake --build build --target fluent-bit-bin flb-it-input_chunk_routes — passed.
• ctest --test-dir build -R '^flb-it-input_chunk_routes$' --output-on-failure — 1 passed.
• .venv\Scripts\python.exe -m pytest scenarios/in_forward/tests/test_in_forward_001.py -q -k storage_limit — 2 passed, 1 platform skip.
• .venv\Scripts\python.exe -m pytest scenarios/in_storage_backlog/tests/test_in_storage_backlog_001.py -q — 2 passed, 2 platform skips.
• git diff --check — passed.

Closes #12220.

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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 filesystem backlog size accounting to prevent underflow and overflow.
  • Added safeguards for invalid or negative chunk-size adjustments.
  • Counters now clamp safely at zero or saturate at the maximum supported value, with warnings for inconsistent values.
  • Improved accounting reliability when chunks are removed or routed through outputs.

• Tests

  • Added coverage verifying safe counter decrements and overflow handling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 0ca9b0eb-5f0e-43fc-9c1e-df5eed743230

📥 Commits

Reviewing files that changed from the base of the PR and between 6315162 and 2dd1c8f.

📒 Files selected for processing (5)

• include/fluent-bit/flb_input_chunk.h
• plugins/in_storage_backlog/sb.c
• src/flb_input_chunk.c
• src/flb_input_log.c
• tests/internal/input_chunk_routes.c

Included review availability: Your plan includes up to 8 reviews per rolling hour; 7 remain after this review.

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📝 Walkthrough

Walkthrough

The change adds bounded filesystem chunk accounting for input outputs and storage backlog. Signed chunk-size deltas prevent invalid negative-size calculations. Routing, cleanup, eviction, and backlog updates now use guarded subtraction and saturating addition. Regression coverage validates these cases.

Changes

Chunk accounting safety

Layer / File(s)	Summary
Accounting contracts and safeguards include/fluent-bit/flb_input_chunk.h, src/flb_input_chunk.c	Adds the subtraction API, changes chunk updates to signed sizes, and adds overflow- and underflow-safe accounting helpers.
Accounting call sites src/flb_input_chunk.c, src/flb_input_log.c, plugins/in_storage_backlog/sb.c	Uses guarded subtraction for chunk removal paths and saturates counters when additions exceed SIZE_MAX.
Accounting regression coverage tests/internal/input_chunk_routes.c	Tests normal updates, underflow protection, oversized subtraction, saturation, and fs_counted state.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 2dd1c

The change prevents chunk-size accounting from wrapping during underflow or overflow and keeps negative adjustments subtractive; no actionable merge-blocking risk remains after normal checks and review.

Possibly related issues

• out_s3: current_buffer_size underflows after deleting buffer files recovered from a previous run — every new chunk is then rejected with "Buffer is full" #12270 — Addresses similar unsigned buffer or chunk accounting underflow handling.

Possibly related PRs

• fluent/fluent-bit#12239 — Both modify filesystem backlog chunk accounting in plugins/in_storage_backlog/sb.c.
• fluent/fluent-bit#12266 — Both add overflow- and underflow-safe buffer or chunk accounting.
• fluent/fluent-bit#12271 — Both clamp unsigned buffer-size counter subtraction to zero.

Suggested reviewers: edsiper

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 18.75% 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 clearly identifies the affected components and the primary underflow and overflow fix.
Linked Issues check	✅ Passed	The changes address issue #12220 by preventing counter underflow, preserving signed adjustments, and adding regression coverage.
Out of Scope Changes check	✅ Passed	All changes support filesystem chunk-accounting protection and related regression tests; no unrelated code changes are evident.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cosmo0920-address-underflowing-chunk-limit-size

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