out_s3: guard current_buffer_size subtraction against underflow

[univbyn-blip]

Fixes #12270

s3_store_file_delete() subtracted s3_file->size from
ctx->current_buffer_size unconditionally. Buffer files recovered from
a previous run are not accounted into current_buffer_size at startup,
so deleting them after upload wraps the unsigned counter around to
~2^64. The store_dir_limit_size check then treats the buffer as
permanently full and rejects every new chunk ("Buffer is full",
"chunk cannot be retried"), causing data loss until restart.

Clamp the subtraction at zero, using the same pattern already applied
on the quarantine accounting path in s3.c.

Observed in production on 5.0.9 (Kubernetes DaemonSet, store_dir on a
hostPath volume surviving pod restarts):

[error] Buffer is full: current_buffer_size=18446744073709314048, new_data=4931, store_dir_limit_size=20000000000 bytes

18446744073709314048 == 2^64 - 237568 — the counter went "negative"
by exactly the size of the recovered-and-deleted files.

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Testing

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

Documentation

• N/A — bug fix, no user-facing configuration change

Backporting

• Backport to latest stable release.

Summary by CodeRabbit

• Bug Fixes
  • Fixed an issue where deleting certain files could cause storage usage tracking to become inaccurate.
  • Storage counters now remain at zero instead of dropping below zero when file sizes are unavailable.

[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: deba4a3b-2f2a-42d3-a59f-ff143ece10f0

📥 Commits

Reviewing files that changed from the base of the PR and between ae51533 and 942e9ca.

📒 Files selected for processing (1)

• plugins/out_s3/s3_store.c

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📝 Walkthrough

Walkthrough

The S3 file deletion logic now prevents current_buffer_size from wrapping when deleting recovered files whose sizes were not included in startup accounting.

Changes

S3 buffer accounting

Layer / File(s)	Summary
Guard file deletion accounting plugins/out_s3/s3_store.c	s3_store_file_delete conditionally subtracts the deleted file size and clamps current_buffer_size to zero when necessary.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related issues

• out_s3: current_buffer_size underflows at startup when store_dir has leftover files — output permanently rejects data ('Buffer is full') #12164 — Describes the same current_buffer_size underflow in s3_store_file_delete.

Possibly related PRs

• fluent/fluent-bit#12266 — Modifies the same S3 buffer accounting path to prevent underflow during file deletion and startup accounting.

Suggested reviewers: cosmo0920, edsiper, leonardo-albertovich

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 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 out_s3 underflow fix, which is the primary change in the pull request.
Linked Issues check	✅ Passed	The change clamps current_buffer_size and directly addresses issue #12270 by preventing underflow after deleting recovered buffer files.
Out of Scope Changes check	✅ Passed	The pull request changes only the relevant out_s3 deletion accounting logic and contains no unrelated scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[cosmo0920]

This will be superseded by #12266.