upstream-sync: atomic writes for checkpoint and dev-cache files

[Liohtml]

Upstream reference

Ported from D4Vinci/Scrapling v0.4.9 – PR #344
"fix(spiders): use os.replace for atomic checkpoint/cache writes on Windows"
(merged 2026-06-07)

Problem

Both ResponseCache::put and CheckpointManager::save used std::fs::write() directly on the target path. std::fs::write truncates the file then fills it, so a process kill (SIGKILL, power loss, OOM) between those two operations leaves a zero-byte or half-written file. On the next run this silently fails to parse, losing the checkpoint or corrupting the dev cache.

Fix

Write the serialized data to a sibling .tmp file, then call std::fs::rename to atomically replace the target. On POSIX, rename(2) is a single syscall — the target either contains the old content or the new content, never a partial write.

# Before (non-atomic):
std::fs::write(&file_path, &data)?;

# After (atomic):
let tmp_path = file_path.with_extension("tmp");   // or .json.tmp
std::fs::write(&tmp_path, &data)?;
std::fs::rename(&tmp_path, &file_path)

This is the direct Rust equivalent of Python's os.replace() used in the upstream fix.

Files changed

File	Change
src/spiders/cache.rs	ResponseCache::put — atomic write via tmp+rename
src/spiders/checkpoint.rs	CheckpointManager::save — atomic write via tmp+rename

Testing

cargo check ✅ cargo test ✅ — all 175 tests pass, no regressions.

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Generated by Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Enhanced data persistence mechanisms for cache and checkpoint storage to prevent corruption and data loss during unexpected application interruptions.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 98a5afd5-69d0-479c-868b-3c5ff79833f1

📥 Commits

Reviewing files that changed from the base of the PR and between 423fcbe and b88a005.

📒 Files selected for processing (3)

• Cargo.toml
• src/spiders/cache.rs
• src/spiders/checkpoint.rs

📝 Walkthrough

Walkthrough

This PR adds atomic write patterns to two persistence operations: response cache and checkpoint management. Both now write to temporary files and rename them to their final destinations, preventing partially written or corrupted files if a process crashes mid-write.

Changes

Atomic write safety for cache and checkpoint

Layer / File(s)	Summary
Response cache atomic write src/spiders/cache.rs	ResponseCache::put now writes JSON to a temporary .tmp file and then renames it to the final cache path instead of writing directly to the destination, ensuring atomicity.
Checkpoint atomic write src/spiders/checkpoint.rs	CheckpointManager::save now writes JSON to checkpoint.json.tmp and then renames it to the final checkpoint.json path with comments indicating crash-safety intent, replacing the prior direct write.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• Liohtml/RUSTScrapling#22: Changes CrawlerEngine to drain in-flight tasks before checkpoint persistence, complementing this PR's atomic checkpoint write safety.

Poem

A rabbit with files writes with care,
Tmp files vanish in the air—
Rename them whole, never halfway,
No crashes truncate the data today! 🐇✨

🚥 Pre-merge checks | ✅ 4 | ❌ 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 (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: atomic writes for both checkpoint and dev-cache files across the two modified files (cache.rs and checkpoint.rs).
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch upstream-sync/atomic-checkpoint-cache-writes

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