fix(storage): 修复 WAL.Clear() nil panic 并添加并发安全保护

[NeverENG]

• Clear() 在 WAL 禁用模式(file==nil)时使用 config.G.WALPath 避免空指针
• 为 WAL 添加 sync.Mutex 保护所有公共方法(Write/Read/Close/Sync/Clear)
• Read() 中使用 io.ReadFull 确保完整读取，消除部分数据风险
• RaftWAL.LoadLogs() 使用 io.ReadFull 修复截断读取
• 统一使用 slog 格式，移除冗余的 [ERROR]/[WARN] 前缀
• 重命名 HEADER_LENGTH→headerLength(非导出标识符)

Summary by CodeRabbit

• Bug Fixes

  • Improved reliability of log loading operations by ensuring complete file reads for each log entry
  • Enhanced log clearing operations with better error handling and file cleanup management
  • Better error propagation when closing and managing log files

• Refactor

  • Added thread-safety protections to write-ahead logging operations
  • Improved log reading and writing mechanisms to ensure data integrity across concurrent operations
  • Strengthened file synchronization to prevent data loss

[coderabbitai]

Caution

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Pull request was closed or merged during review

📝 Walkthrough

Walkthrough

This PR hardens WAL (write-ahead log) reliability across two layers. The core storage/zstorage/WAL.go adds mutex-protected operations, enforces full-length I/O reads via io.ReadFull, and improves state cleanup and error propagation. The Raft/raft_wal.go layer adopts the same full-length read guarantees and improves error handling during log loading and clearing.

Changes

WAL Reliability and Thread-Safety

Layer / File(s)	Summary
WAL Struct and Type Signature storage/zstorage/WAL.go	Header size constant renamed from HEADER_LENGTH (exported) to headerLength (unexported). WAL struct adds sync.Mutex field for thread-safety and updates header buffer to use new constant.
Write Operation with Locking storage/zstorage/WAL.go	Write now locks, explicitly checks errors for each header/key/value write, and calls file.Sync() on success; disabled-mode no-op behavior preserved.
Read Operation with Locking storage/zstorage/WAL.go	Read now locks, seeks to start, uses io.ReadFull for complete header/key/value blocks, treats EOF/short-read as end-of-log, and logs corruption on CRC mismatch while returning collected entries.
Close and Sync Operations storage/zstorage/WAL.go	Close and Sync acquire lock, preserve nil-file short-circuiting, and return underlying file errors only when enabled.
Clear State Management storage/zstorage/WAL.go	Clear now locks and defensively handles both modes: removes WAL path and reopens when disabled, or closes/removes/reopens when enabled with explicit sync.
Constructor Logging Updates storage/zstorage/WAL.go	NewWAL log messages switch from bracketed [INFO]/[WARN]/[ERROR] style to plain slog messages.
Raft WAL Integration Raft/raft_wal.go	Added io package import; LoadLogs uses io.ReadFull for command payload reads to fail on incomplete data; Clear checks w.Close() error, sets w.file = nil, removes state files while tolerating missing files, and recreates log file.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 The logs now read with careful grace,
Full bytes in their proper place,
With mutex locks to keep them sound,
No partial reads shall here be found,
hop hop toward reliability!

🚥 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 directly addresses the main objectives: fixing WAL.Clear() nil panic and adding concurrency protection with mutex, which are core changes in both files.
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 docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/wal-concurrency

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