feat: add FileBackend for filesystem-based caching #18
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Implements a pure Python filesystem cache backend with: Core Features: - BaseBackend protocol compliance (get/set/delete/exists/health_check) - Thread-safe operations via RLock + fcntl.flock/msvcrt.locking - Atomic writes via write-then-rename pattern - LRU eviction at 90% capacity to 70% target - TTL support with expiration enforcement on read - 14-byte versioned header format for corruption detection Security Hardening: - blake2b key hashing prevents path traversal - O_NOFOLLOW on all file opens prevents symlink attacks - lstat() in cleanup/eviction prevents symlink manipulation - ELOOP handling for symlink detection at runtime - Default permissions 0o600/0o700 with security warnings - TTL bounds validation (0 to 10 years max) - Entry count limits prevent disk exhaustion - TOCTOU elimination via "ask forgiveness" pattern Configuration (via CACHEKIT_FILE_* env vars): - cache_dir, max_size_mb, max_value_mb - max_entry_count, lock_timeout_seconds - permissions, dir_permissions Test Coverage: - 89 unit tests (91% coverage) - 7 integration tests (concurrency, atomicity, eviction) - 5 performance benchmarks - 4 critical path tests - Security-specific tests for symlink attacks, TOCTOU, TTL overflow
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Summary
Implements a pure Python filesystem cache backend (
FileBackend) that provides:Security Hardening
Configuration
Via
CACHEKIT_FILE_*environment variables:cache_dir,max_size_mb,max_value_mbmax_entry_count,lock_timeout_secondspermissions,dir_permissionsTest Coverage
Files Changed
src/cachekit/backends/file/- New backend implementationtests/unit/backends/test_file_*.py- Unit teststests/integration/test_file_backend_integration.py- Integration teststests/performance/test_file_backend_perf.py- Benchmarkstests/critical/test_file_backend_critical.py- Critical path testsdocs/guides/backend-guide.md- DocumentationTest plan
make quick-checkpasses