fix: cache race conditions and time underflow bugs - #58
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Issue: Veirt#49 reported that detached cache tasks can stack during slow disk I/O. The previous implementation spawned unbounded tokio tasks for each cache write, which could accumulate faster than the filesystem could process them. Solution: Introduce a bounded mpsc channel (32 tasks) with a single writer worker that serializes all cache writes. When the buffer is full, try_send drops writes instead of blocking, providing backpressure. Changes: - Add CacheWriteTask enum for location/weather/geocode writes - Add static CACHE_WRITER with OnceLock for single worker initialization - Refactor save_* functions to use try_send instead of spawn - Extract write_* async functions for actual I/O operations - Add test demonstrating rapid cache writes complete without hang The bounded worker prevents memory exhaustion under sustained slow I/O while maintaining non-blocking behavior for callers.
Gemini bug hunt found 3 critical/high severity bugs:
1. Cache thrashing: Multiple locations overwrote single cache files.
Fix: Use location-specific filenames (weather_{location}_{provider}.json,
geocode_{location}_{language}.json) to support multiple cached locations.
2. Read/write race: Concurrent reads could see truncated/corrupted JSON during writes.
Fix: Atomic write-then-rename pattern using temp files to prevent partial reads.
3. Time underflow panic: Clock adjustments backward caused arithmetic underflow.
Fix: Use saturating_sub() instead of plain subtraction for cache expiration checks.
All three fixes maintain backward compatibility — old single-file caches still work,
new multi-file caches enable proper multi-location support.
Deferred (low severity, design decisions):
- Silent error discarding (would need logging framework)
- Wasted create_dir_all calls (performance, not correctness)
- Redundant write collapsing (enhancement, not bug)
- Graceful shutdown (requires broader coordination)
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Summary
Gemini bug hunt found 3 critical/high severity bugs:
Cache thrashing: Multiple locations overwrote single cache files.
Fix: Use location-specific filenames (weather_{location}{provider}.json,
geocode{location}_{language}.json) to support multiple cached locations.
Read/write race: Concurrent reads could see truncated/corrupted JSON during writes.
Fix: Atomic write-then-rename pattern using temp files to prevent partial reads.
Time underflow panic: Clock adjustments backward caused arithmetic underflow.
Fix: Use saturating_sub() instead of plain subtraction for cache expiration checks.
All three fixes maintain backward compatibility — old single-file caches still work,
new multi-file caches enable proper multi-location support.
Deferred (low severity, design decisions):
Test plan