Fix #768: TinyUfo weight underflow panic from evict race condition#823
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cobyfrombrooklyn-bot wants to merge 1 commit intocloudflare:mainfrom
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Fix #768: TinyUfo weight underflow panic from evict race condition#823cobyfrombrooklyn-bot wants to merge 1 commit intocloudflare:mainfrom
cobyfrombrooklyn-bot wants to merge 1 commit intocloudflare:mainfrom
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…anic A race condition exists where one thread evicts an item (subtracting its weight) before another thread finishes adding the weight after insertion. This causes fetch_sub to underflow the atomic usize, wrapping to usize::MAX, which then causes 'attempt to add with overflow' panic in evict_to_limit. Replace all fetch_sub on weight atomics with fetch_update using saturating_sub, and use saturating_add in evict_to_limit's weight comparison. This prevents the underflow without adding locks. Fixes cloudflare#768
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A race condition exists in TinyUfo where one thread evicts an item (subtracting its weight via
fetch_sub) before another thread finishes adding the weight after insertion. This causes the atomicusizeto underflow (wrap tousize::MAX), which then triggersattempt to add with overflowpanic inevict_to_limit.Fix
fetch_subcalls onsmall_weightandmain_weightwithfetch_updateusingsaturating_sub- prevents underflow from wrapping.saturating_addinevict_to_limitweight comparison - prevents overflow when summing weights.This is a minimal, lock-free fix that preserves the existing concurrent design.
Tests
Added two tests:
test_weight_saturating_sub_no_underflow: Directly verifies that weight atomics handle underflow gracefully by setting weight to 0 then subtracting, and thatevict_to_limithandles near-usize::MAXweights without panicking.test_concurrent_put_no_overflow_panic: Stress test with 20 threads doing concurrent put/get operations (reproduces the original race from the issue).All 16 TinyUfo tests pass on macOS ARM (Apple Silicon).
Fixes #768