dtourolleandClaude Opus 4.8
bceccc3406
Channel<T>::pop() surfaced the out-of-band sentinel from its empty branch using the tail_ snapshot taken at the top of the loop. Under contention the producer can push more values *and* the sentinel in the window between that snapshot and take_sentinel(), so pop() could return the sentinel while real values still sat in the ring — the sentinel jumping ahead of values pushed before it. No value was lost (a consumer that keeps draining still receives them, and approx_size() keeps counting them so a PoolNode reschedules), but a consumer treating the sentinel as a hard "last message" barrier would act on EOF early. Re-confirm emptiness against a fresh tail_ load before taking the sentinel. Costs one acquire-load on the empty-ring path only; never runs in steady state. The spin and post-spin takes already reload tail_ on the line above them; try_pop_now() already reads tail_ fresh in the same branch — both were correct and are unchanged. The two sentinel stress cases now assert the strict "sentinel is last, after every value" ordering (previously relaxed to avoid the flake this fixes). Verified TSan-clean (2606 assertions, no data races) over repeated runs. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>