.tip

Threads that start staggered rarely collide, so the test passes and production does not. A barrier makes all of them hit the same line at the same instant.

barrier = threading.Barrier(16)
def worker():
    barrier.wait()
    for _ in range(50_000): record("invoice")

Assert on the aggregate. On the free-threaded build this fails against a naive counter every time, which is exactly the point.