checkCancel

fn checkCancel(userdata: ?*anyopaque) Io.Cancelable!void {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    _ = ev;
    const fiber = Thread.current().currentFiber();
    switch (fiber.cancel_protection.check()) {
        .unblocked => {
            const cancel_status = @atomicLoad(Fiber.CancelStatus, &fiber.cancel_status, .monotonic);
            assert(cancel_status.awaiting == .nothing);
            if (cancel_status.requested) {
                @branchHint(.unlikely);
                fiber.cancel_protection.acknowledge();
                return error.Canceled;
            }
        },
        .blocked => {},
    }
}