await

    fn await(cancel_region: *CancelRegion, awaiting: Fiber.CancelStatus.Awaiting) Io.Cancelable!void {
        if (!cancel_region.status.requested) {
            @branchHint(.unlikely);
            return;
        }
        const status: Fiber.CancelStatus = .{ .requested = true, .awaiting = awaiting };
        if (cancel_region.fiber.cancel_status.changeAwaiting(
            cancel_region.status.awaiting,
            status.awaiting,
        )) {
            @branchHint(.unlikely);
            cancel_region.fiber.cancel_protection.acknowledge();
            cancel_region.status = .unrequested;
            return error.Canceled;
        }
        cancel_region.status = status;
    }