checkCancel

    fn checkCancel(s: AlertableSyscall) Io.Cancelable!void {
        comptime assert(is_windows);
        const thread = s.thread orelse return;
        const old_status = thread.status.fetchOr(.{
            .cancelation = @fromBackingInt(@intCast(0b010)),
            .awaitable = .null,
        }, .monotonic);
        switch (old_status.cancelation) {
            .none => unreachable,
            .parked => unreachable,
            .blocked => unreachable,
            .blocked_canceling => unreachable,
            .canceling => unreachable,
            .canceled => unreachable,
            .blocked_alertable => {}, // new status is `.blocked_alertable` (unchanged)
            .blocked_alertable_canceling => {
                // New status is `.canceling`---change to `.canceled` before return.
                thread.status.store(.{ .cancelation = .canceled, .awaitable = old_status.awaitable }, .monotonic);
                return error.Canceled;
            },
        }
    }