start

    fn start() Io.Cancelable!AlertableSyscall {
        const thread = Thread.current orelse return .{ .thread = null };
        switch (thread.cancel_protection) {
            .blocked => return .{ .thread = null },
            .unblocked => {},
        }
        const old_status = thread.status.fetchOr(.{
            .cancelation = @fromBackingInt(@intCast(0b010)),
            .awaitable = .null,
        }, .monotonic);
        switch (old_status.cancelation) {
            .parked => unreachable,
            .blocked => unreachable,
            .blocked_alertable => unreachable,
            .blocked_canceling => unreachable,
            .blocked_alertable_canceling => unreachable,
            .none => return .{ .thread = thread }, // new status is `.blocked_alertable`
            .canceling => {
                // Status is unchanged (still `.canceling`)---change to `.canceled` before return.
                thread.status.store(.{ .cancelation = .canceled, .awaitable = old_status.awaitable }, .monotonic);
                return error.Canceled;
            },
            .canceled => return .{ .thread = null }, // new status is `.canceled` (unchanged)
        }
    }