sleepNanosleep

fn sleepNanosleep(t: *Threaded, timeout: Io.Timeout) Io.Cancelable!void {
    const t_io = io(t);
    const sec_type = @typeInfo(posix.timespec).@"struct".field_types[0];
    const nsec_type = @typeInfo(posix.timespec).@"struct".field_types[1];

    var timespec: posix.timespec = t: {
        const d = timeout.toDurationFromNow(t_io) orelse break :t .{
            .sec = std.math.maxInt(sec_type),
            .nsec = std.math.maxInt(nsec_type),
        };
        break :t timestampToPosix(d.raw.toNanoseconds());
    };
    const syscall: Syscall = try .start();
    while (true) {
        switch (posix.errno(posix.system.nanosleep(&timespec, &timespec))) {
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            // This prong handles success as well as unexpected errors.
            else => return syscall.finish(),
        }
    }
}