dirOpenDirPosix

fn dirOpenDirPosix(
    userdata: ?*anyopaque,
    dir: Dir,
    sub_path: []const u8,
    options: Dir.OpenOptions,
) Dir.OpenError!Dir {
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    _ = t;

    if (is_windows) {
        const sub_path_w = try sliceToPrefixedFileW(dir.handle, sub_path, .{});
        return dirOpenDirWindows(dir, sub_path_w.span(), options);
    }

    var path_buffer: [posix.PATH_MAX]u8 = undefined;
    const sub_path_posix = try pathToPosix(sub_path, &path_buffer);

    var flags: posix.O = switch (native_os) {
        .wasi => .{
            .read = true,
            .NOFOLLOW = !options.follow_symlinks,
            .DIRECTORY = true,
        },
        else => .{
            .ACCMODE = .RDONLY,
            .NOFOLLOW = !options.follow_symlinks,
            .DIRECTORY = true,
            .CLOEXEC = true,
        },
    };

    if (@hasField(posix.O, "PATH") and !options.iterate)
        flags.PATH = true;

    const mode: posix.mode_t = 0;

    const syscall: Syscall = try .start();
    while (true) {
        const rc = openat_sym(dir.handle, sub_path_posix, flags, mode);
        switch (posix.errno(rc)) {
            .SUCCESS => {
                syscall.finish();
                return .{ .handle = @intCast(rc) };
            },
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            .INVAL => return syscall.fail(error.BadPathName),
            .ACCES => return syscall.fail(error.AccessDenied),
            .LOOP => return syscall.fail(error.SymLinkLoop),
            .MFILE => return syscall.fail(error.ProcessFdQuotaExceeded),
            .NAMETOOLONG => return syscall.fail(error.NameTooLong),
            .NFILE => return syscall.fail(error.SystemFdQuotaExceeded),
            .NODEV => return syscall.fail(error.NoDevice),
            .NOENT => return syscall.fail(error.FileNotFound),
            .NOMEM => return syscall.fail(error.SystemResources),
            .NOTDIR => return syscall.fail(error.NotDir),
            .PERM => return syscall.fail(error.PermissionDenied),
            .NXIO => return syscall.fail(error.NoDevice),
            .ILSEQ => return syscall.fail(error.BadPathName),
            .FAULT => |err| return syscall.errnoBug(err),
            .BADF => |err| return syscall.errnoBug(err), // File descriptor used after closed.
            .BUSY => |err| return syscall.errnoBug(err), // O_EXCL not passed
            else => |err| return syscall.unexpectedErrno(err),
        }
    }
}