dirOpenDir

fn dirOpenDir(
    userdata: ?*anyopaque,
    dir: Dir,
    sub_path: []const u8,
    options: Dir.OpenOptions,
) Dir.OpenError!Dir {
    const ev: *Evented = @ptrCast(@alignCast(userdata));

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

    var cancel_region: CancelRegion = .init();
    defer cancel_region.deinit();
    return .{
        .handle = ev.openat(&cancel_region, dir.handle, sub_path_posix, .{
            .ACCMODE = .RDONLY,
            .DIRECTORY = true,
            .NOFOLLOW = !options.follow_symlinks,
            .CLOEXEC = true,
            .PATH = !options.iterate,
        }, 0) catch |err| switch (err) {
            error.IsDir => return errnoBug(.ISDIR),
            error.WouldBlock => return errnoBug(.AGAIN),
            error.FileTooBig => return errnoBug(.FBIG),
            error.NoSpaceLeft => return errnoBug(.NOSPC),
            error.DeviceBusy => return errnoBug(.BUSY), // EXCL unset.
            error.FileBusy => return errnoBug(.TXTBSY),
            error.PathAlreadyExists => return errnoBug(.EXIST), // Not creating.
            error.OperationUnsupported => return errnoBug(.OPNOTSUPP), // No TMPFILE, no locks.
            error.ReadOnlyFileSystem => return errnoBug(.ROFS), // Not creating.
            else => |e| return e,
        },
    };
}