dirOpenDir

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

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

    const flags: c.O = .{
        .ACCMODE = .RDONLY,
        .NOFOLLOW = !options.follow_symlinks,
        .DIRECTORY = true,
        .CLOEXEC = true,
    };

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