Re: [PATCH v6 0/4] MR6855: server: Fix renaming file to a hardlink of itself.

[Jinoh Kang (＠iamahuman)]

Jinoh Kang (@iamahuman) commented about server/fd.c:
> - {
> -
>    if (!(dirname = memdup( dst, dstname - dst )))
> -
>        return;
> -
>    dirname[dstname - dst - 1] = '\0';
> -
>    if ((res = stat( dirname, &st )))
> -
>        file_set_error();
> -
>    free( dirname );
> -
>    if (res)
> -
>        return;
> - }
> -
> - srcname = strrchr( src, '/' ) + 1;
> - if (!(dirname = memdup( src, srcname - src )))
> -
>    return;
> - dirname[srcname - src - 1] = '\0';
> - if ((dirfd = open( dirname, O_RDONLY | O_NONBLOCK )) == -1)

Better open the destination file's directory instead of the source file's directory.

1. We may only have search access to src directory but not read access (--x).
2. `O_PATH` is only available on Linux-like OSes. `O_SEARCH` is also not cross platform. Notably, macOS lacks `O_PATH`.

To achieve this, we can switch the order: do stat() on src dirname, and open/fstat on dest dirname.

If different directories, it must be a hardlink, so we simply remove the source and bail out. If the same, switching order did nothing, and we can proceed as normal.