Re: [PATCH v11 0/4] MR6855: server: Fix renaming file to a hardlink of itself.

[Jinoh Kang (＠iamahuman)]

Jinoh Kang (@iamahuman) commented about server/fd.c:
> - if (!is_dir)
> - {
> -
>    /* we can't just do rename(2) here; renaming a file to its hardlink is no-op */
> -
>    if (linkat( dirfd, tmpname, dirfd, dstname, 0 ) && errno != EEXIST)
> -
>    {
> -
>        if (errno == EPERM || errno == EOPNOTSUPP)
> -
>        {
> -
>            /* FIXME: filesystem doesn't support hardlinks, entire operation might be a no-op on casefold dirs */
> -
>            if (renameat( dirfd, tmpname, dirfd, dstname ))
> -
>                file_set_error();
> -
>            goto ret;
> -
>        }
> -
> -
>        file_set_error();
> -
>        /* revert the temporary rename */
> -
>        renameat( dirfd, tmpname, dirfd, srcname );

Let's make this a common logic for all cases (hardlink, no hardlink, & dir).

Something like this (untested):

```c
    int undo_tmp_rename = 0;

(...)

undo_tmp_rename = 1;

/* directories can't have hardlinks */
    if (!is_dir)
    {
        /* we can't just do rename(2) here; renaming a file to its hardlink is no-op */
        if (linkat( dirfd, tmpname, dirfd, dstname, 0 ) && errno != EEXIST)
        {
            if (errno != EPERM && errno != EOPNOTSUPP)
            {
                file_set_error();
                goto ret;
            }

/* Filesystem doesn't support hardlinks, fall back to renaming */
        }
        else
        {
            undo_tmp_rename = 0;

if (unlinkat( dirfd, tmpname, 0 ))
                file_set_error();

goto ret;  /* We're done */
        }
    }

/* Renaming a directory, or filesystem doesn't support hardlinks */
    if (renameat( dirfd, tmpname, dirfd, dstname ))
        file_set_error();
    else
        undo_tmp_rename = 0;

ret:
    if (undo_tmp_rename)
    {
        /* revert the temporary rename */
        renameat( dirfd, tmpname, dirfd, srcname );
    }
```