>>I think there is no need to explicitly check whether the directory
>>is writable.  What would be the purpose of that?
> 
> 
> I didn't write that patch, but I guess there could be a purpose for it.
> Namely it could help deleting a file in vain. If there are Filesystems
> that let you delete a writable file even though you can't create new
> ones in a non-writable dir, then without those lines, emacs would delete
> an old backup only to find out that it can't create a new one.
> 
> Most likely these would be cases where the old backup file was writable,
> so if you had some logic to write to existing files and only delete them
> if they are not writable, then the check whether the directory is
> writable should really be superfluous. Otherwise I'm not so sure.

Wouldn't it be simpler to try something like the attached patch.