archive-unique-fname is a compiled Lisp function in `arc-mode.el'.

(archive-unique-fname FNAME DIR)

Make sure a file FNAME can be created uniquely in directory DIR.

If FNAME can be uniquely created in DIR, it is returned unaltered.
If FNAME is something our underlying filesystem can't grok, or if another
file by that name already exists in DIR, a unique new name is generated
using `make-temp-file', and the generated name is returned.