> Once I considered this approach, but problems was that
> nFileIndexLow/High can change every time file handle is opened, so
> it's not unique.

Ah, I see, then your approach makes sense.

There's another part of your patch that I don't understand:

+except ImportError: # not running on Windows - mock up something sensible
+    from posixpath import samefile # XXX

In what situations will this happen, and are we guaranteed in those
cases that samefile will be found in posixpath?