bug#4674: 23.1; UNC paths and file-relative-directory

[Richard Copley <rcopley@gmail.com>]

The function `file-relative-directory' has logic to detect when the
two given paths are part of two separate directory trees, and to
return the absolute file name in such cases. That logic does not catch
the case when the two given paths are UNC paths on different servers.
For example, on my present network (where there are computers named
IOBATES and KERES), the form (file-relative-name "//iobates/e/temp"
"//keres/e/temp") returns "../../../iobates/e/temp", which is not a
valid relative path to "//iobates/e/temp" from "//keres/e/temp". This
is therefore a bug.

As a workaround, I have the following piece of advice in my
site-start. However I'm not sure that it is wise to mess with remote
file handling in this way.

(defadvice file-remote-p (around unc-host-and-share activate)
  "For UNC paths, return the first two components."
  (let ((file (ad-get-arg 0)))
    (save-match-data
      (if (and (eq system-type 'windows-nt)
               (string-match "\\`\\(//[^:/\\\\]+/[^:/\\\\]+\\)" file))
          (setq ad-return-value (match-string 1 file))
        ad-do-it))))

In GNU Emacs 23.1.1 (i386-mingw-nt5.1.2600)
 of 2009-07-30 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4)'

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ENG
  value of $XMODIFIERS: nil
  locale-coding-system: cp1252
  default-enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  tool-bar-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  blink-cursor-mode: t
  global-auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
M-: ( f i l e - r e l a t i v e - n a m e SPC " / /
i o b a t e s / e / t e m p " SPC " / / k e r e s /
e / t e m p " ) <return> M-x r e p o r t - e m a c
s - b u g <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
"../../../iobates/e/temp"