Fix bug #4674 with UNCs in file-relative-name.

 files.el (file-relative-name): Handle UNC file names on DOS/Windows.
 Also fixes bug#4673.

2 files changed, 25 insertions, 2 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index f6514c16577..aaa55e9abfe 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,8 @@
+2010-12-04  Eli Zaretskii  <eliz@gnu.org>
+
+        * files.el (file-relative-name): Handle UNC file names on
+        DOS/Windows.  (Bug#4674)
+
 2010-12-02  Glenn Morris  <rgm@gnu.org>
 
         * ps-print.el (ps-line-lengths-internal, ps-nb-pages):
diff --git a/lisp/files.el b/lisp/files.el
index 4901c3872cd..97dce0fc1e5 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -4054,11 +4054,29 @@ on a DOS/Windows machine, it returns FILENAME in expanded form."
           (dremote (file-remote-p directory)))
       (if ;; Conditions for separate trees
           (or
-           ;; Test for different drives on DOS/Windows
+           ;; Test for different filesystems on DOS/Windows
            (and
             ;; Should `cygwin' really be included here?  --stef
             (memq system-type '(ms-dos cygwin windows-nt))
-            (not (eq t (compare-strings filename 0 2 directory 0 2))))
+            (or
+             ;; Test for different drive letters
+             (not (eq t (compare-strings filename 0 2 directory 0 2)))
+             ;; Test for UNCs on different servers
+             (not (eq t (compare-strings
+                         (progn
+                           (if (string-match "\\`//\\([^:/]+\\)/" filename)
+                               (match-string 1 filename)
+                             ;; Windows file names cannot have ? in
+                             ;; them, so use that to detect when
+                             ;; neither FILENAME nor DIRECTORY is a
+                             ;; UNC.
+                             "?"))
+                         0 nil
+                         (progn
+                           (if (string-match "\\`//\\([^:/]+\\)/" directory)
+                               (match-string 1 directory)
+                             "?"))
+                         0 nil t)))))
            ;; Test for different remote file system identification
            (not (equal fremote dremote)))
           filename