Fix backing up remote files in local directories on MS-Windows

* lisp/files.el (make-backup-file-name-1): Support remote file
names correctly when they are backed up into a local directory on
MS-Windows and MS-DOS.  (Bug#29440)

1 files changed, 29 insertions, 19 deletions

diff --git a/lisp/files.el b/lisp/files.el
index 8021e1bbed5..1bdb6d38ab9 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -4653,25 +4653,35 @@ The function `find-backup-file-name' also uses this."
               ;; "/drive_x".
               (or (file-name-absolute-p file)
                   (setq file (expand-file-name file))) ; make defaults explicit
-              ;; Replace any invalid file-name characters (for the
-              ;; case of backing up remote files).
-              (setq file (expand-file-name (convert-standard-filename file)))
-              (if (eq (aref file 1) ?:)
-                  (setq file (concat "/"
-                                     "drive_"
-                                     (char-to-string (downcase (aref file 0)))
-                                     (if (eq (aref file 2) ?/)
-                                         ""
-                                       "/")
-                                     (substring file 2)))))
-            ;; Make the name unique by substituting directory
-            ;; separators.  It may not really be worth bothering about
-            ;; doubling `!'s in the original name...
-            (expand-file-name
-             (subst-char-in-string
-              ?/ ?!
-              (replace-regexp-in-string "!" "!!" file))
-             backup-directory))
+              (cond
+               ((file-remote-p file)
+                ;; Remove the leading slash, if any, to prevent
+                ;; convert-standard-filename from converting that to a
+                ;; backslash.
+                (and (memq (aref file 0) '(?/ ?\\))
+                     (setq file (substring file 1)))
+                ;; Replace any invalid file-name characters, then
+                ;; prepend the leading slash back.
+                (setq file (concat "/" (convert-standard-filename file))))
+               (t
+                ;; Replace any invalid file-name characters.
+                (setq file (expand-file-name (convert-standard-filename file)))
+                (if (eq (aref file 1) ?:)
+                    (setq file (concat "/"
+                                       "drive_"
+                                       (char-to-string (downcase (aref file 0)))
+                                       (if (eq (aref file 2) ?/)
+                                           ""
+                                         "/")
+                                       (substring file 2))))))
+              ;; Make the name unique by substituting directory
+              ;; separators.  It may not really be worth bothering about
+              ;; doubling `!'s in the original name...
+              (expand-file-name
+               (subst-char-in-string
+                ?/ ?!
+                (replace-regexp-in-string "!" "!!" file))
+               backup-directory)))
         (expand-file-name (file-name-nondirectory file)
                           (file-name-as-directory abs-backup-directory))))))