From 66ec92af0060952ef7b53a8f1c07177cf38b3145 Mon Sep 17 00:00:00 2001
From: Eli Zaretskii <eliz@gnu.org>
Date: Sat, 2 Dec 2017 10:57:15 +0200
Subject: [PATCH] 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)
---
 lisp/files.el | 48 +++++++++++++++++++++++++++++-------------------
 1 file 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))))))
 
-- 
2.39.2