Handle empty strings in Tramp's expand-file-name implementations

* lisp/net/tramp.el (tramp-handle-expand-file-name):
* lisp/net/tramp-gvfs.el (tramp-gvfs-handle-expand-file-name):
* lisp/net/tramp-sh.el (tramp-sh-handle-expand-file-name):
* lisp/net/tramp-smb.el (tramp-smb-handle-expand-file-name):
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-handle-expand-file-name):
Handle empty NAME.

* test/lisp/net/tramp-tests.el (tramp-test01-file-name-syntax)
(tramp-test01-file-name-syntax-simplified)
(tramp-test01-file-name-syntax-separate): Use neutral IPv4 address.
(tramp-test05-expand-file-name): Check also "." and "".

6 files changed, 29 insertions, 7 deletions

diff --git a/lisp/net/tramp-gvfs.el b/lisp/net/tramp-gvfs.el
index 569fdb55615..2d8f42004a8 100644
--- a/lisp/net/tramp-gvfs.el
+++ b/lisp/net/tramp-gvfs.el
@@ -883,6 +883,8 @@ file names."
   "Like `expand-file-name' for Tramp files."
   ;; If DIR is not given, use DEFAULT-DIRECTORY or "/".
   (setq dir (or dir default-directory "/"))
+  ;; Handle empty NAME.
+  (when (zerop (length name)) (setq name "."))
   ;; Unless NAME is absolute, concat DIR and NAME.
   (unless (file-name-absolute-p name)
     (setq name (concat (file-name-as-directory dir) name)))
diff --git a/lisp/net/tramp-sh.el b/lisp/net/tramp-sh.el
index 58c2fe5fd3f..ee16138f700 100644
--- a/lisp/net/tramp-sh.el
+++ b/lisp/net/tramp-sh.el
@@ -2716,6 +2716,8 @@ If the localname part of the given file name starts with \"/../\" then
 the result will be a local, non-Tramp, file name."
   ;; If DIR is not given, use `default-directory' or "/".
   (setq dir (or dir default-directory "/"))
+  ;; Handle empty NAME.
+  (when (zerop (length name)) (setq name "."))
   ;; Unless NAME is absolute, concat DIR and NAME.
   (unless (file-name-absolute-p name)
     (setq name (concat (file-name-as-directory dir) name)))
diff --git a/lisp/net/tramp-smb.el b/lisp/net/tramp-smb.el
index 64a8fa22507..e6e2485ea0f 100644
--- a/lisp/net/tramp-smb.el
+++ b/lisp/net/tramp-smb.el
@@ -692,6 +692,8 @@ PRESERVE-UID-GID and PRESERVE-EXTENDED-ATTRIBUTES are completely ignored."
   "Like `expand-file-name' for Tramp files."
   ;; If DIR is not given, use DEFAULT-DIRECTORY or "/".
   (setq dir (or dir default-directory "/"))
+  ;; Handle empty NAME.
+  (when (zerop (length name)) (setq name "."))
   ;; Unless NAME is absolute, concat DIR and NAME.
   (unless (file-name-absolute-p name)
     (setq name (concat (file-name-as-directory dir) name)))
diff --git a/lisp/net/tramp-sudoedit.el b/lisp/net/tramp-sudoedit.el
index 42deaf37975..0d9e04d0bd1 100644
--- a/lisp/net/tramp-sudoedit.el
+++ b/lisp/net/tramp-sudoedit.el
@@ -353,6 +353,8 @@ If the localname part of the given file name starts with \"/../\" then
 the result will be a local, non-Tramp, file name."
   ;; If DIR is not given, use `default-directory' or "/".
   (setq dir (or dir default-directory "/"))
+  ;; Handle empty NAME.
+  (when (zerop (length name)) (setq name "."))
   ;; Unless NAME is absolute, concat DIR and NAME.
   (unless (file-name-absolute-p name)
     (setq name (concat (file-name-as-directory dir) name)))
diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index b35b36eac7d..97ec5e174b4 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -3069,6 +3069,8 @@ User is always nil."
   "Like `expand-file-name' for Tramp files."
   ;; If DIR is not given, use DEFAULT-DIRECTORY or "/".
   (setq dir (or dir default-directory "/"))
+  ;; Handle empty NAME.
+  (when (zerop (length name)) (setq name "."))
   ;; Unless NAME is absolute, concat DIR and NAME.
   (unless (file-name-absolute-p name)
     (setq name (concat (file-name-as-directory dir) name)))
diff --git a/test/lisp/net/tramp-tests.el b/test/lisp/net/tramp-tests.el
index f470385be9f..69d5ba8b7df 100644
--- a/test/lisp/net/tramp-tests.el
+++ b/test/lisp/net/tramp-tests.el
@@ -238,8 +238,8 @@ properly.  BODY shall not contain a timeout."
   (should (tramp-tramp-file-p "/method:user@[::1]:"))
 
   ;; Using an IPv4 mapped IPv6 address.
-  (should (tramp-tramp-file-p "/method:[::ffff:192.168.0.1]:"))
-  (should (tramp-tramp-file-p "/method:user@[::ffff:192.168.0.1]:"))
+  (should (tramp-tramp-file-p "/method:[::ffff:1.2.3.4]:"))
+  (should (tramp-tramp-file-p "/method:user@[::ffff:1.2.3.4]:"))
 
   ;; Local file name part.
   (should (tramp-tramp-file-p "/method:::"))
@@ -268,7 +268,7 @@ properly.  BODY shall not contain a timeout."
   (should-not (tramp-tramp-file-p "/1.2.3.4:"))
   (should-not (tramp-tramp-file-p "/[]:"))
   (should-not (tramp-tramp-file-p "/[::1]:"))
-  (should-not (tramp-tramp-file-p "/[::ffff:192.168.0.1]:"))
+  (should-not (tramp-tramp-file-p "/[::ffff:1.2.3.4]:"))
   (should-not (tramp-tramp-file-p "/host:/:"))
   (should-not (tramp-tramp-file-p "/host1|host2:"))
   (should-not (tramp-tramp-file-p "/user1@host1|user2@host2:"))
@@ -318,8 +318,8 @@ properly.  BODY shall not contain a timeout."
 	  (should (tramp-tramp-file-p "/user@[::1]:"))
 
 	  ;; Using an IPv4 mapped IPv6 address.
-	  (should (tramp-tramp-file-p "/[::ffff:192.168.0.1]:"))
-	  (should (tramp-tramp-file-p "/user@[::ffff:192.168.0.1]:"))
+	  (should (tramp-tramp-file-p "/[::ffff:1.2.3.4]:"))
+	  (should (tramp-tramp-file-p "/user@[::ffff:1.2.3.4]:"))
 
 	  ;; Local file name part.
 	  (should (tramp-tramp-file-p "/host::"))
@@ -372,8 +372,8 @@ properly.  BODY shall not contain a timeout."
 	  (should (tramp-tramp-file-p "/[method/user@::1]"))
 
 	  ;; Using an IPv4 mapped IPv6 address.
-	  (should (tramp-tramp-file-p "/[method/::ffff:192.168.0.1]"))
-	  (should (tramp-tramp-file-p "/[method/user@::ffff:192.168.0.1]"))
+	  (should (tramp-tramp-file-p "/[method/::ffff:1.2.3.4]"))
+	  (should (tramp-tramp-file-p "/[method/user@::ffff:1.2.3.4]"))
 
 	  ;; Local file name part.
 	  (should (tramp-tramp-file-p "/[method/]"))
@@ -1988,6 +1988,18 @@ properly.  BODY shall not contain a timeout."
   (should
    (string-equal
     (expand-file-name "/method:host:/path/../file") "/method:host:/file"))
+  (should
+   (string-equal
+    (expand-file-name "/method:host:/path/.") "/method:host:/path"))
+  (should
+   (string-equal
+    (expand-file-name "/method:host:/path/..") "/method:host:/"))
+  (should
+   (string-equal
+    (expand-file-name "." "/method:host:/path/") "/method:host:/path"))
+  (should
+   (string-equal
+    (expand-file-name "" "/method:host:/path/") "/method:host:/path"))
   ;; Quoting local part.
   (should
    (string-equal