Fix make-temp-file bug with ""/"."/".." prefix

The bug with "." and ".." has been present for a while; I
introduced the bug with "" earlier today in my patch for Bug#28023.
* lisp/files.el (make-temp-file): Do not use expand-file-name if
PREFIX is empty or "." or "..", as it does the wrong thing.
Compute absolute-prefix here ...
(files--make-magic-temp-file): ... instead of here ...
* src/fileio.c (Fmake_temp_file_internal): ... or here.

* lisp/files.el (make-temp-file): If the prefix is empty, append
"/" to the absolute prefix so that the new files are children
rather than siblings of temporary-file-directory.  This fixes a
bug introduced in the previous change.
* test/lisp/files-tests.el (files-test-make-temp-file-empty-prefix):
New test, for the bug.

lisp/files.el

@@ -1407,14 +1407,17 @@ You can then use `write-region' to write new data into the file.
 If DIR-FLAG is non-nil, create a new empty directory instead of a file.
 
 If SUFFIX is non-nil, add that at the end of the file name."
-  (let ((absolute-prefix (expand-file-name prefix temporary-file-directory)))
+  (let ((absolute-prefix
+	 (if (or (zerop (length prefix)) (member prefix '("." "..")))
+	     (concat (file-name-as-directory temporary-file-directory) prefix)
+	   (expand-file-name prefix temporary-file-directory))))
     (if (find-file-name-handler absolute-prefix 'write-region)
-        (files--make-magic-temp-file prefix dir-flag suffix)
+        (files--make-magic-temp-file absolute-prefix dir-flag suffix)
       (make-temp-file-internal absolute-prefix
 			       (if dir-flag t) (or suffix "")))))
 
-(defun files--make-magic-temp-file (prefix &optional dir-flag suffix)
+(defun files--make-magic-temp-file (absolute-prefix &optional dir-flag suffix)
-  "Implement (make-temp-file PREFIX DIR-FLAG SUFFIX).
+  "Implement (make-temp-file ABSOLUTE-PREFIX DIR-FLAG SUFFIX).
 This implementation works on magic file names."
   ;; Create temp files with strict access rights.  It's easy to
   ;; loosen them later, whereas it's impossible to close the
@@ -1423,13 +1426,7 @@ This implementation works on magic file names."
     (let (file)
       (while (condition-case ()
 		 (progn
-		   (setq file
+		   (setq file (make-temp-name absolute-prefix))
-			 (make-temp-name
-			  (if (zerop (length prefix))
-			      (file-name-as-directory
-			       temporary-file-directory)
-			    (expand-file-name prefix
-					      temporary-file-directory))))
 		   (if suffix
 		       (setq file (concat file suffix)))
 		   (if dir-flag

src/fileio.c

@@ -662,17 +662,16 @@ DEFUN ("make-temp-file-internal", Fmake_temp_file_internal,
 Return the name of the generated file.  If DIR-FLAG is zero, do not
 create the file, just its name.  Otherwise, if DIR-FLAG is non-nil,
 create an empty directory.  The file name should end in SUFFIX.
+Do not expand PREFIX; a non-absolute PREFIX is relative to the Emacs
+working directory.
 
 Signal an error if the file could not be created.
 
 This function does not grok magic file names.  */)
   (Lisp_Object prefix, Lisp_Object dir_flag, Lisp_Object suffix)
 {
-  bool make_temp_name = EQ (dir_flag, make_number (0));
+  CHECK_STRING (prefix);
   CHECK_STRING (suffix);
-  if (!make_temp_name)
-    prefix = Fexpand_file_name (prefix, Vtemporary_file_directory);
-
   Lisp_Object encoded_prefix = ENCODE_FILE (prefix);
   Lisp_Object encoded_suffix = ENCODE_FILE (suffix);
   ptrdiff_t prefix_len = SBYTES (encoded_prefix);
@@ -686,7 +685,7 @@ This function does not grok magic file names.  */)
   memset (data + prefix_len, 'X', nX);
   memcpy (data + prefix_len + nX, SSDATA (encoded_suffix), suffix_len);
   int kind = (NILP (dir_flag) ? GT_FILE
-	      : make_temp_name ? GT_NOCREATE
+	      : EQ (dir_flag, make_number (0)) ? GT_NOCREATE
 	      : GT_DIR);
   int fd = gen_tempname (data, suffix_len, O_BINARY | O_CLOEXEC, kind);
   if (fd < 0 || (NILP (dir_flag) && emacs_close (fd) != 0))

test/lisp/files-tests.el

@@ -166,6 +166,20 @@ form.")
 	    (should (eq buffer-file-coding-system 'iso-2022-7bit-unix))))
       (delete-file tempfile))))
 
+(ert-deftest files-test-make-temp-file-empty-prefix ()
+  "Test make-temp-file with an empty prefix."
+  (let ((tempfile (make-temp-file ""))
+        (tempdir (make-temp-file "" t))
+        (tempfile-. (make-temp-file "."))
+        (tempdir-. (make-temp-file "." t))
+        (tempfile-.. (make-temp-file ".."))
+        (tempdir-.. (make-temp-file ".." t)))
+    (dolist (file (list tempfile tempfile-. tempfile-..))
+      (should file)
+      (delete-file file))
+    (dolist (dir (list tempdir tempdir-. tempdir-..))
+      (should dir)
+      (delete-directory dir))))
 
 ;; Stop the above "Local Var..." confusing Emacs.