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.

1 files changed, 8 insertions, 11 deletions

diff --git a/lisp/files.el b/lisp/files.el
index 19573cdf7b2..b05d453b0e7 100644
--- a/lisp/files.el
+++ b/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)
-  "Implement (make-temp-file PREFIX DIR-FLAG SUFFIX).
+(defun files--make-magic-temp-file (absolute-prefix &optional 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
-                         (make-temp-name
-                          (if (zerop (length prefix))
-                              (file-name-as-directory
-                               temporary-file-directory)
-                            (expand-file-name prefix
-                                              temporary-file-directory))))
+                   (setq file (make-temp-name absolute-prefix))
                    (if suffix
                        (setq file (concat file suffix)))
                    (if dir-flag