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, 14 insertions, 0 deletions

diff --git a/test/lisp/files-tests.el b/test/lisp/files-tests.el
index 7bfdca53e08..4a17e0d4697 100644
--- a/test/lisp/files-tests.el
+++ b/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.