Fix custom-available-themes file expansion

For discussion, see thread starting at
https://lists.gnu.org/archive/html/emacs-devel/2018-05/msg00222.html.
* lisp/custom.el: (custom-available-themes): Use directory-files
instead of performing arbitrary wildcard expansion in file names.
(custom-theme--load-path): Document return value.
* test/lisp/custom-tests.el: New file.
(custom-theme--load-path): New test.

2 files changed, 103 insertions, 10 deletions

diff --git a/lisp/custom.el b/lisp/custom.el
index b8ea8811a2a..4536788eb20 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -1311,19 +1311,25 @@ The returned symbols may not correspond to themes that have been
 loaded, and no effort is made to check that the files contain
 valid Custom themes.  For a list of loaded themes, check the
 variable `custom-known-themes'."
-  (let (sym themes)
+  (let ((suffix "-theme\\.el\\'")
+        themes)
     (dolist (dir (custom-theme--load-path))
-      (when (file-directory-p dir)
-        (dolist (file (file-expand-wildcards
-                       (expand-file-name "*-theme.el" dir) t))
-          (setq file (file-name-nondirectory file))
-          (and (string-match "\\`\\(.+\\)-theme.el\\'" file)
-               (setq sym (intern (match-string 1 file)))
-               (custom-theme-name-valid-p sym)
-               (push sym themes)))))
-    (nreverse (delete-dups themes))))
+      ;; `custom-theme--load-path' promises DIR exists and is a
+      ;; directory, but `custom.el' is loaded too early during
+      ;; bootstrap to use `cl-lib' macros, so guard with
+      ;; `file-directory-p' instead of calling `cl-assert'.
+      (dolist (file (and (file-directory-p dir)
+                         (directory-files dir nil suffix)))
+        (let ((theme (intern (substring file 0 (string-match-p suffix file)))))
+          (and (custom-theme-name-valid-p theme)
+               (not (memq theme themes))
+               (push theme themes)))))
+    (nreverse themes)))
 
 (defun custom-theme--load-path ()
+  "Expand `custom-theme-load-path' into a list of directories.
+Members of `custom-theme-load-path' that either don't exist or
+are not directories are omitted from the expansion."
   (let (lpath)
     (dolist (f custom-theme-load-path)
       (cond ((eq f 'custom-theme-directory)
diff --git a/test/lisp/custom-tests.el b/test/lisp/custom-tests.el
new file mode 100644
index 00000000000..96887f8f5fe
--- /dev/null
+++ b/test/lisp/custom-tests.el
@@ -0,0 +1,87 @@
+;;; custom-tests.el --- tests for custom.el  -*- lexical-binding: t -*-
+
+;; Copyright (C) 2018 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+
+(ert-deftest custom-theme--load-path ()
+  "Test `custom-theme--load-path' behavior."
+  (let ((tmpdir (file-name-as-directory (make-temp-file "custom-tests-" t))))
+    (unwind-protect
+        ;; Create all temporary files under the same deletable parent.
+        (let ((temporary-file-directory tmpdir))
+          ;; Path is empty.
+          (let ((custom-theme-load-path ()))
+            (should (null (custom-theme--load-path))))
+
+          ;; Path comprises non-existent file.
+          (let* ((name (make-temp-name tmpdir))
+                 (custom-theme-load-path (list name)))
+            (should (not (file-exists-p name)))
+            (should (null (custom-theme--load-path))))
+
+          ;; Path comprises existing file.
+          (let* ((file (make-temp-file "file"))
+                 (custom-theme-load-path (list file)))
+            (should (file-exists-p file))
+            (should (not (file-directory-p file)))
+            (should (null (custom-theme--load-path))))
+
+          ;; Path comprises existing directory.
+          (let* ((dir (make-temp-file "dir" t))
+                 (custom-theme-load-path (list dir)))
+            (should (file-directory-p dir))
+            (should (equal (custom-theme--load-path) custom-theme-load-path)))
+
+          ;; Expand `custom-theme-directory' path element.
+          (let ((custom-theme-load-path '(custom-theme-directory)))
+            (let ((custom-theme-directory (make-temp-name tmpdir)))
+              (should (not (file-exists-p custom-theme-directory)))
+              (should (null (custom-theme--load-path))))
+            (let ((custom-theme-directory (make-temp-file "file")))
+              (should (file-exists-p custom-theme-directory))
+              (should (not (file-directory-p custom-theme-directory)))
+              (should (null (custom-theme--load-path))))
+            (let ((custom-theme-directory (make-temp-file "dir" t)))
+              (should (file-directory-p custom-theme-directory))
+              (should (equal (custom-theme--load-path)
+                             (list custom-theme-directory)))))
+
+          ;; Expand t path element.
+          (let ((custom-theme-load-path '(t)))
+            (let ((data-directory (make-temp-name tmpdir)))
+              (should (not (file-exists-p data-directory)))
+              (should (null (custom-theme--load-path))))
+            (let ((data-directory tmpdir)
+                  (themedir (expand-file-name "themes" tmpdir)))
+              (should (not (file-exists-p themedir)))
+              (should (null (custom-theme--load-path)))
+              (with-temp-file themedir)
+              (should (file-exists-p themedir))
+              (should (not (file-directory-p themedir)))
+              (should (null (custom-theme--load-path)))
+              (delete-file themedir)
+              (make-directory themedir)
+              (should (file-directory-p themedir))
+              (should (equal (custom-theme--load-path) (list themedir))))))
+      (when (file-directory-p tmpdir)
+        (delete-directory tmpdir t)))))
+
+;;; custom-tests.el ends here