(derived-mode-all-parents): Fix handling of cycles

* lisp/subr.el (derived-mode-all-parents): Fix the handling of cycles
so that it doesn't fill the cache with incorrect results.
(merge-ordered-lists): Improve docstring.
(provided-mode-derived-p): Swap the loops since `modes` is usually
shorter than `ps`.

* test/lisp/subr-tests.el (subr-tests--parent-mode): Simplify.
(subr-tests--mode-A, subr-tests--mode-B, subr-tests--mode-C): New funs.
(subt-tests--derived-mode-add-parents): New test.

1 files changed, 24 insertions, 2 deletions

diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index db327056533..03eb0d5bf8c 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -345,8 +345,7 @@
 
 ;;;; Mode hooks.
 
-(defalias 'subr-tests--parent-mode
-  (if (fboundp 'prog-mode) 'prog-mode 'fundamental-mode))
+(defalias 'subr-tests--parent-mode #'prog-mode)
 
 (define-derived-mode subr-tests--derived-mode-1 prog-mode "test")
 (define-derived-mode subr-tests--derived-mode-2 subr-tests--parent-mode "test")
@@ -360,6 +359,29 @@
                                    'subr-tests--parent-mode))
   (should (provided-mode-derived-p 'subr-tests--derived-mode-2 'prog-mode)))
 
+
+(define-derived-mode subr-tests--mode-A subr-tests--derived-mode-1 "t")
+(define-derived-mode subr-tests--mode-B subr-tests--mode-A "t")
+(defalias 'subr-tests--mode-C #'subr-tests--mode-B)
+(derived-mode-add-parents 'subr-tests--mode-A '(subr-tests--mode-C))
+
+(ert-deftest subt-tests--derived-mode-add-parents ()
+  ;; The Right Answer is somewhat unclear in the presence of cycles,
+  ;; but let's make sure we get tolerable answers.
+  ;; FIXME: Currently `prog-mode' doesn't always end up at the end :-(
+  (let ((set-equal (lambda (a b)
+                     (not (or (cl-set-difference a b)
+                              (cl-set-difference b a))))))
+    (dolist (mode '(subr-tests--mode-A subr-tests--mode-B subr-tests--mode-C))
+      (should (eq (derived-mode-all-parents mode)
+                  (derived-mode-all-parents mode)))
+      (should (eq mode (car (derived-mode-all-parents mode))))
+      (should (funcall set-equal
+                       (derived-mode-all-parents mode)
+                       '(subr-tests--mode-A subr-tests--mode-B prog-mode
+                         subr-tests--mode-C subr-tests--derived-mode-1))))))
+
+
 (ert-deftest number-sequence-test ()
   (should (= (length
               (number-sequence (1- most-positive-fixnum) most-positive-fixnum))