Avoid face inheritance cycles

* src/xfaces.c (face_inheritance_cycle): New function.
(Finternal_set_lisp_face_attribute): Signal an error if the
':inherit' attribute of a face is modified in a way that will
cause it to inherit from itself.  (Bug#79672)

* test/src/xfaces-tests.el (xfaces-test-circular-inheritance): New
test.

* etc/NEWS: Announce the incompatible change.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/src/xfaces-tests.el b/test/src/xfaces-tests.el
index a4c4f148af7..6129018262c 100644
--- a/test/src/xfaces-tests.el
+++ b/test/src/xfaces-tests.el
@@ -52,6 +52,19 @@
   (should (equal (color-values-from-color-spec "rgbi:0/0x0/0") nil))
   (should (equal (color-values-from-color-spec "rgbi:0/+0x1/0") nil)))
 
+(ert-deftest xfaces-test-circular-inheritance ()
+  "Test that bug#79672 remains solved."
+  (let ((buf (get-buffer-create "xfaces-test")))
+    (with-current-buffer buf
+      (font-lock-mode -1)
+      (set-face-attribute 'button nil :inherit 'link)
+      (should (equal
+               (should-error (set-face-attribute 'link nil :inherit 'button))
+               (list 'error
+                     "Face inheritance results in inheritance cycle"
+                     'button))))
+    (kill-buffer buf)))
+
 (provide 'xfaces-tests)
 
 ;;; xfaces-tests.el ends here