Fix print.c infloop on circular lists

Fix infinite loops in print.c when a circular list is passed
to command-error-default-function or to error-message-string.
* src/print.c (print_error_message):
Use FOR_EACH_TAIL to avoid infloop on circular lists.
(print_object): Use FOR_EACH_TAIL_SAFE, as it uses
Brent’s teleporting tortoise-hare algorithm which is
asymptotically better than the classic tortoise-hare
algorithm that the code wsas using.
* test/src/print-tests.el (print-circle-2): When print-circle
is nil, do not insist on a particular cycle-detection heuristic.
(error-message-string-circular): New test.

1 files changed, 5 insertions, 1 deletions

diff --git a/test/src/print-tests.el b/test/src/print-tests.el
index 26d49a5ffba..77371a1b4ce 100644
--- a/test/src/print-tests.el
+++ b/test/src/print-tests.el
@@ -345,11 +345,15 @@ otherwise, use a different charset."
    ;; Bug#31146.
   (let ((x '(0 . #1=(0 . #1#))))
     (let ((print-circle nil))
-      (should (string-match "\\`(0 0 . #[0-9])\\'"
+      (should (string-match "\\`(0\\( 0\\)* . #[0-9]+)\\'"
                             (print-tests--prin1-to-string x))))
     (let ((print-circle t))
       (should (equal "(0 . #1=(0 . #1#))" (print-tests--prin1-to-string x))))))
 
+(print-tests--deftest error-message-string-circular ()
+  (let ((err (list 'error)))
+    (setcdr err err)
+    (should-error (error-message-string err) :type 'circular-list)))
 
 (provide 'print-tests)
 ;;; print-tests.el ends here