* simple.el (undo): Prevent insertion of identity mapping into

undo-equiv-table so as undo-only does not inf loop in the presence
of consecutive nils in undo list.

Fixes: debbugs:17236

2 files changed, 12 insertions, 1 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index e90693180a8..f5f3208256e 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,9 @@
+2014-04-12  Barry O'Reilly  <gundaetiapo@gmail.com>
+
+        * simple.el (undo): Prevent insertion of identity mapping into
+        undo-equiv-table so as undo-only does not inf loop in the presence
+        of consecutive nils in undo list.
+
 2014-04-12  Matthias Dahl  <matthias.dahl@binary-island.eu>
 
         * faces.el (make-face): Deprecate optional argument as it is no
diff --git a/lisp/simple.el b/lisp/simple.el
index 68bfd7b14f4..a5e8381eaaa 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -2143,7 +2143,12 @@ as an argument limits undo to changes within the current region."
       ;; above when checking.
       (while (eq (car list) nil)
         (setq list (cdr list)))
-      (puthash list (if undo-in-region t pending-undo-list)
+      (puthash list
+               ;; Prevent identity mapping.  This can happen if
+               ;; consecutive nils are erroneously in undo list.
+               (if (or undo-in-region (eq list pending-undo-list))
+                   t
+                 pending-undo-list)
                undo-equiv-table))
     ;; Don't specify a position in the undo record for the undo command.
     ;; Instead, undoing this should move point to where the change is.