* src/lisp.h (FOR_EACH_TAIL): New macro.

* src/fns.c (Fdelq): Use it to avoid inf-loops; remove QUIT.

3 files changed, 21 insertions, 9 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index c88f48af517..2a4da678b56 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,8 @@
 2013-12-04  Stefan Monnier  <monnier@iro.umontreal.ca>
 
+        * lisp.h (FOR_EACH_TAIL): New macro.
+        * fns.c (Fdelq): Use it to avoid inf-loops; remove QUIT.
+
         * window.c (select_window): Call second wset_redisplay before we change
         selected_window (bug#16034).
 
diff --git a/src/fns.c b/src/fns.c
index e705bdc58e9..b13bb8d80dc 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -1537,15 +1537,12 @@ Write `(setq foo (delq element foo))' to be sure of correctly changing
 the value of a list `foo'.  */)
   (register Lisp_Object elt, Lisp_Object list)
 {
-  register Lisp_Object tail, prev;
-  register Lisp_Object tem;
+  Lisp_Object tail, tortoise, prev = Qnil;
+  bool skip;
 
-  tail = list;
-  prev = Qnil;
-  while (CONSP (tail))
+  FOR_EACH_TAIL (tail, list, tortoise, skip)
     {
-      CHECK_LIST_CONS (tail, list);
-      tem = XCAR (tail);
+      Lisp_Object tem = XCAR (tail);
       if (EQ (elt, tem))
         {
           if (NILP (prev))
@@ -1555,8 +1552,6 @@ the value of a list `foo'.  */)
         }
       else
         prev = tail;
-      tail = XCDR (tail);
-      QUIT;
     }
   return list;
 }
diff --git a/src/lisp.h b/src/lisp.h
index 60cbc98daf8..d8cf6b6960e 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4443,6 +4443,20 @@ extern void *record_xmalloc (size_t);
       memory_full (SIZE_MAX);                                  \
   } while (0)
 
+/* Loop over all tails of a list, checking for cycles.
+   FIXME: Make tortoise and n internal declarations.
+   FIXME: Unroll the loop body so we don't need `n'.  */
+#define FOR_EACH_TAIL(hare, list, tortoise, n)  \
+  for (tortoise = hare = (list), n = true;                      \
+       CONSP (hare);                                            \
+       (hare = XCDR (hare), n = !n,                             \
+        (n                                                      \
+         ? ((EQ (hare, tortoise)                                \
+             && (xsignal1 (Qcircular_list, (list)), 0)))        \
+         /* Move tortoise before the next iteration, in case */ \
+         /* the next iteration does an Fsetcdr.  */             \
+         : (tortoise = XCDR (tortoise), 0))))
+
 /* Do a `for' loop over alist values.  */
 
 #define FOR_EACH_ALIST_VALUE(head_var, list_var, value_var)             \