Signal list cycles in ‘length’ etc.

Use macros like FOR_EACH_TAIL instead of maybe_quit to
catch list cycles automatically instead of relying on the
user becoming impatient and typing C-g (Bug#25606).
* src/fns.c (Flength, Fmember, Fmemq, Fmemql, Fassq, Fassoc, Frassq)
(Frassoc, Fdelete, Freverse):
Use FOR_EACH_TAIL instead of maybe_quit.
(Fnreverse): Use simple EQ to check for circular list instead
of rarely_quit, as this suffices in this unusual case.
(Fplist_put, Flax_plist_put, Flax_plist_put):
Use FOR_EACH_TAIL_CONS instead of maybe_quit.
(internal_equal): Use FOR_EACH_TAIL_CONS to check lists, instead
of by-hand tail recursion that did not catch cycles.
* src/fns.c (Fsafe_length, Fplist_get):
* src/xdisp.c (display_mode_element):
Use FOR_EACH_TAIL_SAFE instead of by-hand Floyd’s algorithm.
* src/lisp.h (QUIT_COUNT_HEURISTIC): Remove; no longer needed.
(rarely_quit): Simply count toward USHRT_MAX + 1, since the
fancier versions are no longer needed.
(FOR_EACH_TAIL_CONS, FOR_EACH_TAIL_SAFE)
(FOR_EACH_TAIL_INTERNAL): New macros, the last with definiens
mostly taken from FOR_EACH_TAIL.
(FOR_EACH_TAIL): Rewrite in terms of FOR_EACH_TAIL_INTERNAL.

1 files changed, 24 insertions, 11 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 102e8bd70ef..13fca0b29e0 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3129,20 +3129,14 @@ extern void maybe_quit (void);
 
 #define QUITP (!NILP (Vquit_flag) && NILP (Vinhibit_quit))
 
-/* Heuristic on how many iterations of a tight loop can be safely done
-   before it's time to do a quit.  This must be a power of 2.  It
-   is nice but not necessary for it to equal USHRT_MAX + 1.  */
-
-enum { QUIT_COUNT_HEURISTIC = 1 << 16 };
-
 /* Process a quit rarely, based on a counter COUNT, for efficiency.
-   "Rarely" means once per QUIT_COUNT_HEURISTIC or per USHRT_MAX + 1
-   times, whichever is smaller (somewhat arbitrary, but often faster).  */
+   "Rarely" means once per USHRT_MAX + 1 times; this is somewhat
+   arbitrary, but efficient.  */
 
 INLINE void
 rarely_quit (unsigned short int count)
 {
-  if (! (count & (QUIT_COUNT_HEURISTIC - 1)))
+  if (! count)
     maybe_quit ();
 }
 
@@ -4598,13 +4592,32 @@ enum
    http://maths-people.anu.edu.au/~brent/pd/rpb051i.pdf  */
 
 #define FOR_EACH_TAIL(list)                                             \
+  FOR_EACH_TAIL_INTERNAL (list, CHECK_LIST_END (li.tail, list),         \
+                          circular_list (list))
+
+/* Like FOR_EACH_TAIL (LIST), except check only for cycles.  */
+
+#define FOR_EACH_TAIL_CONS(list) \
+  FOR_EACH_TAIL_INTERNAL (list, (void) 0, circular_list (list))
+
+/* Like FOR_EACH_TAIL (LIST), except check for neither dotted lists
+   nor cycles.  */
+
+#define FOR_EACH_TAIL_SAFE(list) \
+  FOR_EACH_TAIL_INTERNAL (list, (void) 0, (void) (li.tail = Qnil))
+
+/* Like FOR_EACH_TAIL (LIST), except evaluate DOTTED or CYCLE,
+   respectively, if a dotted list or cycle is found.  This is an
+   internal macro intended for use only by the above macros.  */
+
+#define FOR_EACH_TAIL_INTERNAL(list, dotted, cycle)                     \
   for (struct { Lisp_Object tail, tortoise; intptr_t n, max; } li       \
          = { list, list, 2, 2 };                                        \
-       CONSP (li.tail) || (CHECK_LIST_END (li.tail, list), false);      \
+       CONSP (li.tail) || (dotted, false);                              \
        (li.tail = XCDR (li.tail),                                       \
         (li.n-- == 0                                                    \
          ? (void) (li.n = li.max <<= 1, li.tortoise = li.tail)          \
-         : EQ (li.tail, li.tortoise) ? circular_list (list) : (void) 0)))
+         : EQ (li.tail, li.tortoise) ? (cycle) : (void) 0)))
 
 /* Do a `for' loop over alist values.  */