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, 3 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index cbf2b70c821..4d8ae091a7c 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -900,6 +900,9 @@ collection).
 ** 'if-let*', 'when-let*', and 'and-let*' are new in subr-x.el.
 The incumbent 'if-let' and 'when-let' are now aliases.
 
+** Low-level list functions like 'length' and 'member' now do a better
+job of signaling list cycles instead of looping indefinitely.
+
 +++
 ** The new functions 'make-nearby-temp-file' and 'temporary-file-directory'
 can be used for creation of temporary files of remote or mounted directories.