Fix core dump in substitute-object-in-subtree

Without this fix, (substitute-object-in-subtree #0=(#0# 'a) 'a)
would dump core, since the C code would recurse indefinitely through
the infinite structure.  This patch adds an argument to the function,
and renames it to lread--substitute-object-in-subtree as the function
is not general-purpose and should not be relied on by outside code.
See Bug#23660.
* src/intervals.c (traverse_intervals_noorder): ARG is now void *,
not Lisp_Object, so that callers need not cons unnecessarily.
All callers changed.  Also, remove related #if-0 code that was
“temporary” in the early 1990s and has not been compilable for
some time.
* src/lread.c (struct subst): New type, for substitution closure data.
(seen_list): Remove this static var, as this info is now part of
struct subst.  All uses removed.
(Flread__substitute_object_in_subtree): Rename from
Fsubstitute_object_in_subtree, and give it a 3rd arg so that it
doesn’t dump core when called from the top level with an
already-cyclic structure.  All callers changed.
(SUBSTITUTE): Remove.  All callers expanded and then simplified.
(substitute_object_recurse): Take a single argument SUBST rather
than a pair OBJECT and PLACEHOLDER, so that its address can be
passed around as part of a closure; this avoids the need for an
AUTO_CONS call.  All callers changed.  If the COMPLETED component
is t, treat every subobject as potentially circular.
(substitute_in_interval): Take a struct subst * rather than a
Lisp_Object, for the closure data.  All callers changed.
* test/src/lread-tests.el (lread-lread--substitute-object-in-subtree):
New test, to check that the core dump does not reoccur.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/alloc.c b/src/alloc.c
index ac3de83b2b6..2d785d5b9a4 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -1553,7 +1553,7 @@ make_interval (void)
 /* Mark Lisp objects in interval I.  */
 
 static void
-mark_interval (register INTERVAL i, Lisp_Object dummy)
+mark_interval (INTERVAL i, void *dummy)
 {
   /* Intervals should never be shared.  So, if extra internal checking is
      enabled, GC aborts if it seems to have visited an interval twice.  */
@@ -1567,7 +1567,7 @@ mark_interval (register INTERVAL i, Lisp_Object dummy)
 #define MARK_INTERVAL_TREE(i)                                   \
   do {                                                          \
     if (i && !i->gcmarkbit)                                     \
-      traverse_intervals_noorder (i, mark_interval, Qnil);      \
+      traverse_intervals_noorder (i, mark_interval, NULL);      \
   } while (0)
 
 /***********************************************************************