GH-18572: infinite stack recursion in fallback object comparison.

With nested objects and recursive comparisons, it is for now unavoidable
to have a stack overflow we do some early damage control attempt early
on with zend.max_allowed_stack_size check but ultimately more a band-aid
than a definitive solution.

Author: devnexen

Zend/zend_object_handlers.c

@@ -42,6 +42,15 @@
 #define IN_UNSET	ZEND_GUARD_PROPERTY_UNSET
 #define IN_ISSET	ZEND_GUARD_PROPERTY_ISSET
 
+static zend_always_inline bool zend_objects_check_stack_limit(void)
+{
+#ifdef ZEND_CHECK_STACK_LIMIT
+	return zend_call_stack_overflowed(EG(stack_limit));
+#else
+	return false;
+#endif
+}
+
 /*
   __X accessors explanation:
 
@@ -1714,6 +1723,11 @@ ZEND_API int zend_std_compare_objects(zval *o1, zval *o2) /* {{{ */
 {
 	zend_object *zobj1, *zobj2;
 
+	if (zend_objects_check_stack_limit()) {
+		zend_throw_error(NULL, "Object compare - stack limit reached");
+		return ZEND_UNCOMPARABLE;
+	}
+
 	if (Z_TYPE_P(o1) != Z_TYPE_P(o2)) {
 		/* Object and non-object */
 		zval *object;