Restore half of Matt's fix.

I don't quite understand why it's needed, but it immediately
failed CI.

Author: byroot

gc.c

@@ -1810,6 +1810,16 @@ static const rb_data_type_t id_to_obj_tbl_type = {
     .flags = RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_FREE_IMMEDIATELY
 };
 
+bool
+rb_gc_update_id_to_obj_table(VALUE obj, VALUE id)
+{
+    if (id_to_obj_tbl && !st_lookup(id_to_obj_tbl, id, 0)) {
+        st_insert(id_to_obj_tbl, id, obj);
+        return true;
+    }
+    return false;
+}
+
 static VALUE
 object_id(VALUE obj)
 {

internal/gc.h

@@ -262,6 +262,8 @@ void ruby_sized_xfree(void *x, size_t size);
 const char *rb_gc_active_gc_name(void);
 int rb_gc_modular_gc_loaded_p(void);
 
+bool rb_gc_update_id_to_obj_table(VALUE obj, VALUE id);
+
 RUBY_SYMBOL_EXPORT_END
 
 int rb_ec_stack_check(struct rb_execution_context_struct *ec);

variable.c

@@ -1590,6 +1590,13 @@ rb_evict_fields_to_hash(VALUE obj)
 
     // Evacuate all previous values from shape into id_table
     rb_obj_copy_fields_to_hash_table(obj, table);
+    rb_shape_t *shape = rb_shape_get_shape(obj);
+    if (rb_shape_has_object_id(shape)) {
+        // We need to ensure the object ID is registered in id_to_obj_table
+        // before transitioning to too complex.
+        rb_gc_update_id_to_obj_table(obj, id);
+    }
+
     obj_transition_too_complex(obj, table);
 
     RUBY_ASSERT(rb_shape_obj_too_complex(obj));