[Bug #19469] Fix crash when resizing generic iv list

The following script can sometimes trigger a crash:

```ruby
GC.stress = true

class Array
  def foo(bool)
    if bool
      @a = 1
      @b = 2
      @c = 1
    else
      @c = 1
    end
  end
end

obj = []
obj.foo(true)

obj2 = []
obj2.foo(false)

obj3 = []
obj3.foo(true)
```

This is because vm_setivar_default calls rb_ensure_generic_iv_list_size
to resize the iv list. However, the call to gen_ivtbl_resize reallocs
the iv list, and then inserts into the generic iv table. If the
st_insert triggers a GC then the old iv list will be read during
marking, causing a use-after-free bug.

Co-Authored-By: Jemma Issroff <jemmaissroff@gmail.com>

internal/variable.h

@@ -57,7 +57,7 @@ VALUE rb_gvar_defined(ID);
 void rb_const_warn_if_deprecated(const rb_const_entry_t *, VALUE, ID);
 rb_shape_t * rb_grow_iv_list(VALUE obj);
 void rb_ensure_iv_list_size(VALUE obj, uint32_t len, uint32_t newsize);
-struct gen_ivtbl * rb_ensure_generic_iv_list_size(VALUE obj, uint32_t newsize);
+struct gen_ivtbl *rb_ensure_generic_iv_list_size(VALUE obj, rb_shape_t *shape, uint32_t newsize);
 attr_index_t rb_obj_ivar_set(VALUE obj, ID id, VALUE val);
 MJIT_SYMBOL_EXPORT_END
 

variable.c

@@ -66,7 +66,10 @@ static st_table *generic_iv_tbl_;
 struct ivar_update {
     struct gen_ivtbl *ivtbl;
     uint32_t iv_index;
-    rb_shape_t* shape;
+    uint32_t max_index;
+#if !SHAPE_IN_BASIC_FLAGS
+    rb_shape_t *shape;
+#endif
 };
 
 void
@@ -1009,7 +1012,7 @@ generic_ivar_update(st_data_t *k, st_data_t *v, st_data_t u, int existing)
         }
     }
     FL_SET((VALUE)*k, FL_EXIVAR);
-    ivtbl = gen_ivtbl_resize(ivtbl, ivup->shape->next_iv_index);
+    ivtbl = gen_ivtbl_resize(ivtbl, ivup->max_index);
     // Reinsert in to the hash table because ivtbl might be a newly resized chunk of memory
     *v = (st_data_t)ivtbl;
     ivup->ivtbl = ivtbl;
@@ -1272,7 +1275,10 @@ generic_ivar_set(VALUE obj, ID id, VALUE val)
         RUBY_ASSERT(index == (shape->next_iv_index - 1));
     }
 
+    ivup.max_index = shape->next_iv_index;
+#if !SHAPE_IN_BASIC_FLAGS
     ivup.shape = shape;
+#endif
 
     RB_VM_LOCK_ENTER();
     {
@@ -1373,15 +1379,22 @@ rb_ensure_iv_list_size(VALUE obj, uint32_t current_capacity, uint32_t new_capaci
 }
 
 struct gen_ivtbl *
-rb_ensure_generic_iv_list_size(VALUE obj, uint32_t newsize)
+rb_ensure_generic_iv_list_size(VALUE obj, rb_shape_t *shape, uint32_t newsize)
 {
     struct gen_ivtbl * ivtbl = 0;
 
     RB_VM_LOCK_ENTER();
     {
         if (UNLIKELY(!gen_ivtbl_get_unlocked(obj, 0, &ivtbl) || newsize > ivtbl->numiv)) {
-            ivtbl = gen_ivtbl_resize(ivtbl, newsize);
-            st_insert(generic_ivtbl_no_ractor_check(obj), (st_data_t)obj, (st_data_t)ivtbl);
+            struct ivar_update ivup = {
+                .iv_index = newsize - 1,
+                .max_index = newsize,
+#if !SHAPE_IN_BASIC_FLAGS
+                .shape = shape
+#endif
+            };
+            st_update(generic_ivtbl_no_ractor_check(obj), (st_data_t)obj, generic_ivar_update, (st_data_t)&ivup);
+            ivtbl = ivup.ivtbl;
             FL_SET_RAW(obj, FL_EXIVAR);
         }
     }

vm_insnhelper.c

@@ -1416,11 +1416,11 @@ vm_setivar_default(VALUE obj, ID id, VALUE val, shape_id_t dest_shape_id, attr_i
         shape_id_t source_shape_id = dest_shape->parent_id;
 
         if (shape_id == source_shape_id && dest_shape->edge_name == id && dest_shape->type == SHAPE_IVAR) {
-            ivtbl = rb_ensure_generic_iv_list_size(obj, index + 1);
+            ivtbl = rb_ensure_generic_iv_list_size(obj, dest_shape, index + 1);
 #if SHAPE_IN_BASIC_FLAGS
             RBASIC_SET_SHAPE_ID(obj, dest_shape_id);
 #else
-            ivtbl->shape_id = dest_shape_id;
+            RUBY_ASSERT(ivtbl->shape_id == dest_shape_id);
 #endif
         }
         else {