# Liveness barriers and finalization

**URL:** <https://es.discourse.group/t/liveness-barriers-and-finalization/1082>\
**Category:** I have questions\
**Created:** [November 14, 2021, 10:24am UTC](https://es.discourse.group/t/liveness-barriers-and-finalization/1082 "2021-11-14T10:24:41Z")\
**Posts on this page:** 1\
**Showing post:** 7

<div class="post-metadata">

**Author:** ![felix.s](https://yyz2.discourse-cdn.com/free1/user_avatar/es.discourse.group/felix.s/32/1109_2.png) [@felix.s](https://es.discourse.group/u/felix.s)\
**Post date:** [November 15, 2021, 12:26pm UTC](https://es.discourse.group/t/liveness-barriers-and-finalization/1082/7 "2021-11-15T12:26:21Z")

</div>

> [@mhofman](#):
>
> In any external usage, the program will require keeping a reference to the resource object itself, and the identity will be observed by the object's method call, which has to use `this` while reaching the private field.

The PR ticket you linked specifically calls out early dispatch and inlining as allowed. The specification doesn’t distinguish ‘internal’ and ‘external’ usage: all user code is equivalent, and an engine can perform transformations on it that would be otherwise impossible to express due to name resolution and scoping rules. That should make cross-module inlining allowed.

If you what you claim here by ‘the object's method call […] has to use `this` while reaching the private field’ is that private field accesses observe object identity, then even the original example cannot trigger a use-after-free. But the [PrivateGet](https://tc39.es/ecma262/#sec-privateget) algorithm does not look like it observes object identity any more than [OrdinaryGet](https://tc39.es/ecma262/#sec-ordinaryget) does, so scalar replacement performed against one should be as valid as against the other.

In practice, I’d expect the biggest obstacle against such shenanigans to be the reason I had to add `Object.freeze(Resource.prototype)` to the second example: an engine must consider the fact that in the meantime, i.e. before the asynchronous job completes, someone may have monkey-patched `Resource.prototype.use` so that it observe the identity of `this` after all (whatever that may mean).

> **A particularly ironic interpretation of ‘observing object identity’**
>
> If we take [ECMA-262 12th Ed. §9.9.2](https://262.ecma-international.org/12.0/#sec-liveness) Note 4 seriously, then it seems even the motivating example in that PR can fail:
> 
> ```javascript
> const o = {};
> const wr = new WeakRef(o);
> await 1;
> /* (A) */
> assert(wr.deref() === o);
> 
> ```
> 
> In a WeakRef-oblivious execution with respect to { `o` }, we have `wr.deref()` return undefined. The subsequent strict comparison between undefined and `o` therefore returns false because of a type mismatch. This does not constitute ‘observing a strict equality comparison between **objects** ’ (note plural); you don’t need to know which object `o` is to know it is not undefined. Therefore, `o` is eligible for collection at (A)!

---

_[View the full topic](https://es.discourse.group/t/liveness-barriers-and-finalization/1082)._