Bug 228892 – [JSC] Custom getter / setter functions can be collected

• Status: NEW
• Component: JavaScriptCore

[Yusuke Suzuki]

It is weakly held. This means that they can be collected if nobody references it.
So,

{
    var getter = Object.getOwnPropertyDescriptor(RegExp, "rightContext").get;
    getter.mark = true;
}

// Make GC happens.

{
    var getter = Object.getOwnPropertyDescriptor(RegExp, "rightContext").get;
    print(getter.mark); // => undefined
}

We need to keep these accessors strongly in the holder.

[Alexey Shvayka]

Wow, this is a great catch!

Since there could be a lot of `Object.getOwnPropertyDescriptors(%WebIDLInterface%.prototype)` out there in the web, my first approach would be creating separate HashSet on JSGlobalObject that is strong, and leverage Structure::didTransitionFromThisStructure() (m_transitionWatchpointSet) to copy JSCustom{Getter,Setter}Function objects there, so we can avoid overriding MethodTable::destroy() for performance reasons.

[Alexey Shvayka]

So the idea is to keep them weak unless they transition from initial structure.

[Alexey Shvayka]

Hmm, a watchpoint won't help preserve the first marked getter / setter correctly as we have no place to check it except JSObject::getOwnPropertyDescriptor().

Instead, we should subclass WeakHandleOwner, check for modified structure in its finalize(), and tweak WeakGCSet to accept it.

[Radar WebKit Bug Importer]

<rdar://problem/81928455>