[Harmony Proxies] LazyReadCopy experiment and invariant checking for [[Extensible]]=false

David Bruant david.bruant at labri.fr
Thu Jul 14 09:20:10 PDT 2011

+ Mark to discuss ES5.1 invariants.

> I'm working on an implementation. It will be independent from the
> FixedHandler one. It should be possible to combine both easily though.
> I'll give a follow-up here when I'm done.
So, here it is:
I have just commited but not tested. The reason is that I wondered if
all this effort was worth.
I've re-read the invariants in ES5 (last one on extensible not allowed
to go from false to true omitted):
* If the value of the host object’s [[Extensible]] internal property is
has been observed by ECMAScript code to be false, then if a call to
[[GetOwnProperty]] describes a property as non-existent all subsequent
calls must also describe that property as non-existent.
=> To implement this, the handler just has to keep a record of "already
observed as non-existent properties" and make sure [[GetOwnProperty]]
respects the invariant by a lookup in this record.

* The [[DefineOwnProperty]] internal method of a host object must not
permit the addition of a new property to a host object if the
[[Extensible]] internal property of that host object has been observed
by ECMAScript code to be false.
=> This one is a bit more tricky. "must not permit the addition of a new
property" seems to assume that an object has a set of properties. But
that's not really true for proxies. The engine has no way to say whether
a property is defined in a proxy or not. Consequently, it cannot tell if
[[DefineOwnProperty]] is called on a "new" or "old" property.
This could be compensated by explicitely returning a list of property
names in the fix/preventExtension trap.

Also, none of the 3 invariants regarding [[Extensible]]=false are about
property names enumeration. Is there just no invariant to hold on
enumeration? If there is none, well, no need to do O(n)-ish enforcements!

I'll update and test my code based on conclusions of this thread.


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