494637 - inconsistent parent for null and flat closures

Status: RESOLVED DUPLICATE of bug 659577

(Core :: JavaScript Engine, defect)

Description

[Igor Bukanov]

Currently js_Interpter, when creating null and flat closures, uses fp->scopeChain as their parent. This means that null closure may or may not get a Call or Block on their parent chain depending on the current runtime situation. For example, the following examples prints [object global], null because for the second invocation of f the null closure will have the Call object on its scope chain:

function f(x)
{
    if (x)
        x("1");
    return function() {}
}

print(f().__parent__);
print(f(eval).__parent__);

At the very least the parent chain should be similar for both cases. I suggest to use always callee's parent when creating null or flat closures. Besides the consistency, it will also avoid rooting Call or Block objects that may created during closure's parent execution or debugging.

Comment 1

[Tom S [:evilpie]]

__parent__ has been removed.

Comment 2

[Brendan Eich [:brendan]]

Tom, this isn't about __parent__, that was just used to demo the inconsistency.

/be

Comment 3

[Brendan Eich [:brendan]]

So, the comment 0 code would look like this nowadays:

function f(x)
{
    if (x)
        x("1");
    return function() {}
}

print(parent(f()));
print(parent(f(eval)));

But this now prints [object global] twice, because indirect eval is global eval (no Call object is created during the second call to f).

Igor's point about the debugger or some equivalent way of forcing Call object creation selectively and unpredictably at runtime stands.

Looks like I'm going to fix this for bug 592202.

/be

Comment 4

[Nochum Sossonko [:Natch]]

(In reply to comment #3)
> Looks like I'm going to fix this for bug 592202.

Fixed?

Comment 5

[Brendan Eich [:brendan]]

Not fixed.

/be

Comment 6

[Luke Wagner [:luke]]

Bug 659577 provides deterministic scope chain depth; debugger gets a faux-scope-chain of proxies that doesn't mutate fp->scopeChain ever.