throw/catch(e) abandons a Document

[Simon Fraser (smfr)]

If you load LayoutTests/fast/css/invalid-import-rule-insertion.html, then load another page and clear the page cache, the Document for LayoutTests/fast/css/invalid-import-rule-insertion.html is never released. It seems to be trapped in a retain cycle.

[Radar WebKit Bug Importer]

<rdar://problem/40743441>

[Simon Fraser (smfr)]

This is triggered by the throw and a catch that references the exception. Minimal test case: 

<script>
    try
    {
        throw('The document is abandoned');
    }
    catch(e)
    {
    }
</script>

[Simon Fraser (smfr)]

My heap inspector tool shows a reference chain thus:

Exception (GC root—VM exceptions) -> Internal -> 
ProgramCodeBlock  ->  Internal -> 
Window  -> Variable document  -> 
HTMLDocument  “file:///Volumes/Data/Development/apple/webkit/OpenSource/LayoutTests/fast/css/invalid-import-rule-insertion.html”

m_vm->lastException() is a GC root (see slotVisitor.appendUnbarriered(m_vm->lastException()) in Heap.cpp).

Seems like we should clear this at some point.

[Simon Fraser (smfr)]

I guess we clear lastException() next time we run script (in the VMEntryScope constructor).

[Simon Fraser (smfr)]

Maybe GCController should call clearLastException() in some code paths.

[Geoffrey Garen]

Two promising places to clear lastException:

(1) ~JSLock()

(2) A zero-delay timer

The purpose of lastException is to provide an out-of-band accessor to the exception thrown by the current task / micro task. So, it's OK to throw it away after the task / micro task ends.

[Keith Miller]

*** This bug has been marked as a duplicate of bug 186277 ***