Bug 77488 – Replace JSArray destructor with finalizer

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Mark Hahnenberg]

We want JSArray to be in the imminent no-destructor heap, and the only thing preventing us from doing so is the SparseArrayValueMap pointer in JSArray. We can instead add a finalizer whenever we allocate one of these maps, which is very rarely so the performance hit should be negligible. There is also a void* subclassData, but any classes that use this should have destructors anyways.

[Mark Hahnenberg]

Created attachment 124854 [details]
Patch

[Geoffrey Garen]

Comment on attachment 124854 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=124854&action=review

> Source/JavaScriptCore/runtime/JSArray.cpp:314
> +    if (!map) {
>          map = m_sparseValueMap = new SparseArrayValueMap;
> +        globalData.heap.addFinalizer(this, finalize);

m_sparseValueMap can oscillate between null and non-null. In such a case, this code will register one finalizer for each oscillation, which will result in a double delete.

Please write a test case for this condition.

You can fix this by setting thisObject->m_sparseValueMap to 0 after deleting it, adding a comment that the finalizer can run more than once, or by never setting m_sparseValueMap to 0, even if it becomes empty.

[Mark Hahnenberg]

Created attachment 125001 [details]
Patch

[Geoffrey Garen]

Comment on attachment 125001 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=125001&action=review

r=me

> Source/JavaScriptCore/runtime/JSArray.cpp:203
> +void JSArray::finalize(JSCell* cell)

Please add a comment here noting that this function can be called more than once, so a future developer doesn't add unsafe code.

[WebKit Review Bot]

Comment on attachment 125001 [details]
Patch

Attachment 125001 [details] did not pass chromium-ews (chromium-xvfb):
Output: http://queues.webkit.org/results/11393541

New failing tests:
fast/js/sparse-array.html

[Mark Hahnenberg]

Committed r106496: <http://trac.webkit.org/changeset/106496>