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<b><a class="bz_bug_link
bz_status_NEW "
title="NEW - Regression(r197648): JSObject::setPrototypeWithCycleCheck() allows for cycles but the rest of the code base does not"
href="https://bugs.webkit.org/show_bug.cgi?id=161534#c1">Comment # 1</a>
on <a class="bz_bug_link
bz_status_NEW "
title="NEW - Regression(r197648): JSObject::setPrototypeWithCycleCheck() allows for cycles but the rest of the code base does not"
href="https://bugs.webkit.org/show_bug.cgi?id=161534">bug 161534</a>
from <span class="vcard"><a class="email" href="mailto:sbarati@apple.com" title="Saam Barati <sbarati@apple.com>"> <span class="fn">Saam Barati</span></a>
</span></b>
<pre>(In reply to <a href="show_bug.cgi?id=161534#c0">comment #0</a>)
<span class="quote">> JSObject::setPrototypeWithCycleCheck() allows for cycles but the rest of the
> code base does not deal properly with cycles.
>
> This is because of the following check that was added:
> if (UNLIKELY(asObject(nextPrototype)->methodTable(vm)->getPrototype !=
> defaultGetPrototype))
> break;
>
> This was added to match the EcmaScript spec:
> - <a href="https://tc39.github.io/ecma262/#sec-ordinarysetprototypeof">https://tc39.github.io/ecma262/#sec-ordinarysetprototypeof</a> (step 8)
>
> However, if you create a cycles, we end up with an infinite loop later on
> under:
> Structure::anyObjectInChainMayInterceptIndexedAccesses()
>
> This is likely not the only place we traverse the prototype chain and except
> there is no cycle.
>
> I noticed this when running
> html/browsers/history/the-location-interface/
> allow_prototype_cycle_through_location.sub.html with the patch for Bug
> 161455 applied.</span >
Yeah, we should teach the rest of the engine that it's OK to have cycles
in the "getPrototypeDirect" chain.
Currently, inside JSC itself, this will never happen. However, I guess we need to
teach it this w.r.t the HTML spec.</pre>
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