When we just had CycleCollectedJSContext (and no CycleCollectedJSRuntime) a
weird thing happened at shutdown:
1. We would call JS_DestroyContext from ~CycleCollectedJSContext. By that time,
the ~XPCJSContext destructor had already finished.
2. Destroying the context runs a final GC. That GC would call back into various
GC callbacks, such as TraceBlackJS and TraceGrayJS.
3. These callbacks would do a virtual method call:
http://searchfox.org/mozilla-central/rev/876c7dd30586f9c6f9c99ef7444f2d73c7acfe7c/xpcom/base/CycleCollectedJSRuntime.cpp#791
4. Normally this method call would call into
XPCContext::TraceNativeBlackRoots. However, C++ changes the vtable for an
object during destruction. So we would only call CycleCollectedJSContext's
version of TraceNativeBlackRoots, which is empty. So we never traced anything.
When I moved this code into the runtime, we actually do call into
XPCJSRuntime::TraceNativeBlackRoots at that time. So the behavior changed, and
that was causing crashes once I nulled out the TLS as you asked. So I removed
these callbacks for the last GC.
MozReview-Commit-ID: 3do13bjpwQj
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