修复miniblink一处内存泄漏的bug
最后的结论很简单，是我绑定v8 function的时候没释放。但查找问题的过程比较艰难，因为
v8的代码实在太难读了。
下面先大概了解下v8的垃圾回收机制。
v8\src\global-handles.cc里有个GlobalHandles类，管理了所有使用v8::Persistent<XXX>形式声明的永久性
v8变量。说永久性的意思是，除非自己主动调用v8::Persistent::reset，不然就一直占稳不释放了。
但有个例外，就是v8::Persistent::SetWeak形式设置了释放回调，或者叫弱回调（这里有点存疑）。
看段堆栈：
node.dll!v8::internal::GlobalHandles::NodeBlock::IncreaseUses C++
node.dll!v8::internal::GlobalHandles::Node::IncreaseBlockUses C++
node.dll!v8::internal::GlobalHandles::Node::Acquire C++
node.dll!v8::internal::GlobalHandles::Create C++
node.dll!v8::V8::GlobalizeReference C++
node.dll!v8::PersistentBase<v8::ObjectTemplate>::New C++
> node.dll!v8::PersistentBase<v8::ObjectTemplate>::Reset<v8::ObjectTemplate> C++
node.dll!blink::V8Window::getShadowObjectTemplate C++
node.dll!blink::WindowProxy::createContext C++
node.dll!blink::WindowProxy::initialize C++
node.dll!blink::WindowProxy::initializeIfNeeded C++
node.dll!blink::ScriptController::windowProxy C++
node.dll!blink::LocalFrame::windowProxy C++
node.dll!blink::toV8ContextEvenIfDetached C++
node.dll!blink::ScriptState::forWorld C++
node.dll!blink::ScriptState::forMainWorld C++
node.dll!blink::WebLocalFrameImpl::mainWorldScriptContext C++
node.dll!wke::CWebView::globalExec C++
node.dll!wkeGlobalExec C++
wkexe.exe!handleDocumentReady C++
node.dll!wke::CWebWindow::_onDocumentReady C++
node.dll!wke::CWebWindow::_staticOnDocumentReady C++
node.dll!content::WebFrameClientImpl::didFinishDocumentLoad C++
node.dll!blink::FrameLoaderClientImpl::dispatchDidFinishDocumentLoad C++
node.dll!blink::FrameLoader::finishedParsing C++
node.dll!blink::Document::finishedParsing C++
node.dll!blink::HTMLConstructionSite::finishedParsing C++
node.dll!blink::HTMLTreeBuilder::finished C++
node.dll!blink::HTMLDocumentParser::end C++
node.dll!blink::HTMLDocumentParser::attemptToRunDeferredScriptsAndEnd C++
node.dll!blink::HTMLDocumentParser::prepareToStopParsing C++
node.dll!blink::HTMLDocumentParser::processParsedChunkFromBackgroundParser C++
node.dll!blink::HTMLDocumentParser::pumpPendingSpeculations C++
node.dll!blink::HTMLDocumentParser::resumeParsingAfterYield C++
node.dll!blink::HTMLParserScheduler::continueParsing C++
node.dll!WTF::FunctionWrapper
node.dll!WTF::PartBoundFunctionImpl
node.dll!blink::CancellableTaskFactory::CancellableTask::run C++
node.dll!content::WebTimerBase::fired C++
node.dll!content::WebThreadImpl::schedulerTasks C++
node.dll!content::WebThreadImpl::fire C++
node.dll!wkeWake C++
wkexe.exe!RunMessageLoop C++这显示了如何把永久对象放到GlobalHandles里。
当垃圾回收的时刻到来时，
> node.dll!v8::internal::GlobalHandles::IdentifyWeakHandles C++
node.dll!v8::internal::MarkCompactCollector::MarkLiveObjects C++
node.dll!v8::internal::MarkCompactCollector::CollectGarbage C++
node.dll!v8::internal::Heap::MarkCompact C++
node.dll!v8::internal::Heap::PerformGarbageCollection C++
node.dll!v8::internal::Heap::CollectGarbage C++
node.dll!v8::internal::Heap::CollectAllAvailableGarbage C++
node.dll!v8::Isolate::LowMemoryNotification C++
node.dll!content::BlinkPlatformImpl::doGarbageCollected C++
node.dll!content::BlinkPlatformImpl::garbageCollectedTimer C++
node.dll!blink::Timer<content::BlinkPlatformImpl>::fired C++可以看到GlobalHandles::IdentifyWeakHandles这里会遍历所有v8::Persistent永久化对象。
当设置了弱回调的时候，这些永久化对象就靠v8自己的垃圾处理机制了。此时就有个问题，v8如何知道这个对象无人引用了呢？
下面看几个v8的数据结构：
class GlobalHandles::Node
class Object
class HeapObject
class Marking
要读懂垃圾回收机制，连得理清楚这几个对象的关系。
实际上是这样：
Object相对于对应js里的Object。不过注意下，v8分外部导出到头文件的Object和内部Object，这两货其实是一样的，只是
为了工程上的严谨性。
这个GlobalHandles::Node代表每个存在GlobalHandles里的对象，如上面那个持久化东西。Object也被存在这个node里。
但这里有个特别要强调的是，一个object可以放在多个 node。这点对后来解决内存泄漏比较关键。
每个object，其实是个以HeapObject开头的内存。这个HeapObject就是为了方便内存管理而设计的。
HeapObject的头部，通过一系列位运算，地址运算，得到了Marking对象。这过程我没怎么看懂，因为都是类似
static_cast<uint32_t>(addr - this->address()) >> kPointerSizeLog2;这种风格的运算…实在不想搞明白算出真实Marking地址了。
总之得到Marking地址后，就可以通过Marking::IsBlack，Marking::IsWhite来判断Marking的状态（或者叫颜色）了。
black表示内存被占用，white表示内存没人占用。
（插一句，其实准确的说，应该是Marking里的cell类来记录这个颜色，不同object最后是在cell里标记的）
那问题来了，什么时候会去设置这些颜色呢？
最终是通过Marking::WhiteToBlack、Marking::MarkBlack来标记的。下个断点，
> node.dll!v8::internal::Marking::WhiteToBlack C++
node.dll!v8::internal::MarkCompactCollector::MarkObject C++
node.dll!v8::internal::MarkCompactMarkingVisitor::MarkObjectByPointer C++
node.dll!v8::internal::MarkCompactMarkingVisitor::VisitPointers C++
node.dll!v8::internal::StaticMarkingVisitor<v8::internal::MarkCompactMarkingVisitor>::VisitMap C++
node.dll!v8::internal::StaticMarkingVisitor<v8::internal::MarkCompactMarkingVisitor>::IterateBody C++
node.dll!v8::internal::MarkCompactCollector::EmptyMarkingDeque C++
node.dll!v8::internal::MarkCompactCollector::ProcessMarkingDeque C++
node.dll!v8::internal::MarkCompactCollector::PrepareForCodeFlushing C++
node.dll!v8::internal::MarkCompactCollector::MarkLiveObjects C++
node.dll!v8::internal::MarkCompactCollector::CollectGarbage C++
node.dll!v8::internal::Heap::MarkCompact C++
node.dll!v8::internal::Heap::PerformGarbageCollection C++
node.dll!v8::internal::Heap::CollectGarbage C++
node.dll!v8::internal::Heap::CollectAllAvailableGarbage C++
node.dll!v8::Isolate::LowMemoryNotification C++
node.dll!content::BlinkPlatformImpl::doGarbageCollected C++其实关键就是这句
v8::internal::MarkCompactMarkingVisitor::VisitPointersv8构造了一个图结构，通过不停遍历里面的节点，如果访问到的节点，就标记为block。
那回到最上面的问题，
一个object可以放在多个 node。如果一个node没人引用了，会被标记为white。这是下面堆栈做的：
> node.dll!v8::internal::Sweep<0,0,1,0> C++
node.dll!v8::internal::MarkCompactCollector::SweepSpace C++
node.dll!v8::internal::MarkCompactCollector::SweepSpaces C++
node.dll!v8::internal::MarkCompactCollector::CollectGarbage C++
node.dll!v8::internal::Heap::MarkCompact C++
node.dll!v8::internal::Heap::PerformGarbageCollection C++
node.dll!v8::internal::Heap::CollectGarbage C++
node.dll!v8::internal::Heap::CollectAllAvailableGarbage C++
node.dll!v8::Isolate::LowMemoryNotification C++但如果一个object放在两个node呢？比如有两个v8::Persistent<Objcet>持有了这个object，那就是一个object放在两个node里。
当第一个node在v8::internal::Sweep<0,0,1,0>里被标记white时，
下面堆栈可能又因为另个node在引用这个object，而导致同样的Marking又被标记成black：
> node.dll!v8::internal::MarkBit::Set()
node.dll!v8::internal::Marking::WhiteToBlack
node.dll!v8::internal::MarkCompactCollector::SetMark
node.dll!v8::internal::RootMarkingVisitor::MarkObjectByPointer
node.dll!v8::internal::RootMarkingVisitor::VisitPointer
node.dll!v8::internal::GlobalHandles::IterateStrongRoots
node.dll!v8::internal::Heap::IterateStrongRoots
node.dll!v8::internal::MarkCompactCollector::MarkRoots
node.dll!v8::internal::MarkCompactCollector::MarkLiveObjects
node.dll!v8::internal::MarkCompactCollector::CollectGarbage
node.dll!v8::internal::Heap::MarkCompact
node.dll!v8::internal::Heap::PerformGarbageCollection
node.dll!v8::internal::Heap::CollectGarbage
node.dll!v8::internal::Heap::CollectAllAvailableGarbage
node.dll!v8::Isolate::LowMemoryNotification
node.dll!content::BlinkPlatformImpl::doGarbageCollected
node.dll!content::BlinkPlatformImpl::garbageCollectedTimer
node.dll!blink::Timer最后跟踪到这里时，才发现miniblink是因为同个object被两个node引用导致没释放。问题解决。