摘要：介紹是一個擴(kuò)展，主要用來觀察內(nèi)存泄露問題，基本用法如下實現(xiàn)分析分析的版本為。的入口函數(shù)在文件中，通過進(jìn)行聲明。下面分析下函數(shù)的具體實現(xiàn)函數(shù)內(nèi)部會遞歸調(diào)用函數(shù)得到最終堆快照的結(jié)果。回調(diào)的觸發(fā)與的鉤子有關(guān)，詳細(xì)看下一節(jié)分析。

memwatch是一個c++擴(kuò)展，主要用來觀察nodejs內(nèi)存泄露問題，基本用法如下：

const memwatch = require("@airbnb/memwatch");
function LeakingClass() {
}

memwatch.gc();
var arr = [];
var hd = new memwatch.HeapDiff();
for (var i = 0; i < 10000; i++) arr.push(new LeakingClass);
var hde = hd.end();
console.log(JSON.stringify(hde, null, 2));

分析的版本為@airbnb/memwatch。首先從binding.gyp開始入手:

{
  "targets": [
    {
      "target_name": "memwatch",
      "include_dirs": [
        "
這份配置表示其生成的目標(biāo)是memwatch.node,源碼是src目錄下的heapdiff.cc、init.cc、memwatch.cc、util.cc,在項目編譯的過程中還需要include額外的nan目錄，nan目錄通過執(zhí)行node -e "require("nan")按照node模塊系統(tǒng)尋找nan依賴， 表示后面是一條指令。

memwatch的入口函數(shù)在init.cc文件中，通過NODE_MODULE(memwatch, init);進(jìn)行聲明。當(dāng)執(zhí)行require("@airbnb/memwatch")的時候會首先調(diào)用init函數(shù)：
void init (v8::Handle target)
{
    Nan::HandleScope scope;
    heapdiff::HeapDiff::Initialize(target);

    Nan::SetMethod(target, "upon_gc", memwatch::upon_gc);
    Nan::SetMethod(target, "gc", memwatch::trigger_gc);

    Nan::AddGCPrologueCallback(memwatch::before_gc);
    Nan::AddGCEpilogueCallback(memwatch::after_gc);
}
init函數(shù)的入口參數(shù)v8:Handle target可以類比nodejs中的module.exports的exports對象。函數(shù)內(nèi)部做的實現(xiàn)可以分為三塊，初始化target、給target綁定upon_gc和gc兩個函數(shù)、在nodejs的gc前后分別掛上對應(yīng)的鉤子函數(shù)。
Initialize實現(xiàn)
到heapdiff.cc文件中來看heapdiff::HeapDiff::Initialize(target);的實現(xiàn)。
void heapdiff::HeapDiff::Initialize ( v8::Handle target )
{
    Nan::HandleScope scope;
    
    v8::Local t = Nan::New(New);
    t->InstanceTemplate()->SetInternalFieldCount(1);
    t->SetClassName(Nan::New("HeapDiff").ToLocalChecked());

    Nan::SetPrototypeMethod(t, "end", End);
    target->Set(Nan::New("HeapDiff").ToLocalChecked(), t->GetFunction());
}
Initialize函數(shù)中創(chuàng)建一個叫做HeapDiff的函數(shù)t，同時在t的原型鏈上綁了end方法，使得js層面可以執(zhí)行vat hp = new memwatch.HeapDiff();hp.end()。

new memwatch.HeapDiff實現(xiàn)
當(dāng)js執(zhí)行new memwatch.HeapDiff();的時候，c++層面會執(zhí)行heapdiff::HeapDiff::New函數(shù)，去掉注釋和不必要的宏，New函數(shù)精簡如下：
NAN_METHOD(heapdiff::HeapDiff::New)
{
    if (!info.IsConstructCall()) {
        return Nan::ThrowTypeError("Use the new operator to create instances of this object.");
    }

    Nan::HandleScope scope;

    HeapDiff * self = new HeapDiff();
    self->Wrap(info.This());

    s_inProgress = true;
    s_startTime = time(NULL);
    
    self->before = v8::Isolate::GetCurrent()->GetHeapProfiler()->TakeHeapSnapshot(NULL);

    s_inProgress = false;

    info.GetReturnValue().Set(info.This());
}
可以看到用戶在js層面執(zhí)行var hp = new memwatch.HeapDiff();的時候，c++層面會調(diào)用nodejs中的v8的api對對堆上內(nèi)存打一個snapshot保存到self->before中，并將當(dāng)前對象返回出去。

memwatch.HeapDiff.End實現(xiàn)
當(dāng)用戶執(zhí)行hp.end()的時候，會執(zhí)行原型鏈上的end方法，也就是c++的heapdiff::HeapDiff::End方法。同樣去掉冗余的注釋以及宏，End方法可以精簡如下：
NAN_METHOD(heapdiff::HeapDiff::End)
{
    Nan::HandleScope scope;

    HeapDiff *t = Unwrap( info.This() );

    if (t->ended) {
        return Nan::ThrowError("attempt to end() a HeapDiff that was already ended");
    }
    t->ended = true;

    s_inProgress = true;
    t->after = v8::Isolate::GetCurrent()->GetHeapProfiler()->TakeHeapSnapshot(NULL);
    s_inProgress = false;

    v8::Local comparison = compare(t->before, t->after);
    
    ((HeapSnapshot *) t->before)->Delete();
    t->before = NULL;
    ((HeapSnapshot *) t->after)->Delete();
    t->after = NULL;

    info.GetReturnValue().Set(comparison);
}
在End函數(shù)中，拿到當(dāng)前的HeapDiff對象之后，再對當(dāng)前的堆上內(nèi)存再打一個snapshot，調(diào)用compare函數(shù)對前后兩個snapshot對比后得到comparison后，將前后兩次snapshot對象釋放掉，并將結(jié)果通知給js。
下面分析下compare函數(shù)的具體實現(xiàn)：
compare函數(shù)內(nèi)部會遞歸調(diào)用buildIDSet函數(shù)得到最終堆快照的diff結(jié)果。
static v8::Local
compare(const v8::HeapSnapshot * before, const v8::HeapSnapshot * after)
{
    Nan::EscapableHandleScope scope;
    int s, diffBytes;

    Local