SSR: possible scopeId interference between two components in rendering

[HerringtonDarkholme]

Vue.js version

2.0.2

Steps to reproduce

import Vue from 'vue/dist/vue.common.js'
import { createRenderer } from 'vue-server-renderer'
const { renderToStream } = createRenderer()

const padding = (new Array(20000)).join('x')
const component1 = new Vue({
  template: `<div>${padding}<div></div></div>`,
  _scopeId: '_component1'
})
const component2 = new Vue({
  template: `<div></div>`,
  _scopeId: '_component2'
})
var a = renderToStream(component1)
var b = renderToStream(component2)
var res = ''
a.on('data', (text) => {
  res += text.toString('utf-8').replace(/x/g, '')
})
a.on('end', () => {
  console.log(res)
})
a.read(1)
b.read(1)

What is Expected?

ScopeId should be independent

Thus should log

<div server-rendered="true" _component1><div _component1></div></div>

What is actually happening?

Component2's scopeId goes into Component1

<div server-rendered="true" _component1><div _component2 _component1></div></div>

I found this when reading server rendering code. The problem is activeInstance in src/server/render.js is shared across all render functions returned by createRenderFunction.

I don't know how nodejs schedules stream flowing, but this might be a pernicious bug if two streams' _read calls were interleaved....