fix(ssr): Memory leak in poll method (fix #2606).

[ronald-d-rogers]

Fix #2606.

This fix depends on the following being merged first to work: vuejs/vue#9479

Fixes memory leak in poll method when a Vue app is created per request, which is typical in SSR.

In particular, the leak happens when:

1. The router-view is conditioned to appear upon a variable that never becomes true.

<div id="app">
  <div v-if="condition">
    <router-view />
  </div>
</div>

2. A beforeRouteEnter guard is defined in the view in the following way:

export default {
  beforeRouteEnter(to, from, next) {
    // Poll happens only if you pass in a function to next(...)
    next(vm => {})
  }
}

[ronald-d-rogers]

Note I haven't had time to test this again after all of the changes that have been made to both projects. Will do this tomorrow if possible.

[ronald-d-rogers]

There still appears to be a leak. I've gone ahead and updated https://github.com/ronald-d-rogers/vue-router-ssr-memory-leak with the latest versions of vue and vue-router, and included instructions on testing the fix and an explanation how it works. Hope this helps.

[posva]

Thanks for the detailed report @ronald-d-rogers. I will try to give it a look next week

[ronald-d-rogers]

@posva Note that I don't think this a great fix. I think the solution is probably to find a way to get rid of the poll, which was just added to fix a bug where the router-view hasn't yet appeared because the previous route is transitioning out, because we don't have a vm to pass to the beforeRouteEnter hook's next callback if the hook was defined with one (next(vm => { ... })), at the time the callbacks are called.

One way to get around it may be by putting the callback on the matching RouteRecord, adding some method like registerComponentInstance(key, vm) and registerBeforeRouterEnterCb(key, cb) to it, and only firing when both have been registered if (this.hasCb(key) && this.hasInstance(key)) { /* fire */ }.

But I can't tell if that would work here or not. Maybe I will give it a go today or tomorrow.

[ryouaki]

This issues will block on poll function. So you should let poll exit at right time。

[posva]

Let's close this in favor of #2875