[WIP] Fix for fiber root scheduling memory leak

[trueadm]

Not to be merged

After some investigation work for a memory leak on React Native, we found that nextRoot was never set to null and left in that state upon the view fulling being unmounted. I believe that is has to do with the fact that we set nextRoot and nextUnitOfWork to null, then immediately call requestWork() which sets them back again.

This results in a memory leak as the node tree of nextRoot never gets garbage collected.

This is more of a temporary research fix, rather than an explicit fix. I'm sure @acdlite will have a concrete fix based from this.

[gaearon]

Nothing here?

[trueadm]

No idea what my brain was thinking then. Sorry, pushed the code.

[gaearon]

I don't have enough context on which variables should be reset when but my hunch is that they don't all need to be reset twice. Let's leave this to @acdlite.

[acdlite]

Thanks for tracking this down. I think I have a better idea for how to fix it. What's the best way to reproduce the error, and confirm that it's fixed?

[trueadm]

@acdlite I used create-react-app and then did ReactDOM.render(null, ...) in a setTimeout(...) after the initial render – unmounting the entire UI. I then used the Chrome dev tools to see the memory trace hadn't really decreased that much as nextRoot still held a reference to the initial fiber tree.

[clemmy]

We'll come back to this with a more concrete implementation from @acdlite later. Merging so that 16.2 doesn't ship with memory leak.

[Rebsos]

Thx for the fix. My React Native App needs now less memory. Just wanted to let some real world data here:

React Native	React	Total (average)
0.49.5	16.0.0-beta.5	164 MB
0.51.1	16.0.0	199 MB
0.52.3	16.2.0	173 MB