dillon1000/react

[Experiment] Add feature flag for more aggressive memory clean-up of deleted fiber trees (#21039)

* Add feature flag: enableStrongMemoryCleanup

Add a feature flag that will test doing a recursive clean of an unmount
node. This will disconnect the fiber graph making leaks less severe.

* Detach sibling pointers in old child list

When a fiber is deleted, it's still part of the previous (alternate)
parent fiber's list of children. Because children are a linked list, an
earlier sibling that's still alive will be connected to the deleted
fiber via its alternate:


  live fiber
  --alternate--> previous live fiber
  --sibling--> deleted fiber

We can't disconnect `alternate` on nodes that haven't been deleted
yet, but we can disconnect the `sibling` and `child` pointers.

Will use this feature flag to test the memory impact.

* Combine into single enum flag

I combined `enableStrongMemoryCleanup` and `enableDetachOldChildList`
into a single enum flag. The flag has three possible values. Each level
is a superset of the previous one and performs more aggressive clean up.

We will use this to compare the memory impact of each level.

* Add Flow type to new host config method

* Re-use existing recursive clean up path

We already have a recursive loop that visits every deleted fiber. We
can re-use that one for clean up instead of adding another one.

Co-authored-by: Andrew Clark <[email protected]>