Commits on Jan 22, 2021

1. Store interleaved updates on separate queue until end of render …

   ## Motivation
   
   An *interleaved* update is one that is scheduled while a render is
   already in progress, typically from a concurrent user input event.
   
   We have to take care not to process these updates during the current
   render, because a multiple interleaved updates may have been scheduled
   across many components; to avoid tearing, we cannot render some of
   those updates without rendering all of them.
   
   ## Old approach
   
   What we currently do when we detect an interleaved update is assign a
   lane that is not part of the current render.
   
   This has some unfortunate drawbacks. For example, we will eventually run
   out of lanes at a given priority level. When this happens, our last
   resort is to interrupt the current render and start over from scratch.
   If this happens enough, it can lead to starvation.
   
   More concerning, there are a suprising number of places that must
   separately account for this case, often in subtle ways. The maintenance
   complexity has led to a number of tearing bugs.
   
   ## New approach
   
   I added a new field to the update queue, `interleaved`. It's a linked
   list, just like the `pending` field. When an interleaved update is
   scheduled, we add it to the `interleaved` list instead of `pending`.
   
   Then we push the entire queue object onto a global array. When the
   current render exits, we iterate through the array of interleaved queues
   and transfer the `interleaved` list to the `pending` list.
   
   So, until the current render has exited (whether due to a commit or an
   interruption), it's impossible to process an interleaved update, because
   they have not yet been enqueued.
   
   In this new approach, we don't need to resort to clever lanes tricks to
   avoid inconsistencies. This should allow us to simplify a lot of the
   logic that's currently in ReactFiberWorkLoop and ReactFiberLane,
   especially `findUpdateLane` and `getNextLanes`. All the logic for
   interleaved updates is isolated to one place.

   

   acdlite committed Jan 22, 2021
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