Commits on Jan 26, 2021

1. Experiment: Unsuspend all lanes on update …

   Adds a feature flag to tweak the internal heuristic used to "unsuspend"
   lanes when a new update comes in.
   
   A lane is "suspended" if we couldn't finish rendering it because it was
   missing data, and we chose not to commit the fallback. (In this context,
   "suspended" does not include updates that finished with a fallback.)
   
   When we receive new data in the form of an update, we need to retry
   rendering the suspended lanes, since the new data may have unblocked the
   previously suspended work. For example, the new update could navigate
   back to an already loaded route.
   
   It's impractical to retry every combination of suspended lanes, so we
   need some heuristic that decides which lanes to retry and in
   which order.
   
   The existing heuristic roughly approximates the old Expiration Times
   model. It unsuspends all lower priority lanes, but leaves higher
   priority lanes suspended.
   
   Then when we start rendering, we choose the lanes that have the highest
   LanePriority and render those -- and then we add to that all the lanes
   that are highher priority.
   
   If this sounds terribly confusing, it's because it barely makes sense.
   (It made more sense in the Expiration Times world, I promise, but it
   was still confusing.) I don't think it's worth me trying to explain the
   old behavior too much because the point here is that we can replace it
   with something simpler.
   
   The new heurstic is to unsuspend all suspended lanes whenever there's
   an update.
   
   This is effectively what we already do except in a few very specific
   edge cases, ever since we removed the delayed suspense feature from
   everything that's not a refresh transition.
   
   We can optimize this in the future to only unsuspend lanes that are
   either 1) in the `lanes` or `subtreeLanes` of the node that was updated,
   or 2) in the `lanes` of the return path of the node that was updated.
   This would exclude lanes that are only located in unrelated sibling
   trees. But, this optimization wouldn't be useful currently because we
   assign the same transition lane to all transitions. It will become
   relevant again once we start assigning arbitrary lanes to transitions
   -- but that in turn requires us to implement entanglement of overlapping
   transitions, one of our planned projects.
   
   So to sum up: the goal here is to remove the weird edge cases and switch
   to a simpler model, on top of which we can make more substantial
   improvements.
   
   I put it behind a flag so I can run an A/B test and confirm it doesn't
   cause a regression.

   

   acdlite committed Jan 26, 2021
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