Commits on Feb 20, 2020

1. Bugfix: memo drops lower pri updates on bail out …

   Fixes a bug where lower priority updates on a components wrapped with
   `memo` are sometimes left dangling in the queue without ever being
   processed, if they are preceded by a higher priority bailout.
   
   Cause
   -----
   
   The pending update priority field is cleared at the beginning of
   `beginWork`. If there is remaining work at a lower priority level, it's
   expected that it will be accumulated on the work-in-progress fiber
   during the begin phase.
   
   There's an exception where this assumption doesn't hold:
   SimpleMemoComponent contains a bailout that occurs *before* the
   component is evaluated and the update queues are processed, which means
   we don't accumulate the next priority level. When we complete the fiber,
   the work loop is left to believe that there's no remaining work.
   
   Mitigation
   ----------
   
   Since this only happens in a single case, a late bailout in
   SimpleMemoComponent, I've mitigated the bug in that code path by
   restoring the original update priority from the current fiber.
   
   This same case does not apply to MemoComponent, because MemoComponent
   fibers do not contain hooks or update queues; rather, they wrap around
   an inner fiber that may contain those. However, I've added a test case
   for MemoComponent to protect against a possible future regression.
   
   Possible next steps
   -------------------
   
   We should consider moving the update priority assignment in `beginWork`
   out of the common path and into each branch, to avoid similar bugs in
   the future.

   

   acdlite committed Feb 20, 2020
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