dillon1000/react

Detect and prevent render starvation, per lane (#18864)

* Detect and prevent render starvation, per lane

If an update is CPU-bound for longer than expected according to its
priority, we assume it's being starved by other work on the main thread.

To detect this, we keep track of the elapsed time using a fixed-size
array where each slot corresponds to a lane. What we actually store is
the event time when the lane first became CPU-bound.

Then, when receiving a new update or yielding to the main thread, we
check how long each lane has been pending. If the time exceeds a
threshold constant corresponding to its priority, we mark it as expired
to force it to synchronously finish.

We don't want to mistake time elapsed while an update is IO-bound
(waiting for data to resolve) for time when it is CPU-bound. So when a
lane suspends, we clear its associated event time from the array. When
it receives a signal to try again, either a ping or an update, we assign
a new event time to restart the clock.

* Store as expiration time, not start time

I originally stored the start time because I thought I could use this
in the future to also measure Suspense timeouts. (Event times are
currently stored on each update object for this purpose.) But that
won't work because in the case of expiration times, we reset the clock
whenever the update becomes IO-bound. So to replace the per-update
field, I'm going to have to track those on the room separately from
expiration times.