Add dedicated LanePriority for Suspense retries #19287
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A "retry" is a special type of update that attempts to flip a Suspense boundary from its placeholder state back to its primary/resolved state. Currently, retries are given default priority, using the same algorithm as normal updates and occupying range of lanes. This adds a new range of lanes dedicated specifically to retries, and gives them lower priority than normal updates. A couple of existing tests were affected because retries are no longer batched with normal updates; they commit in separate batches. Not totally satisfied with this design, but I think things will snap more into place once the rest of the Lanes changes (like how we handle parallel Suspense transitions) have settled.
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packages/react-reconciler/src/__tests__/ReactSuspenseCallback-test.internal.js
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| // lanes, leading to the extra intermediate render. This is because when | ||
| // we schedule the fourth update, we're already in the middle of rendering | ||
| // the three others. Since three are only three lanes in the default | ||
| // range, the fourth lane is shifted to slightly lower priority. This |
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the fourth lane is shifted to slightly lower priority
Could you expand in this more? I would think that the "priority" doesn't change, the work just gets pushed out further?
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The "lane priority" of a bit is determined by which range of bits it belongs to. We can't assign a new update to a lane that's already in the middle of rendering. In this (fairly contrived) test scenario, we ran out of lanes in the Default range so we had to pick from the next available lane in a lower priority range. Another option would be to throw out the in-progress work and start over — then we're free to pick any lane we want. tl;dr this is a batching heuristic that doesn't matter that much in most scenarios and probably isn't the last time this will change. It's only observable in tests if you do clever stuff like partial rendering, which we don't officially support and is only used in our own test suite.
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I think what's confusing to me in "the fourth lane is shifted to slightly lower priority" is that the lane isn't what changes priorities? Do you mean "the fourth update is shifted to a (lane in a) slightly lower priority"?
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Oh right. Yeah I meant fourth update :D
| @@ -3923,7 +3923,6 @@ describe('Profiler', () => { | |||
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| expect(onPostCommit).toHaveBeenCalledTimes(1); | |||
| expect(onPostCommit.mock.calls[0][4]).toMatchInteractions([ | |||
| initialRenderInteraction, | |||
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tl;dr the initial render is no longer entangled with the Suspense retry. See rationale from previous comment.
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Note to whoever does this next sync: this is moderately risky. If I weren't possessed of such a daring personality, I might put it behind a feature flag. |
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| // from its placeholder state to its primary/resolved state. | ||
| let lane = findLane(RetryRangeStart, RetryRangeEnd, wipLanes); | ||
| if (lane === NoLane) { | ||
| lane = pickArbitraryLane(RetryLanes); |
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Late to the party here (sorry, oncall this week and a bit behind) but catching up now, I had a small thought.
The naming of pickArbitraryLane suggests (to me) that it could return a different value each time it's called. Its current implementation always happens to return a predictable/consistent value, but do we think this might ever change? If it did, the usage of it would be at conflict with the above comment:
This should be pure function, so that it always returns the same lane for given inputs.
...in which case maybe calling getLowestPriorityLane directly would be "better"
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“arbitrary” in this case means it doesn’t matter if it’s the lowest priority lane or highest priority lane or anything in between.
For example, it communicates if there’s a refactor where the most significant lane is available for some other reason, you could use that instead without affecting the algorithm.
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Now that Suspense retries have their own dedicated set of lanes (facebook#19287), we can determine if a render includes only retries by checking if its lanes are a subset of the retry lanes. Previously we inferred this by checking `workInProgressRootLatestProcessedEventTime`. If it's not set, that implies that no updates were processed in the current render, which implies it must be a Suspense retry. The eventual plan is to get rid of `workInProgressRootLatestProcessedEventTime` and instead track event times on the root; this change is one the steps toward that goal. The relevant tests were originally added in facebook#15769.
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Now that Suspense retries have their own dedicated set of lanes (#19287), we can determine if a render includes only retries by checking if its lanes are a subset of the retry lanes. Previously we inferred this by checking `workInProgressRootLatestProcessedEventTime`. If it's not set, that implies that no updates were processed in the current render, which implies it must be a Suspense retry. The eventual plan is to get rid of `workInProgressRootLatestProcessedEventTime` and instead track event times on the root; this change is one the steps toward that goal. The relevant tests were originally added in #15769.
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A "retry" is a special type of update that attempts to flip a Suspense boundary from its placeholder state back to its primary/resolved state.
Currently, retries are given default priority, using the same algorithm as normal updates and occupying range of lanes.
This adds a new range of lanes dedicated specifically to retries, and gives them lower priority than normal updates.
A couple of existing tests were affected because retries are no longer batched with normal updates; they commit in separate batches.
Not totally satisfied with this design, but I think things will snap more into place once the rest of the Lanes changes (like how we handle parallel Suspense transitions) have settled.