bugfix: useSyncExternalStore() seems to be not solid in avoiding tearing

[JSerZANP]

Summary

Currently the update branch of useSyncExternalStore() - updateSyncExternalStore() sets up consistency check conditionally, while for mountSyncExternalStore() it is unconditionally.

This results in tearing still being possible when store mutates during rendering in concurrent mode,
if the mutation is not with emitting(which is possible considering the throttling of continuous events).

Here is the repro on stackblitz

Proposed Fix

Same as mountSyncExternalStore(), we should add consistency check without condition.

How did you test this change?

Added one more test case, which didn't pass before the fix

we see 'A0B0C1' being flushed - tearing happened

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[JSerZANP]

I just moved the consistency check out from the if condition, nothing more.

[react-sizebot]

Comparing: 493f72b...edd152a

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	164.38 kB	164.38 kB	=	51.77 kB	51.77 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	171.79 kB	171.79 kB	=	53.99 kB	53.98 kB
facebook-www/ReactDOM-prod.classic.js	=	567.40 kB	567.41 kB	+0.03%	100.10 kB	100.13 kB
facebook-www/ReactDOM-prod.modern.js	=	551.20 kB	551.21 kB	+0.04%	97.26 kB	97.30 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against edd152a

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[rickhanlonii]

I don't think I understand the use case for the store not emitting? I believe uSES requires that when the store changes the change is emitted.

[JSerZANP]

@rickhanlonii thank you for taking a look!

Maybe this was not a good example, but I was thinking about some throttled event stream like scroll or mouse movement.

I believe uSES requires that when the store changes the change is emitted.

If we have this assumption, then I have a question on the necessity of consistency check.

Since inside the subscribe callback from uSES, React schedules SyncLane update on target fiber nodes.
This will be flushed synchronously, so the tearing issue is actually already avoided even without consistency check?

https://github.com/facebook/react/blob/main/packages/react-reconciler/src/ReactFiberHooks.js#L1686-L1719

If I use set() rather than setWithoutEmit and remove the consistency check from updateSynExternalStore(),
the test actually passes, meaning consistency check might not be needed at all.

[rickhanlonii]

The consistency check is needed only when getSnapshot or subscribe changes during render. Check out this test, which fails when you comment out the consistency check, even though the subscriptions are emitted: https://github.com/facebook/react/blob/62cd5af08e2ac8b1d4691e75252487083cf7a4aa/packages/react-reconciler/src/__tests__/useSyncExternalStore-test.js#L83C1-L85

Notice that without the consistency check, React would commit the torn values with A0, B0, and C1. With the check, React restarts the render and renders A1 B1 and C1 together.

[JSerZANP]

@rickhanlonii thanks for the reply.
looks like I misunderstood what consistency check does!