[Fresh] Don't ignore dependencies for render phase updates

[gaearon]

During a Fresh update, we ignore dependencies. We let the effects re-run, even if deps are [], and free the memo'd values.

This is important because it prevents us from referencing stale code. For example, if we edit top-level computeExpensive, we still want useMemo(() => computeExpensive(), []) to re-evaluate.

Sometimes this may cause problems, but arguably this points to other flaws in components or Hooks that should be fixed regardless.

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There is a failure case though. Generally, while components shouldn’t rely on useMemo for semantics, there is still an expectation that memo chains can “settle”. For example, useSubscription does a render-phase update if its arguments differ from its state:

react/packages/use-subscription/src/useSubscription.js

Lines 42 to 45 in 0da7bd0

	if (
	state.getCurrentValue !== getCurrentValue ||
	state.subscribe !== subscribe
	) {

It has to rely on something like useMemo in the calling code to keep the source eventually stable.

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The problem with our current approach is that during a Fresh update, dependencies are ignored completely. So even with useMemo, things never truly “settle”. This breaks the pattern relied upon by useSubscription, and likely, other similar mechanisms.

The fix is to only ignore dependencies during the first render after the patch. By that point it has run, all memo'd values have been recreated with fresh code. So it makes sense that we can turn off the cache busting flag for all subsequent render-phase updates of this component, thus letting it settle. This fixes the infinite loop in the regression test I've added.

[sizebot]

No significant bundle size changes to report.

Generated by 🚫 dangerJS

[bvaughn]

Nice explanation. Good test.