LayoutRouter: Support segment value of Promise<null> to asynchronously bail out and trigger a server patch

[acdlite]

If the data for a segment is missing when LayoutRouter renders, it initiates a lazy fetch to patch the cache. This is how all dynamic data fetching works in the pre-PPR implementation.

For PPR, we won't use this mechanism anymore for regular navigations, but (at least for now) we will still use it as a fallback behavior if the server response does not match what we expected to receive.

This commit adds support for asynchronously triggering a lazy fetch, by unwrapping the segment data promise inside LayoutRouter to check if it's missing. If so, it will trigger the lazy fetch mechanism.

When PPR is not enabled this should not observably impact behavior.

Closes NEXT-1893

[ijjk]

Stats from current PR

Default Build

General

	vercel/next.js canary	acdlite/next.js async-trigger-lazy-server-patch	Change
buildDuration	12.8s	12.7s	N/A
buildDurationCached	7s	6.2s	N/A
nodeModulesSize	200 MB	200 MB	N/A
nextStartRea..uration (ms)	420ms	426ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	acdlite/next.js async-trigger-lazy-server-patch	Change
170-HASH.js gzip	26.9 kB	26.9 kB	N/A
199.HASH.js gzip	181 B	185 B	N/A
3f784ff6-HASH.js gzip	53.3 kB	53.3 kB	✓
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main-HASH.js gzip	31.7 kB	31.7 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	N/A
Overall change	98.5 kB	98.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	acdlite/next.js async-trigger-lazy-server-patch	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

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1afbb74e6ecf..834.css gzip	106 B	106 B	✓
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Client Build Manifests

	vercel/next.js canary	acdlite/next.js async-trigger-lazy-server-patch	Change
_buildManifest.js gzip	484 B	482 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	acdlite/next.js async-trigger-lazy-server-patch	Change
index.html gzip	529 B	526 B	N/A
link.html gzip	542 B	540 B	N/A
withRouter.html gzip	523 B	522 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

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edge-ssr.js gzip	93.7 kB	93.7 kB	N/A
page.js gzip	146 kB	146 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

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middleware-b..fest.js gzip	628 B	622 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	37.4 kB	37.4 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	2.07 kB	2.07 kB	✓

Next Runtimes

	vercel/next.js canary	acdlite/next.js async-trigger-lazy-server-patch	Change
app-page-exp...dev.js gzip	168 kB	168 kB	✓
app-page-exp..prod.js gzip	94.1 kB	94.1 kB	✓
app-page-tur..prod.js gzip	94.8 kB	94.8 kB	✓
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app-page.run...dev.js gzip	138 kB	138 kB	✓
app-page.run..prod.js gzip	88.7 kB	88.7 kB	✓
app-route-ex...dev.js gzip	24 kB	24 kB	✓
app-route-ex..prod.js gzip	16.6 kB	16.6 kB	✓
app-route-tu..prod.js gzip	16.6 kB	16.6 kB	✓
app-route-tu..prod.js gzip	16.2 kB	16.2 kB	✓
app-route.ru...dev.js gzip	23.4 kB	23.4 kB	✓
app-route.ru..prod.js gzip	16.2 kB	16.2 kB	✓
pages-api-tu..prod.js gzip	9.37 kB	9.37 kB	✓
pages-api.ru...dev.js gzip	9.64 kB	9.64 kB	✓
pages-api.ru..prod.js gzip	9.37 kB	9.37 kB	✓
pages-turbo...prod.js gzip	21.9 kB	21.9 kB	✓
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pages.runtim..prod.js gzip	21.9 kB	21.9 kB	✓
server.runti..prod.js gzip	49.4 kB	49.4 kB	✓
Overall change	931 kB	931 kB	✓

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Commit: 9cd3d65