Update app-index to only ever construct the initial data response once

[gnoff]

There was a bug where if the root hydrates and then an update happens the intitial server data response can be replaced by one that is completely empty and will never resolve. This can lead to a frozen hydration that blocks interactivity. This udpate makes it so it is impossible for the initial data response to ever be created more than once.

Making a regression test is tricky because this relies on subtle timing of hydration, updates, and when the inline chunks arrive in the stream. The original implementation is just not safe in that it violates the rules of react and the new one is self-evidently unable to produce a similar situation so as long as our existing test suite passes that must be sufficient absent a good alternative to making a specific regression test

Closes NEXT-2420

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build

General

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
buildDuration	11.6s	11.7s	⚠️ +132ms
buildDurationCached	6.4s	5s	N/A
nodeModulesSize	196 MB	196 MB	N/A
nextStartRea..uration (ms)	426ms	433ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
3f784ff6-HASH.js gzip	53.5 kB	53.5 kB	N/A
423.HASH.js gzip	185 B	181 B	N/A
68-HASH.js gzip	29.7 kB	29.6 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	238 B	238 B	✓
main-HASH.js gzip	31.9 kB	31.9 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	✓
Overall change	47.2 kB	47.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
_app-HASH.js gzip	194 B	195 B	N/A
_error-HASH.js gzip	182 B	181 B	N/A
amp-HASH.js gzip	502 B	501 B	N/A
css-HASH.js gzip	320 B	322 B	N/A
dynamic-HASH.js gzip	2.5 kB	2.5 kB	N/A
edge-ssr-HASH.js gzip	255 B	256 B	N/A
head-HASH.js gzip	350 B	349 B	N/A
hooks-HASH.js gzip	368 B	369 B	N/A
image-HASH.js gzip	4.19 kB	4.18 kB	N/A
index-HASH.js gzip	257 B	256 B	N/A
link-HASH.js gzip	2.67 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	310 B	311 B	N/A
script-HASH.js gzip	384 B	383 B	N/A
withRouter-HASH.js gzip	306 B	308 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	106 B	106 B	✓

Client Build Manifests

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
_buildManifest.js gzip	483 B	485 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
index.html gzip	527 B	527 B	✓
link.html gzip	542 B	540 B	N/A
withRouter.html gzip	524 B	523 B	N/A
Overall change	527 B	527 B	✓

Edge SSR bundle Size

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
edge-ssr.js gzip	94.4 kB	94.4 kB	N/A
page.js gzip	150 kB	150 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
middleware-b..fest.js gzip	619 B	624 B	N/A
middleware-r..fest.js gzip	151 B	149 B	N/A
middleware.js gzip	47.4 kB	47.4 kB	N/A
edge-runtime..pack.js gzip	1.94 kB	1.94 kB	✓
Overall change	1.94 kB	1.94 kB	✓

Next Runtimes

	vercel/next.js canary	gnoff/next.js fix-initial-flight-data	Change
app-page-exp...dev.js gzip	166 kB	166 kB	✓
app-page-exp..prod.js gzip	95.4 kB	95.4 kB	✓
app-page-tur..prod.js gzip	97.2 kB	97.2 kB	✓
app-page-tur..prod.js gzip	91.6 kB	91.6 kB	✓
app-page.run...dev.js gzip	136 kB	136 kB	✓
app-page.run..prod.js gzip	90.2 kB	90.2 kB	✓
app-route-ex...dev.js gzip	22 kB	22 kB	✓
app-route-ex..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.7 kB	14.7 kB	✓
app-route.ru...dev.js gzip	21.7 kB	21.7 kB	✓
app-route.ru..prod.js gzip	14.7 kB	14.7 kB	✓
pages-api-tu..prod.js gzip	9.43 kB	9.43 kB	✓
pages-api.ru...dev.js gzip	9.7 kB	9.7 kB	✓
pages-api.ru..prod.js gzip	9.43 kB	9.43 kB	✓
pages-turbo...prod.js gzip	22 kB	22 kB	✓
pages.runtim...dev.js gzip	22.7 kB	22.7 kB	✓
pages.runtim..prod.js gzip	22 kB	22 kB	✓
server.runti..prod.js gzip	49.9 kB	49.9 kB	✓
Overall change	924 kB	924 kB	✓

Diff details

Diff for 68-HASH.js

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Commit: 9747dce