[Segment Cache] Respond with 204 on cache miss

[acdlite]

Currently, when the client prefetches a segment, the server responds with a 404 if it cannot fulfill the request. This updates it to respond with a 204 No Content instead, since it's not an error for the client to request a segment whose prefetch hasn't been generated.

When responding with 204, the server also sends the 'x-nextjs-postponed' header, but only if PPR is enabled for the route. The client can use the absence of this header to distinguish between a regular cache miss and a miss due to PPR being disabled. In a later PR, I will update the client to use this information to fall back to the non-PPR behavior.

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
buildDuration	23.4s	21.8s	N/A
buildDurationCached	20.7s	18.1s	N/A
nodeModulesSize	409 MB	409 MB	⚠️ +15.9 kB
nextStartRea..uration (ms)	540ms	539ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
1187-HASH.js gzip	50.2 kB	50.2 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.3 kB	5.3 kB	N/A
bccd1874-HASH.js gzip	53 kB	53 kB	N/A
framework-HASH.js gzip	57.5 kB	57.5 kB	N/A
main-app-HASH.js gzip	232 B	235 B	N/A
main-HASH.js gzip	33.8 kB	33.7 kB	N/A
webpack-HASH.js gzip	1.71 kB	1.71 kB	N/A
Overall change	0 B	0 B	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Overall change	39.4 kB	39.4 kB	✓

Client Pages

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
_app-HASH.js gzip	193 B	193 B	✓
_error-HASH.js gzip	193 B	193 B	✓
amp-HASH.js gzip	512 B	510 B	N/A
css-HASH.js gzip	343 B	342 B	N/A
dynamic-HASH.js gzip	1.84 kB	1.84 kB	✓
edge-ssr-HASH.js gzip	265 B	265 B	✓
head-HASH.js gzip	363 B	362 B	N/A
hooks-HASH.js gzip	393 B	392 B	N/A
image-HASH.js gzip	4.44 kB	4.43 kB	N/A
index-HASH.js gzip	268 B	268 B	✓
link-HASH.js gzip	2.35 kB	2.34 kB	N/A
routerDirect..HASH.js gzip	328 B	328 B	✓
script-HASH.js gzip	397 B	397 B	✓
withRouter-HASH.js gzip	323 B	326 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.59 kB	3.59 kB	✓

Client Build Manifests

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
_buildManifest.js gzip	747 B	745 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
index.html gzip	524 B	523 B	N/A
link.html gzip	538 B	538 B	✓
withRouter.html gzip	520 B	520 B	✓
Overall change	1.06 kB	1.06 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
edge-ssr.js gzip	128 kB	128 kB	N/A
page.js gzip	203 kB	203 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
middleware-b..fest.js gzip	671 B	667 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	31 kB	31 kB	N/A
edge-runtime..pack.js gzip	844 B	844 B	✓
Overall change	844 B	844 B	✓

Next Runtimes

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
523-experime...dev.js gzip	322 B	322 B	✓
523.runtime.dev.js gzip	314 B	314 B	✓
app-page-exp...dev.js gzip	322 kB	322 kB	N/A
app-page-exp..prod.js gzip	127 kB	127 kB	N/A
app-page-tur..prod.js gzip	140 kB	140 kB	N/A
app-page-tur..prod.js gzip	135 kB	135 kB	N/A
app-page.run...dev.js gzip	312 kB	312 kB	N/A
app-page.run..prod.js gzip	122 kB	122 kB	N/A
app-route-ex...dev.js gzip	37.1 kB	37.1 kB	✓
app-route-ex..prod.js gzip	25.1 kB	25.1 kB	✓
app-route-tu..prod.js gzip	25.1 kB	25.1 kB	✓
app-route-tu..prod.js gzip	24.9 kB	24.9 kB	✓
app-route.ru...dev.js gzip	38.7 kB	38.7 kB	✓
app-route.ru..prod.js gzip	24.9 kB	24.9 kB	✓
pages-api-tu..prod.js gzip	9.56 kB	9.56 kB	✓
pages-api.ru...dev.js gzip	11.4 kB	11.4 kB	✓
pages-api.ru..prod.js gzip	9.56 kB	9.56 kB	✓
pages-turbo...prod.js gzip	21.3 kB	21.3 kB	✓
pages.runtim...dev.js gzip	27 kB	27 kB	✓
pages.runtim..prod.js gzip	21.3 kB	21.3 kB	✓
server.runti..prod.js gzip	916 kB	916 kB	N/A
Overall change	277 kB	277 kB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js segment-cache-204-on-miss	Change
0.pack gzip	2.04 MB	2.04 MB	⚠️ +241 B
index.pack gzip	145 kB	146 kB	⚠️ +903 B
Overall change	2.18 MB	2.18 MB	⚠️ +1.14 kB

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Commit: 3ae4382

[ijjk]

This could also have been lazily generated in case that matters for context