[metadata] Align prefetch head type with head

[huozhi]

What

Follow up of #73867 where prefetchHead should also be HeadData type like head. The default value of prefetchHead can just be null instead of [null, null] which represents there's no prefetched head, as we're also checking if the value is truthy so that we can do the fallback logic in Head component

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js 12-17-separate_prefetch_head	Change
buildDuration	25.1s	22.6s	N/A
buildDurationCached	21.1s	18.6s	N/A
nodeModulesSize	416 MB	416 MB	⚠️ +1.4 kB
nextStartRea..uration (ms)	589ms	595ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js 12-17-separate_prefetch_head	Change
1187-HASH.js gzip	52.4 kB	52.4 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.36 kB	5.36 kB	N/A
bccd1874-HASH.js gzip	52.8 kB	52.8 kB	N/A
framework-HASH.js gzip	57.5 kB	57.5 kB	N/A
main-app-HASH.js gzip	233 B	235 B	N/A
main-HASH.js gzip	34.1 kB	34.1 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	vercel/next.js 12-17-separate_prefetch_head	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	vercel/next.js 12-17-separate_prefetch_head	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	4.49 kB	4.49 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	vercel/next.js 12-17-separate_prefetch_head	Change
_buildManifest.js gzip	749 B	746 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js 12-17-separate_prefetch_head	Change
index.html gzip	522 B	524 B	N/A
link.html gzip	537 B	537 B	✓
withRouter.html gzip	518 B	520 B	N/A
Overall change	537 B	537 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js 12-17-separate_prefetch_head	Change
edge-ssr.js gzip	129 kB	129 kB	N/A
page.js gzip	206 kB	206 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js 12-17-separate_prefetch_head	Change
middleware-b..fest.js gzip	669 B	666 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	31.3 kB	31.3 kB	N/A
edge-runtime..pack.js gzip	844 B	844 B	✓
Overall change	844 B	844 B	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js 12-17-separate_prefetch_head	Change
274-experime...dev.js gzip	322 B	322 B	✓
274.runtime.dev.js gzip	314 B	314 B	✓
app-page-exp...dev.js gzip	359 kB	359 kB	N/A
app-page-exp..prod.js gzip	129 kB	129 kB	✓
app-page-tur..prod.js gzip	141 kB	141 kB	✓
app-page-tur..prod.js gzip	137 kB	137 kB	✓
app-page.run...dev.js gzip	348 kB	348 kB	✓
app-page.run..prod.js gzip	125 kB	125 kB	✓
app-route-ex...dev.js gzip	37.4 kB	37.4 kB	✓
app-route-ex..prod.js gzip	25.5 kB	25.5 kB	✓
app-route-tu..prod.js gzip	25.5 kB	25.5 kB	✓
app-route-tu..prod.js gzip	25.3 kB	25.3 kB	✓
app-route.ru...dev.js gzip	39.1 kB	39.1 kB	✓
app-route.ru..prod.js gzip	25.3 kB	25.3 kB	✓
pages-api-tu..prod.js gzip	9.69 kB	9.69 kB	✓
pages-api.ru...dev.js gzip	11.6 kB	11.6 kB	✓
pages-api.ru..prod.js gzip	9.68 kB	9.68 kB	✓
pages-turbo...prod.js gzip	21.7 kB	21.7 kB	✓
pages.runtim...dev.js gzip	27.5 kB	27.5 kB	✓
pages.runtim..prod.js gzip	21.7 kB	21.7 kB	✓
server.runti..prod.js gzip	916 kB	916 kB	✓
Overall change	2.08 MB	2.08 MB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	vercel/next.js 12-17-separate_prefetch_head	Change
0.pack gzip	2.08 MB	2.08 MB	⚠️ +1.06 kB
index.pack gzip	73.4 kB	74.4 kB	⚠️ +996 B
Overall change	2.15 MB	2.15 MB	⚠️ +2.06 kB

Diff details

Diff for 1187-HASH.js

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Diff for main-HASH.js

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Commit: 1d70f48

[huozhi]

• [metadata] Merge the metadata resolve apis into one api #74191
• [metadata] Align prefetch head type with head #74161 👈 (View in Graphite)
• canary

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[ztanner]

I'm a bit confused: shouldn't we either standardize on null representing [null, null]? Or consistently always do [null, null]?

Suggested change

	head: (isLeafSegment ? possiblyPartialPrefetchHead : null) ?? [
	null,
	null,
	],
	head: isLeafSegment ? possiblyPartialPrefetchHead : [null, null],

[huozhi]

possiblyPartialPrefetchHead is still nullable. I tried all in [null, null] first but feels it's bit redudant as prefetched data doesn't have to always hold a [null, null] as default value. When you want to check if it's nullable and fallback to current head, [null, null] would be a bit confusing to represent null and to check the values.

This will be a temeporary state, we'll have the [null, null] for short time then metadata will go to stay with page component. Then head and prefetchedHead will be back to single ReactNode