[Segment Cache] Prioritize route trees over segments

[acdlite]

When processing the prefetch queue, we should prioritize fetching the route trees for all the links before we fetch any of the segments.

The reason the route tree is more important is because it tells us the structure of the target page; from this alone, we can determine which segments are already cached and skip them during the navigation. Whereas if the route tree is missing at the time of the navigation, we cannot instruct the server to skip over prefetched segments, because we don't know which ones to skip.

To implement this, I added a phase property to the PrefetchTask type. This is essentially a secondary priority field that changes as the task progresses. There are two phases: RouteTree and Segments. Any task in the RouteTree phase has higher priority than tasks in the Segments phase.

Another way to implement this would be to add a secondary queue for segment prefetches. This would require reference counting to clean up segments when there are no more tasks that depend on them. The ref count could also be used as an additional prioritization heuristic: shared layouts with many dependent tasks could be prioritized over segments with fewer. However, I'm not sure the benefits are worth the extra code/complexity, so I'm starting with this simpler approach.

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js segment-cache-prioritize-route-tree	Change
buildDuration	21.1s	19s	N/A
buildDurationCached	18.2s	15.4s	N/A
nodeModulesSize	419 MB	419 MB	⚠️ +30.8 kB
nextStartRea..uration (ms)	478ms	475ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	acdlite/next.js segment-cache-prioritize-route-tree	Change
5306-HASH.js gzip	54.1 kB	54.1 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.46 kB	5.46 kB	N/A
bccd1874-HASH.js gzip	52.9 kB	52.9 kB	N/A
framework-HASH.js gzip	57.5 kB	57.5 kB	N/A
main-app-HASH.js gzip	240 B	242 B	N/A
main-HASH.js gzip	34.6 kB	34.6 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-prioritize-route-tree	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-prioritize-route-tree	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	4.59 kB	4.58 kB	N/A
index-HASH.js gzip	268 B	268 B	✓
link-HASH.js gzip	2.35 kB	2.35 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-prioritize-route-tree	Change
_buildManifest.js gzip	748 B	747 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	acdlite/next.js segment-cache-prioritize-route-tree	Change
index.html gzip	523 B	522 B	N/A
link.html gzip	538 B	537 B	N/A
withRouter.html gzip	520 B	520 B	✓
Overall change	520 B	520 B	✓

Edge SSR bundle Size

	vercel/next.js canary	acdlite/next.js segment-cache-prioritize-route-tree	Change
edge-ssr.js gzip	129 kB	129 kB	N/A
page.js gzip	209 kB	209 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	acdlite/next.js segment-cache-prioritize-route-tree	Change
middleware-b..fest.js gzip	670 B	667 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	acdlite/next.js segment-cache-prioritize-route-tree	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	376 kB	376 kB	N/A
app-page-exp..prod.js gzip	131 kB	131 kB	N/A
app-page-tur..prod.js gzip	144 kB	144 kB	N/A
app-page-tur..prod.js gzip	140 kB	140 kB	N/A
app-page.run...dev.js gzip	364 kB	364 kB	N/A
app-page.run..prod.js gzip	127 kB	127 kB	N/A
app-route-ex...dev.js gzip	37.6 kB	37.6 kB	✓
app-route-ex..prod.js gzip	25.6 kB	25.6 kB	✓
app-route-tu..prod.js gzip	25.6 kB	25.6 kB	✓
app-route-tu..prod.js gzip	25.4 kB	25.4 kB	✓
app-route.ru...dev.js gzip	39.2 kB	39.2 kB	✓
app-route.ru..prod.js gzip	25.4 kB	25.4 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	21.9 kB	21.9 kB	✓
pages.runtim...dev.js gzip	27.7 kB	27.7 kB	✓
pages.runtim..prod.js gzip	21.9 kB	21.9 kB	✓
server.runti..prod.js gzip	916 kB	916 kB	✓
Overall change	1.2 MB	1.2 MB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js segment-cache-prioritize-route-tree	Change
0.pack gzip	2.1 MB	2.1 MB	N/A
index.pack gzip	75.4 kB	75.7 kB	⚠️ +292 B
Overall change	75.4 kB	75.7 kB	⚠️ +292 B

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Commit: d870c86