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Andrew ClarkandGitHub
3717cd3dfb
In #96074 I put the `partialFallback` behavior behind the `partialPrefetching` flag, so that we don't risk an explosion in ISR costs for apps that haven't opted into Partial Prefetching. But it turns out I only handled this for deploy mode — i.e. the build output consumed by the Vercel adapter. The behavior was still unconditionally on for `next start`. The reason it was easy to miss is that `next start` and the adapter express partial fallback shells through completely separate mechanisms. The adapter emits a `partialFallback` flag into the build output and lets the platform's ISR layer perform the shell upgrade. `next start` has no such flag; the server performs the upgrade itself, in the compiled page runtime. So the gate I added to the adapter output had no effect on the self-hosted path. To understand how the `next start` path was configured, I looked at the PR that removed the original top-level `partialFallbacks` config flag (#93859). Before that PR, the behavior was gated in the runtime; when the flag was removed those gates were deleted and the behavior became always-on. The important subtlety is that only the *upgrade* should be gated, not the shell machinery as a whole. On `next start`, the value that decides whether a shell can be specialized (`remainingPrerenderableParams`) also drives core Cache Components behavior — build-time partial prerendering of params and serving build-time sub-shells — which must stay on regardless of `partialPrefetching`. So this gates only the two things that make up the "upgrade on first request" cost risk: the background ISR revalidation that specializes a shell per request, and the client-facing `isFallbackUpgradeable` signal that tells the client to retry a prefetch waiting for that upgrade. Adds coverage with a `partialPrefetching`-disabled fixture asserting that fallback shells stay shared rather than specializing per request.
19 lines
571 B
TypeScript
19 lines
571 B
TypeScript
import type { NextConfig } from 'next'
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const nextConfig: NextConfig = {
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cacheComponents: true,
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// Fallback shells are only flagged upgradeable (and upgraded) when Partial
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// Prefetching is enabled; this suite exercises that upgrade + client-retry
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// path. The links opt into `prefetch={true}` because a Partial Prefetching
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// app skips the speculative prefetch otherwise.
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partialPrefetching: true,
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experimental: {
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prefetchInlining: true,
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varyParams: true,
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optimisticRouting: true,
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cachedNavigations: true,
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},
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}
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export default nextConfig
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