Introduce prerenderAsyncStorage and refactor app-render

In preparation for a number of new features and rendering modes this PR dramatically refactors app-render and related files to make future contributions easier to land.

Historically we have used AsyncLocalStorage to scope state that is contextual for an entire page export (RSC > SSR > error handling etc...) however it can be useful and is generally a good practice to push the ALS scoping deeper into the render when possible. One case is for tracking dynamic access. in this PR we introduce prerenderAsyncStorage which is wrapped around individual render calls into React. If there is a store during a render then we can infer we are in an App Router prerender.

Longer term we should consider combinging the requestStorage and staticGenerationStorage since these are scoped at the same level and the name of staticGenerationStorage is misleading because it is in scope even when there is no static generation happening.

In addition to storage changes the code paths for app-render are reorganized to use fewer closures and to consolidate argument passing. Additionally we now treat render (dynamic request handling) and prerender (build and revalidate handling) in separate code paths. This allows for clearer login within each pathway and fewer repeated conditionals.

There are still a number of things to further clean up like how we track metadata. One idea is to have the render/prerender functions simply return a RenderResult. Another area is error handling. There is currently a bug where ssr only errors won't fail a build the same way an RSC error would. To keep this PR clean I've reproduced this behavior even if it is buggy so we can land a fix stacked on top

Author: gnoff

packages/next/src/build/templates/app-route.ts

@@ -36,7 +36,10 @@ const { requestAsyncStorage, staticGenerationAsyncStorage, serverHooks } =
   routeModule
 
 function patchFetch() {
-  return _patchFetch({ staticGenerationAsyncStorage, requestAsyncStorage })
+  return _patchFetch({
+    staticGenerationAsyncStorage,
+    requestAsyncStorage,
+  })
 }
 
 export {

packages/next/src/client/components/static-generation-async-storage.external.ts

@@ -3,7 +3,6 @@ import type { IncrementalCache } from '../../server/lib/incremental-cache'
 import type { DynamicServerError } from './hooks-server-context'
 import type { FetchMetrics } from '../../server/base-http'
 import type { Revalidate } from '../../server/lib/revalidate'
-import type { PrerenderState } from '../../server/app-render/dynamic-rendering'
 
 // Share the instance module in the next-shared layer
 import { staticGenerationAsyncStorage } from './static-generation-async-storage-instance' with { 'turbopack-transition': 'next-shared' }
@@ -28,9 +27,6 @@ export interface StaticGenerationStore {
   readonly isRevalidate?: boolean
   readonly isUnstableCacheCallback?: boolean
 
-  // When this exists (is not null) it means we are in a Prerender
-  prerenderState: null | PrerenderState
-
   forceDynamic?: boolean
   fetchCache?:
     | 'only-cache'

packages/next/src/server/app-render/action-handler.ts

@@ -539,7 +539,7 @@ export async function handleAction({
 
         return {
           type: 'done',
-          result: await generateFlight(ctx, {
+          result: await generateFlight(req, ctx, {
             actionResult: promise,
             // if the page was not revalidated, we can skip the rendering the flight tree
             skipFlight: !staticGenerationStore.pathWasRevalidated,
@@ -817,7 +817,7 @@ export async function handleAction({
           requestStore,
         })
 
-        actionResult = await generateFlight(ctx, {
+        actionResult = await generateFlight(req, ctx, {
           actionResult: Promise.resolve(returnVal),
           // if the page was not revalidated, or if the action was forwarded from another worker, we can skip the rendering the flight tree
           skipFlight:
@@ -895,7 +895,7 @@ export async function handleAction({
         }
         return {
           type: 'done',
-          result: await generateFlight(ctx, {
+          result: await generateFlight(req, ctx, {
             skipFlight: false,
             actionResult: promise,
             asNotFound: true,
@@ -928,7 +928,7 @@ export async function handleAction({
 
       return {
         type: 'done',
-        result: await generateFlight(ctx, {
+        result: await generateFlight(req, ctx, {
           actionResult: promise,
           // if the page was not revalidated, or if the action was forwarded from another worker, we can skip the rendering the flight tree
           skipFlight: