fix: don't permamently cache fallback html

src/run/handlers/request-context.cts

@@ -11,7 +11,7 @@ export type RequestContext = {
   responseCacheGetLastModified?: number
   responseCacheKey?: string
   responseCacheTags?: string[]
-  usedFsRead?: boolean
+  usedFsReadForNonFallback?: boolean
   didPagesRouterOnDemandRevalidate?: boolean
   serverTiming?: string
   routeHandlerRevalidate?: NetlifyCachedRouteValue['revalidate']

src/run/headers.ts

@@ -263,7 +263,7 @@ export const setCacheControlHeaders = (
     cacheControl === null &&
     !headers.has('cdn-cache-control') &&
     !headers.has('netlify-cdn-cache-control') &&
-    requestContext.usedFsRead
+    requestContext.usedFsReadForNonFallback
   ) {
     // handle CDN Cache Control on static files
     headers.set('cache-control', 'public, max-age=0, must-revalidate')

src/run/next.cts

@@ -1,8 +1,9 @@
-import fs from 'fs/promises'
+import fs, { readFile } from 'fs/promises'
 import { relative, resolve } from 'path'
 
 // @ts-expect-error no types installed
 import { patchFs } from 'fs-monkey'
+import { type PrerenderManifest } from 'next/dist/build/index.js'
 
 import { getRequestContext } from './handlers/request-context.cjs'
 import { getTracer } from './handlers/tracer.cjs'
@@ -80,6 +81,30 @@ console.timeEnd('import next server')
 
 type FS = typeof import('fs')
 
+function normalizeStaticAssetPath(path: string) {
+  // just skip leading / for now
+  return path.replace(/^\/+/g, '')
+}
+
+let fallbacks: Array<string> | undefined
+async function isFallbackHTML(relPath: string) {
+  if (!fallbacks) {
+    fallbacks = []
+
+    try {
+      const prerenderManifest = JSON.parse(
+        await readFile(resolve('.next/prerender-manifest.json'), 'utf-8'),
+      ) as PrerenderManifest
+      fallbacks = Object.values(prerenderManifest.dynamicRoutes)
+        .map((route) => route.fallback)
+        .filter((fallback) => typeof fallback === 'string')
+        .map(normalizeStaticAssetPath)
+    } catch {}
+  }
+
+  return fallbacks.includes(normalizeStaticAssetPath(relPath))
+}
+
 export async function getMockedRequestHandlers(...args: Parameters<typeof getRequestHandlers>) {
   const tracer = getTracer()
   return tracer.withActiveSpan('mocked request handler', async () => {
@@ -100,9 +125,11 @@ export async function getMockedRequestHandlers(...args: Parameters<typeof getReq
           const relPath = relative(resolve('.next/server/pages'), path)
           const file = await store.get(await encodeBlobKey(relPath))
           if (file !== null) {
-            const requestContext = getRequestContext()
-            if (requestContext) {
-              requestContext.usedFsRead = true
+            if (!(await isFallbackHTML(relPath))) {
+              const requestContext = getRequestContext()
+              if (requestContext) {
+                requestContext.usedFsReadForNonFallback = true
+              }
             }
 
             return file