Fix fallback detection logic when multiple generateStaticParams are n…

…eeded (#47982)

### What?

Our current logic of detecting if a route allows dynamic params or not
(`fallback`) is flawed, and this PR fixes it.

### Why?

Right now, if no `generateStaticParams` is specified we return
`fallback: undefined` during dev. However, for an app with multiple
params, it may have multiple `generateStaticParams` defined in different
levels. If some level isn't covered by any `generateStaticParams`, we
still can't determine the fallback value.

### How?

I added a naive implementation to check if all params are covered by
`generateStaticParams` in the current or inner layers.

Closes NEXT-946

packages/next/src/build/utils.ts

@@ -1086,6 +1086,7 @@ export type AppConfig = {
 }
 export type GenerateParams = Array<{
   config?: AppConfig
+  isDynamicSegment?: boolean
   segmentPath: string
   getStaticPaths?: GetStaticPaths
   generateStaticParams?: any
@@ -1133,9 +1134,11 @@ export const collectGenerateParams = async (
   const config = collectAppConfig(mod)
 
   const isClientComponent = isClientReference(mod)
+  const isDynamicSegment = segment[0] && /^\[.+\]$/.test(segment[0])
 
   const result = {
     isLayout,
+    isDynamicSegment,
     segmentPath: `/${parentSegments.join('/')}${
       segment[0] && parentSegments.length > 0 ? '/' : ''
     }${segment[0]}`,
@@ -1152,7 +1155,11 @@ export const collectGenerateParams = async (
 
   if (result.config || result.generateStaticParams || result.getStaticPaths) {
     generateParams.push(result)
+  } else if (isDynamicSegment) {
+    // It is a dynamic route, but no config was provided
+    generateParams.push(result)
   }
+
   return collectGenerateParams(
     segment[1]?.children,
     parentSegments,
@@ -1243,7 +1250,7 @@ export async function buildAppStaticPaths({
         // if generateStaticParams is being used we iterate over them
         // collecting them from each level
         type Params = Array<Record<string, string | string[]>>
-        let hadGenerateParams = false
+        let hadAllParamsGenerated = false
 
         const buildParams = async (
           paramsItems: Params = [{}],
@@ -1258,9 +1265,15 @@ export async function buildAppStaticPaths({
             typeof curGenerate.generateStaticParams !== 'function' &&
             idx < generateParams.length
           ) {
+            if (curGenerate.isDynamicSegment) {
+              // This dynamic level has no generateStaticParams so we change
+              // this flag to false, but it could be covered by a later
+              // generateStaticParams so it could be set back to true.
+              hadAllParamsGenerated = false
+            }
             return buildParams(paramsItems, idx + 1)
           }
-          hadGenerateParams = true
+          hadAllParamsGenerated = true
 
           const newParams = []
 
@@ -1287,7 +1300,7 @@ export async function buildAppStaticPaths({
           (generate) => generate.config?.dynamicParams === false
         )
 
-        if (!hadGenerateParams) {
+        if (!hadAllParamsGenerated) {
           return {
             paths: undefined,
             fallback:

test/e2e/app-dir/app-static/app-static.test.ts

@@ -54,6 +54,16 @@ createNextDescribe(
           'A required parameter (slug) was not provided as a string received object'
         )
       })
+
+      it('should correctly handle multi-level generateStaticParams when some levels are missing', async () => {
+        const browser = await next.browser('/flight/foo/bar')
+        const v = ~~(Math.random() * 1000)
+        await browser.eval(`document.cookie = "test-cookie=${v}"`)
+        await browser.elementByCss('button').click()
+        await check(async () => {
+          return await browser.elementByCss('h1').text()
+        }, v.toString())
+      })
     }
 
     it('should correctly skip caching POST fetch for POST handler', async () => {
@@ -216,6 +226,7 @@ createNextDescribe(
           'dynamic-error/[id]/page.js',
           'dynamic-no-gen-params-ssr/[slug]/page.js',
           'dynamic-no-gen-params/[slug]/page.js',
+          'flight/[slug]/[slug2]/page.js',
           'force-dynamic-catch-all/[slug]/[[...id]]/page.js',
           'force-dynamic-no-prerender/[id]/page.js',
           'force-dynamic-prerender/[slug]/page.js',
@@ -591,16 +602,6 @@ createNextDescribe(
               '^\\/partial\\-gen\\-params\\-no\\-additional\\-slug\\/([^\\/]+?)\\/([^\\/]+?)(?:\\/)?$'
             ),
           },
-          '/partial-gen-params/[lang]/[slug]': {
-            dataRoute: '/partial-gen-params/[lang]/[slug].rsc',
-            dataRouteRegex: normalizeRegEx(
-              '^\\/partial\\-gen\\-params\\/([^\\/]+?)\\/([^\\/]+?)\\.rsc$'
-            ),
-            fallback: null,
-            routeRegex: normalizeRegEx(
-              '^\\/partial\\-gen\\-params\\/([^\\/]+?)\\/([^\\/]+?)(?:\\/)?$'
-            ),
-          },
           '/force-static/[slug]': {
             dataRoute: '/force-static/[slug].rsc',
             dataRouteRegex: normalizeRegEx(

test/e2e/app-dir/app-static/app/flight/[slug]/[slug2]/page.js

@@ -0,0 +1,13 @@
+import { cookies } from 'next/headers'
+
+import Refresh from './refresh'
+
+export default function Page() {
+  const cookieValue = cookies().get('test-cookie')
+  return (
+    <>
+      <Refresh />
+      <h1>{cookieValue?.value}</h1>
+    </>
+  )
+}

test/e2e/app-dir/app-static/app/flight/[slug]/[slug2]/refresh.js

@@ -0,0 +1,9 @@
+'use client'
+
+import { useRouter } from 'next/navigation'
+
+export default function Refresh() {
+  const router = useRouter()
+
+  return <button onClick={() => router.refresh()}>Refresh</button>
+}

test/e2e/app-dir/app-static/app/flight/[slug]/layout.js

@@ -0,0 +1,7 @@
+export default function Layout({ children }) {
+  return children
+}
+
+export function generateStaticParams() {
+  return [{ slug: 'foo' }]
+}