Fix generateMetadata race condition (#63169)

This ensures we properly catch/handle `generateMetadata` errors during
eager evaluating of nested `generateMetadata` functions in the tree.
Previously if we eager evaluated a child metadata function that threw an
error e.g. `notFound()` and but the parent metadata function took longer
the thrown error would be an unhandled rejection causing the process to
crash depending on the environment.

Fixes: NEXT-2588


Closes NEXT-2786

Author: ijjk

packages/next/src/lib/metadata/resolve-metadata.ts

@@ -620,13 +620,24 @@ function collectMetadataExportPreloading<Data, ResolvedData>(
   dynamicMetadataExportFn: DataResolver<Data, ResolvedData>,
   resolvers: ((value: ResolvedData) => void)[]
 ) {
-  results.push(
-    dynamicMetadataExportFn(
-      new Promise<any>((resolve) => {
-        resolvers.push(resolve)
-      })
-    )
+  const result = dynamicMetadataExportFn(
+    new Promise<any>((resolve) => {
+      resolvers.push(resolve)
+    })
   )
+
+  if (result instanceof Promise) {
+    // since we eager execute generateMetadata and
+    // they can reject at anytime we need to ensure
+    // we attach the catch handler right away to
+    // prevent unhandled rejections crashing the process
+    result.catch((err) => {
+      return {
+        __nextError: err,
+      }
+    })
+  }
+  results.push(result)
 }
 
 async function getMetadataFromExport<Data, ResolvedData>(
@@ -681,6 +692,11 @@ async function getMetadataFromExport<Data, ResolvedData>(
     resolveParent(currentResolvedMetadata)
     metadata =
       metadataResult instanceof Promise ? await metadataResult : metadataResult
+
+    if (metadata && typeof metadata === 'object' && '__nextError' in metadata) {
+      // re-throw caught metadata error from preloading
+      throw metadata['__nextError']
+    }
   } else if (metadataExport !== null && typeof metadataExport === 'object') {
     // This metadataExport is the object form
     metadata = metadataExport

test/e2e/app-dir/metadata/app/(dynamic-meta)/dynamic-meta/page.tsx

@@ -0,0 +1,17 @@
+import type { ResolvingMetadata } from 'next'
+import { notFound } from 'next/navigation'
+
+process.on('unhandledRejection', (rej) => {
+  console.log('unhandledRejection', rej)
+  process.exit(1)
+})
+
+export const revalidate = 0
+
+export default function Page() {
+  return notFound()
+}
+
+export async function generateMetadata(_, __: ResolvingMetadata) {
+  return notFound()
+}

test/e2e/app-dir/metadata/app/(dynamic-meta)/layout.tsx

@@ -0,0 +1,24 @@
+process.on('unhandledRejection', (rej) => {
+  console.log('unhandledRejection', rej)
+  process.exit(1)
+})
+
+export const revalidate = 0
+
+export default function Layout({ children }) {
+  return (
+    <>
+      <p>Layout</p>
+      {children}
+    </>
+  )
+}
+
+export async function generateMetadata() {
+  await new Promise((resolve) => {
+    setTimeout(() => {
+      process.nextTick(resolve)
+    }, 2_000)
+  })
+  return {}
+}

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

@@ -1000,5 +1000,10 @@ createNextDescribe(
       expect(iconHtml).toContain('pages-icon-page')
       expect(ogHtml).toContain('pages-opengraph-image-page')
     })
+
+    it('should not crash from error thrown during preloading nested generateMetadata', async () => {
+      const res = await next.fetch('/dynamic-meta')
+      expect(res.status).toBe(404)
+    })
   }
 )