implement unstable_rethrow (#65831)

This implements an API to re-throw errors that are intended to be caught
by Next.js, so that they are not caught by your code.

RFC: https://github.com/vercel/next.js/discussions/64076

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---------

Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: Ahmed Abdelbaset <A7med3bdulBaset@gmail.com>
Co-authored-by: Janka Uryga <lolzatu2@gmail.com>

docs/02-app/02-api-reference/04-functions/unstable_rethrow.mdx

@@ -0,0 +1,61 @@
+---
+title: unstable_rethrow
+description: API Reference for the unstable_rethrow function.
+---
+
+`unstable_rethrow` can be used to avoid catching internal errors thrown by Next.js when attempting to handle errors thrown from your own application.
+
+For example, given the following code:
+
+```jsx
+import { notFound } from 'next/navigation'
+
+export default async function Component() {
+  try {
+    notFound()
+    await getData()
+  } catch (err) {
+    console.error(err)
+  }
+}
+```
+
+In this scenario, `console.error` will pick up the error thrown by `notFound`, which won't be useful for logging and will prevent
+the `not-found.js` page from rendering. Instead, you can do this:
+
+```jsx
+import { notFound, unstable_rethrow } from 'next/navigation'
+
+export default async function Component() {
+  try {
+    notFound()
+    await getData()
+  } catch (err) {
+    unstable_rethrow(err)
+    console.error(err)
+  }
+}
+```
+
+If the caught error is meant to be handled by Next.js, `unstable_rethrow` will re-throw the error instead of letting it reach your
+error handling code.
+
+The following Next.js APIs rely on throwing an error which should be rethrown and handled by Next.js itself:
+
+- [`notFound()`](/docs/app/api-reference/functions/not-found)
+- [`redirect()`](/docs/app/building-your-application/routing/redirecting#redirect-function)
+- [`permanentRedirect()`](/docs/app/building-your-application/routing/redirecting#permanentredirect-function)
+
+If a route segment is marked to throw an error unless it's static, a dynamic function call will also throw an error that should similarly not be caught by the developer. Note that Partial Prerendering (PPR) affects this behavior as well. These APIs are:
+
+- [`cookies()`](/docs/app/api-reference/functions/cookies)
+- [`headers()`](/docs/app/api-reference/functions/headers)
+- [`searchParams`](/docs/app/api-reference/file-conventions/page#searchparams-optional)
+- `fetch(..., { cache: 'no-store' })`
+- `fetch(..., { next: { revalidate: 0 } })`
+
+> **Good to know**:
+>
+> - This method should be called at the top of the catch block, passing the error object as its only argument. It can also be used within a `.catch` handler of a promise.
+> - If you ensure that your calls to APIs that throw are not wrapped in a try/catch then you don't need to use `unstable_rethrow`
+> - Any resource cleanup (like clearing intervals, timers, etc) would have to either happen prior to the call to `unstable_rethrow` or within a `finally` block.

packages/next/src/client/components/navigation.react-server.ts

@@ -28,4 +28,5 @@ class ReadonlyURLSearchParams extends URLSearchParams {
 
 export { redirect, permanentRedirect, RedirectType } from './redirect'
 export { notFound } from './not-found'
+export { unstable_rethrow } from './unstable-rethrow'
 export { ReadonlyURLSearchParams }

packages/next/src/client/components/navigation.ts

@@ -269,4 +269,5 @@ export {
   permanentRedirect,
   RedirectType,
   ReadonlyURLSearchParams,
+  unstable_rethrow,
 } from './navigation.react-server'

packages/next/src/client/components/unstable-rethrow.ts

@@ -0,0 +1,26 @@
+import { isDynamicUsageError } from '../../export/helpers/is-dynamic-usage-error'
+import { isPostpone } from '../../server/lib/router-utils/is-postpone'
+import { isBailoutToCSRError } from '../../shared/lib/lazy-dynamic/bailout-to-csr'
+import { isNextRouterError } from './is-next-router-error'
+
+/**
+ * This function should be used to rethrow internal Next.js errors so that they can be handled by the framework.
+ * When wrapping an API that uses errors to interrupt control flow, you should use this function before you do any error handling.
+ * This function will rethrow the error if it is a Next.js error so it can be handled, otherwise it will do nothing.
+ *
+ * Read more: [Next.js Docs: `unstable_rethrow`](https://nextjs.org/docs/app/api-reference/functions/unstable_rethrow)
+ */
+export function unstable_rethrow(error: unknown): void {
+  if (
+    isNextRouterError(error) ||
+    isBailoutToCSRError(error) ||
+    isDynamicUsageError(error) ||
+    isPostpone(error)
+  ) {
+    throw error
+  }
+
+  if (error instanceof Error && 'cause' in error) {
+    unstable_rethrow(error.cause)
+  }
+}

test/e2e/app-dir/unstable-rethrow/app/cause/page.tsx

@@ -0,0 +1,22 @@
+import { cookies } from 'next/headers'
+import { unstable_rethrow } from 'next/navigation'
+
+function someFunction() {
+  try {
+    cookies()
+  } catch (err) {
+    throw new Error('Oopsy', { cause: err })
+  }
+}
+
+export default async function Page() {
+  try {
+    someFunction()
+  } catch (err) {
+    console.log('[test assertion]: checking error')
+    unstable_rethrow(err)
+    console.error('[test assertion]: error leaked', err)
+  }
+
+  return <p>hello world</p>
+}

test/e2e/app-dir/unstable-rethrow/app/dynamic-error/page.tsx

@@ -0,0 +1,14 @@
+import { cookies } from 'next/headers'
+import { unstable_rethrow } from 'next/navigation'
+
+export default async function Page() {
+  try {
+    cookies()
+  } catch (err) {
+    console.log('[test assertion]: checking error')
+    unstable_rethrow(err)
+    console.error('[test assertion]: error leaked', err)
+  }
+
+  return <p>hello world</p>
+}

test/e2e/app-dir/unstable-rethrow/app/layout.tsx

@@ -0,0 +1,7 @@
+export default function Root({ children }: { children: React.ReactNode }) {
+  return (
+    <html>
+      <body>{children}</body>
+    </html>
+  )
+}

test/e2e/app-dir/unstable-rethrow/app/not-found-page/page.tsx

@@ -0,0 +1,13 @@
+import { notFound, unstable_rethrow } from 'next/navigation'
+
+export default async function Page() {
+  try {
+    notFound()
+  } catch (err) {
+    console.log('[test assertion]: checking error')
+    unstable_rethrow(err)
+    console.error('[test assertion]: error leaked', err)
+  }
+
+  return <p>hello world</p>
+}

test/e2e/app-dir/unstable-rethrow/app/page.tsx

@@ -0,0 +1,12 @@
+import { notFound, unstable_rethrow } from 'next/navigation'
+
+export default async function Page() {
+  try {
+    notFound()
+  } catch (err) {
+    unstable_rethrow(err)
+    console.error(err)
+  }
+
+  return <p>hello world</p>
+}

test/e2e/app-dir/unstable-rethrow/app/redirect/page.tsx

@@ -0,0 +1,11 @@
+import { redirect, unstable_rethrow } from 'next/navigation'
+
+export default function Page() {
+  try {
+    redirect('/')
+  } catch (err) {
+    console.log('[test assertion]: checking error')
+    unstable_rethrow(err)
+    console.error('[test assertion]: error leaked', err)
+  }
+}

test/e2e/app-dir/unstable-rethrow/next.config.js

@@ -0,0 +1,6 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {}
+
+module.exports = nextConfig

test/e2e/app-dir/unstable-rethrow/unstable-rethrow.test.ts

@@ -0,0 +1,30 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('unstable-rethrow', () => {
+  const { next, isNextStart } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('should correctly trigger the not found page as not found', async () => {
+    const browser = await next.browser('/not-found-page')
+    expect(await browser.elementByCss('body').text()).toContain(
+      'This page could not be found.'
+    )
+  })
+
+  it('should handle an internal error that gets propagated to the `cause` field', async () => {
+    const browser = await next.browser('/cause')
+    expect(await browser.elementByCss('body').text()).toContain('hello world')
+  })
+
+  if (isNextStart) {
+    it('should not log any errors at build time', async () => {
+      expect(next.cliOutput).toContain('[test assertion]: checking error')
+      expect(next.cliOutput).not.toContain('[test assertion]: error leaked')
+    })
+
+    it('should correctly mark the dynamic page as dynamic', async () => {
+      expect(next.cliOutput).toContain('ƒ /dynamic-error')
+    })
+  }
+})