fix: Add stricter check for "use server" exports (#62821)

As mentioned in the new-added error messages, and the [linked
resources](https://react.dev/reference/react/use-server#:~:text=Because%20the%20underlying%20network%20calls%20are%20always%20asynchronous%2C%20%27use%20server%27%20can%20only%20be%20used%20on%20async%20functions.):

> Because the underlying network calls are always asynchronous, 'use
server' can only be used on async functions.
> https://react.dev/reference/react/use-server

It's a requirement that only async functions are allowed to be exported
and annotated with `'use server'`. Currently, we already have compiler
check so this will already error:

```js
'use server'

export function foo () {} // missing async
```

However, since exported values can be very dynamic the compiler can't
catch all mistakes like that. We also have a runtime check for all
exports in a `'use server'` function, but it only covers `typeof value
=== 'function'`.

This PR adds a stricter check for "use server" annotated values to also
make sure they're async functions (`value.constructor.name ===
'AsyncFunction'`).

That said, there are still cases like synchronously returning a promise
to make a function "async", but it's still very different by definition.
For example:

```js
const f = async () => { throw 1; return 1 }
const g = () => { throw 1; return Promise.resolve(1) }
```

Where `g()` can be synchronously caught (`try { g() } catch {}`) but
`f()` can't even if they have the same types. If we allow `g` to be a
Server Action, this behavior is no longer always true but depending on
where it's called (server or client).

Closes #62727.

errors/invalid-use-server-value.mdx

@@ -0,0 +1,43 @@
+---
+title: 'Invalid "use server" Value'
+---
+
+## Why This Error Occurred
+
+This error occurs when a `"use server"` file exports a value that is not an async function. It might happen when you unintentionally export something like a configuration object, an arbitrary value, or missed the `async` keyword in the exported function declaration.
+
+These functions are required to be defined as async, because `"use server"` marks them as [Server Actions](https://nextjs.org/docs/app/building-your-application/data-fetching/server-actions-and-mutations) and they can be invoked directly from the client through a network request.
+
+Examples of incorrect code:
+
+```js
+'use server'
+
+// ❌ This is incorrect: only async functions are allowed.
+export const value = 1
+
+// ❌ This is incorrect: missed the `async` keyword.
+export function getServerData() {
+  return '...'
+}
+```
+
+Correct code:
+
+```js
+'use server'
+
+// ✅ This is correct: an async function is exported.
+export async function getServerData() {
+  return '...'
+}
+```
+
+## Possible Ways to Fix It
+
+Check all exported values in the `"use server"` file (including `export *`) and make sure that they are all defined as async functions.
+
+## Useful Links
+
+- [Server Actions and Mutations - Next.js](https://nextjs.org/docs/app/building-your-application/data-fetching/server-actions-and-mutations)
+- ['use server' directive - React](https://react.dev/reference/react/use-server)

packages/next/src/build/webpack/loaders/next-flight-loader/action-validate.ts

@@ -6,7 +6,22 @@ export function ensureServerEntryExports(actions: any[]) {
     const action = actions[i]
     if (typeof action !== 'function') {
       throw new Error(
-        `A "use server" file can only export async functions, found ${typeof action}.`
+        `A "use server" file can only export async functions, found ${typeof action}.\nRead more: https://nextjs.org/docs/messages/invalid-use-server-value`
+      )
+    }
+
+    if (action.constructor.name !== 'AsyncFunction') {
+      const actionName: string = action.name || ''
+
+      // Note: if it's from library code with `async` being transpiled to returning a promise,
+      // it would be annoying. But users can still wrap it in an async function to work around it.
+      throw new Error(
+        `A "use server" file can only export async functions.${
+          // If the function has a name, we'll make the error message more specific.
+          actionName
+            ? ` Found "${actionName}" that is not an async function.`
+            : ''
+        }\nRead more: https://nextjs.org/docs/messages/invalid-use-server-value`
       )
     }
   }

test/e2e/app-dir/actions/app-action.test.ts

@@ -1,6 +1,6 @@
 /* eslint-disable jest/no-standalone-expect */
 import { createNextDescribe } from 'e2e-utils'
-import { check, waitFor } from 'next-test-utils'
+import { check, waitFor, getRedboxSource, hasRedbox } from 'next-test-utils'
 import type { Request, Response, Route } from 'playwright'
 import fs from 'fs-extra'
 import { join } from 'path'
@@ -508,6 +508,73 @@ createNextDescribe(
     }
 
     if (isNextDev) {
+      describe('"use server" export values', () => {
+        it('should error when exporting non async functions at build time', async () => {
+          const filePath = 'app/server/actions.js'
+          const origContent = await next.readFile(filePath)
+
+          try {
+            const browser = await next.browser('/server')
+
+            const cnt = await browser.elementByCss('h1').text()
+            expect(cnt).toBe('0')
+
+            // This can be caught by SWC directly
+            await next.patchFile(
+              filePath,
+              origContent + '\n\nexport const foo = 1'
+            )
+
+            expect(await hasRedbox(browser)).toBe(true)
+            expect(await getRedboxSource(browser)).toContain(
+              'Only async functions are allowed to be exported in a "use server" file.'
+            )
+          } finally {
+            await next.patchFile(filePath, origContent)
+          }
+        })
+
+        it('should error when exporting non async functions during runtime', async () => {
+          const logs: string[] = []
+          next.on('stdout', (log) => {
+            logs.push(log)
+          })
+          next.on('stderr', (log) => {
+            logs.push(log)
+          })
+
+          const filePath = 'app/server/actions.js'
+          const origContent = await next.readFile(filePath)
+
+          try {
+            const browser = await next.browser('/server')
+
+            const cnt = await browser.elementByCss('h1').text()
+            expect(cnt).toBe('0')
+
+            // This requires the runtime to catch
+            await next.patchFile(
+              filePath,
+              origContent + '\n\nconst f = () => {}\nexport { f }'
+            )
+
+            await check(
+              () =>
+                logs.some((log) =>
+                  log.includes(
+                    'Error: A "use server" file can only export async functions. Found "f" that is not an async function.'
+                  )
+                )
+                  ? 'true'
+                  : '',
+              'true'
+            )
+          } finally {
+            await next.patchFile(filePath, origContent)
+          }
+        })
+      })
+
       describe('HMR', () => {
         it('should support updating the action', async () => {
           const filePath = 'app/server/actions-3.js'