Add flag to disable API warning

This flag is useful when you are using an external API resolver like express when using you are defining your API route, since the native functionality doesn't realize that the API actually sent a response.

A very simple use case example https://github.com/PabloSzx/next-external-api-resolver-example

docs/api-routes/api-middlewares.md

@@ -48,6 +48,16 @@ export const config = {
 }
 ```
 
+`externalResolver` is a explicit flag that tells the server that this route is being handled by an external resolver like _express_ or _connect_ and it shouldn't make misleading warnings
+
+```js
+export const config = {
+  api: {
+    externalResolver: true,
+  },
+}
+```
+
 ## Connect/Express middleware support
 
 You can also use [Connect](https://github.com/senchalabs/connect) compatible middleware.

packages/next/next-server/server/api-utils.ts

@@ -40,6 +40,7 @@ export async function apiResolver(
     }
     const config: PageConfig = resolverModule.config || {}
     const bodyParser = config.api?.bodyParser !== false
+    const externalResolver = config.api?.externalResolver ?? false
 
     apiReq.env = env ? collectEnv(req.url!, env, config.env) : {}
 
@@ -75,7 +76,12 @@ export async function apiResolver(
     // Call API route method
     await resolver(req, res)
 
-    if (process.env.NODE_ENV !== 'production' && !isResSent(res) && !wasPiped) {
+    if (
+      process.env.NODE_ENV !== 'production' &&
+      !externalResolver &&
+      !isResSent(res) &&
+      !wasPiped
+    ) {
       console.warn(
         `API resolved without sending a response for ${req.url}, this may result in stalled requests.`
       )

packages/next/types/index.d.ts

@@ -54,6 +54,12 @@ export type PageConfig = {
      * format supported by `bytes`, for example `1000`, `'500kb'` or `'3mb'`.
      */
     bodyParser?: { sizeLimit?: number | string } | false
+    /**
+     * Flag to disable warning "API page resolved
+     * without sending a response", due to explicitly
+     * using an external API resolver, like express
+     */
+    externalResolver?: true
   }
   env?: Array<string>
 }