Add warning when API resolves without the request being finished

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

@@ -6,6 +6,7 @@ import { parse } from 'content-type'
 import { Params } from './router'
 import { PageConfig } from '../../types'
 import { interopDefault } from './load-components'
+import { isResSent } from '../lib/utils'
 
 export type NextApiRequestCookies = { [key: string]: string }
 export type NextApiRequestQuery = { [key: string]: string | string[] }
@@ -55,6 +56,12 @@ export async function apiResolver(
 
     const resolver = interopDefault(resolverModule)
     await resolver(req, res)
+
+    if (process.env.NODE_ENV !== 'production' && !isResSent(res)) {
+      console.warn(
+        `API resolved without sending a response for ${req.url}, this may result in a stalled requests`
+      )
+    }
   } catch (err) {
     if (err instanceof ApiError) {
       sendError(apiRes, err.statusCode, err.message)

test/integration/api-support/pages/api/no-parsing.js

@@ -5,14 +5,17 @@ export const config = {
 }
 
 export default (req, res) => {
-  if (!req.body) {
-    let buffer = ''
-    req.on('data', chunk => {
-      buffer += chunk
-    })
+  return new Promise(resolve => {
+    if (!req.body) {
+      let buffer = ''
+      req.on('data', chunk => {
+        buffer += chunk
+      })
 
-    req.on('end', () => {
-      res.status(200).json(JSON.parse(Buffer.from(buffer).toString()))
-    })
-  }
+      req.on('end', () => {
+        res.status(200).json(JSON.parse(Buffer.from(buffer).toString()))
+        resolve()
+      })
+    }
+  })
 }

test/integration/api-support/pages/api/test-no-end.js

@@ -0,0 +1,4 @@
+export default (req, res) => {
+  console.log('hi')
+  return {}
+}

test/integration/api-support/test/index.test.js

@@ -2,6 +2,7 @@
 /* global jasmine */
 import fs from 'fs-extra'
 import { join } from 'path'
+import AbortController from 'abort-controller'
 import {
   killApp,
   findPort,
@@ -17,6 +18,7 @@ jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 2
 const appDir = join(__dirname, '../')
 const nextConfig = join(appDir, 'next.config.js')
 let appPort
+let stderr
 let mode
 let app
 
@@ -332,6 +334,19 @@ function runTests(dev = false) {
         )
       ).toBeTruthy()
     })
+
+    it('should show warning when the API resolves without ending the request in dev mode', async () => {
+      const controller = new AbortController()
+      setTimeout(() => {
+        controller.abort()
+      }, 2000)
+      await fetchViaHTTP(appPort, '/api/test-no-end', undefined, {
+        signal: controller.signal,
+      }).catch(() => {})
+      expect(stderr).toContain(
+        `API resolved without sending a response for /api/test-no-end, this may result in a stalled requests`
+      )
+    })
   } else {
     it('should build api routes', async () => {
       const pagesManifest = JSON.parse(
@@ -360,8 +375,13 @@ function runTests(dev = false) {
 describe('API routes', () => {
   describe('dev support', () => {
     beforeAll(async () => {
+      stderr = ''
       appPort = await findPort()
-      app = await launchApp(appDir, appPort)
+      app = await launchApp(appDir, appPort, {
+        onStderr: msg => {
+          stderr += msg
+        },
+      })
     })
     afterAll(() => killApp(app))