Fix catch-all route + index.js in dev when accessed with trailing slash (#10502)

Failing test case for https://github.com/zeit/next.js/issues/10488#issuecomment-584500081

This used to give a 500 in dev environment

packages/next/next-server/server/next-server.ts

@@ -604,7 +604,7 @@ export default class Server {
           const handled = await this.handleApiRequest(
             req as NextApiRequest,
             res as NextApiResponse,
-            pathname!,
+            pathname,
             query
           )
           if (handled) {
@@ -701,7 +701,16 @@ export default class Server {
     // or else it won't be in the manifest yet
     await this.ensureApiPage(page)
 
-    const builtPagePath = await this.getPagePath(page)
+    let builtPagePath
+    try {
+      builtPagePath = await this.getPagePath(page)
+    } catch (err) {
+      if (err.code === 'ENOENT') {
+        return false
+      }
+      throw err
+    }
+
     const pageModule = require(builtPagePath)
     query = { ...query, ...params }
 

test/integration/api-catch-all/pages/api/users/[...slug].js

+export default function(req, res) {
+  res.json(req.query)
+}

test/integration/api-catch-all/pages/api/users/index.js

+export { default } from './[...slug]'

test/integration/api-catch-all/test/index.test.js

+/* eslint-env jest */
+import fs from 'fs-extra'
+import { join } from 'path'
+import {
+  killApp,
+  findPort,
+  launchApp,
+  fetchViaHTTP,
+  nextBuild,
+  nextStart,
+} from 'next-test-utils'
+
+jest.setTimeout(1000 * 60 * 2)
+
+const appDir = join(__dirname, '../')
+const nextConfig = join(appDir, 'next.config.js')
+let appPort
+let app
+
+function runTests() {
+  it('should return data when catch-all', async () => {
+    const data = await fetchViaHTTP(appPort, '/api/users/1', null, {}).then(
+      res => res.ok && res.json()
+    )
+
+    expect(data).toEqual({ slug: ['1'] })
+  })
+
+  it('should 404 when catch-all with index and trailing slash', async () => {
+    const data = await fetchViaHTTP(appPort, '/api/users/', null, {}).then(
+      res => res.status
+    )
+
+    expect(data).toEqual(404)
+  })
+
+  it('should return data when catch-all with index and no trailing slash', async () => {
+    const data = await fetchViaHTTP(appPort, '/api/users', null, {}).then(
+      res => res.ok && res.json()
+    )
+
+    expect(data).toEqual({})
+  })
+}
+
+describe('API routes', () => {
+  describe('dev support', () => {
+    beforeAll(async () => {
+      appPort = await findPort()
+      app = await launchApp(appDir, appPort)
+    })
+    afterAll(() => killApp(app))
+
+    runTests()
+  })
+
+  describe('Server support', () => {
+    beforeAll(async () => {
+      await nextBuild(appDir)
+      appPort = await findPort()
+      app = await nextStart(appDir, appPort)
+    })
+    afterAll(() => killApp(app))
+
+    runTests()
+  })
+
+  describe('Serverless support', () => {
+    beforeAll(async () => {
+      await fs.writeFile(
+        nextConfig,
+        `module.exports = { target: 'serverless' }`
+      )
+      await nextBuild(appDir)
+      appPort = await findPort()
+      app = await nextStart(appDir, appPort)
+    })
+    afterAll(async () => {
+      await killApp(app)
+      await fs.remove(nextConfig)
+    })
+
+    runTests()
+  })
+})