Fix HMR failing when rendering with `/index` (#6541)

Fixes: #6497 

packages/next-server/server/render.tsx

@@ -178,6 +178,7 @@ export async function renderToHTML(
   query: ParsedUrlQuery,
   renderOpts: RenderOpts,
 ): Promise<string | null> {
+  pathname = pathname === '/index' ? '/' : pathname
   const {
     err,
     dev = false,

packages/next/server/next-dev-server.js

@@ -108,7 +108,6 @@ export default class DevServer extends Server {
       }
       if (!this.quiet) console.error(err)
     }
-
     return super.renderToHTML(req, res, pathname, query, options)
   }
 

test/integration/custom-server/server.js

@@ -24,6 +24,10 @@ app.prepare().then(() => {
       app.setAssetPrefix('')
     }
 
+    if (/test-index-hmr/.test(req.url)) {
+      return app.render(req, res, '/index')
+    }
+
     handleNextRequests(req, res)
   })
 

test/integration/custom-server/test/index.test.js

@@ -3,14 +3,19 @@
 import { join } from 'path'
 import getPort from 'get-port'
 import clone from 'clone'
+import webdriver from 'next-webdriver'
 import {
   initNextServerScript,
   killApp,
   renderViaHTTP,
-  fetchViaHTTP
+  fetchViaHTTP,
+  check,
+  File
 } from 'next-test-utils'
 
 const appDir = join(__dirname, '../')
+const indexPg = new File(join(appDir, 'pages/index.js'))
+
 let appPort
 let server
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 2
@@ -84,4 +89,32 @@ describe('Custom Server', () => {
       expect(response.headers.get('etag')).toBeNull()
     })
   })
+
+  describe('HMR with custom server', () => {
+    beforeAll(() => startServer())
+    afterAll(() => {
+      killApp(server)
+      indexPg.restore()
+    })
+
+    it('Should support HMR when rendering with /index pathname', async () => {
+      let browser
+      try {
+        browser = await webdriver(context.appPort, '/test-index-hmr')
+        const text = await browser.elementByCss('#go-asset').text()
+        expect(text).toBe('Asset')
+
+        indexPg.replace('Asset', 'Asset!!')
+
+        await check(
+          () => browser.elementByCss('#go-asset').text(),
+          /Asset!!/
+        )
+      } finally {
+        if (browser) {
+          browser.close()
+        }
+      }
+    })
+  })
 })