fix: handle compression for custom-server render calls (#16378) (#18891)

Co-authored-by: Radosław Grochowski <radoslaw.grochowski@grupawp.pl>
Co-authored-by: Tim Neutkens <timneutkens@me.com>

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

@@ -1315,6 +1315,11 @@ export default class Server {
       return this.handleRequest(req, res, parsedUrl)
     }
 
+    // Custom server users can run `app.render()` which needs compression.
+    if (this.renderOpts.customServer) {
+      this.handleCompression(req, res)
+    }
+
     if (isBlockedPage(pathname)) {
       return this.render404(req, res, parsedUrl)
     }

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

@@ -183,4 +183,17 @@ describe('Custom Server', () => {
       expect(stderr).toContain('Cannot render page with path "dashboard"')
     })
   })
+
+  describe('compression handling', function () {
+    beforeAll(() => startServer())
+    afterAll(() => killApp(server))
+
+    it.each(['/', '/no-query'])(
+      'should handle compression for route %s',
+      async (route) => {
+        const response = await fetchViaHTTP(appPort, route)
+        expect(response.headers.get('Content-Encoding')).toBe('gzip')
+      }
+    )
+  })
 })