fix(express): listen for finish event on response for async express…

… layer #107

plugins/node/opentelemetry-plugin-express/src/express.ts

@@ -218,9 +218,15 @@ export class ExpressPlugin extends BasePlugin<typeof express> {
           };
         }
         const result = original.apply(this, arguments);
-        // If the callback is never called, we need to close the span.
-        setImmediate(() => {
+        /**
+         * As this point if the callback wasn't called, that means either the
+         * layer is asynchronous (so it will call the callback later on) or that
+         * the layer directly end the http response, so we'll hook into the "finish"
+         * event to handle the later case.
+         */
+        req.res?.once('finish', () => {
           if (spanHasEnded === false) {
+            spanHasEnded = true;
             span.end(startTime);
           }
         });

plugins/node/opentelemetry-plugin-express/test/express.test.ts

@@ -90,8 +90,10 @@ describe('Express Plugin', () => {
       });
       const router = express.Router();
       app.use('/toto', router);
-      router.get('/:id', (req, res, next) => {
-        return res.status(200).end();
+      router.get('/:id', (req, res) => {
+        setTimeout(() => {
+          return res.status(200).end();
+        }, 5);
       });
       const server = http.createServer(app);
       await new Promise(resolve => server.listen(0, resolve));