fix(plugin-express): fix double span end open-telemetry#908

packages/opentelemetry-plugin-express/src/express.ts

@@ -221,19 +221,21 @@ export class ExpressPlugin extends BasePlugin<typeof express> {
          *      - however it will not represent the actual time that the span took
          *      - we would need to hook into the http response's `end` event.
          *
-         * The fix for case 4) is to hook into the `end` event, but it would apply
-         * to case 2) and 3), which would cost quite a lot of memory for big express apps.
-         *
-         * The workaround is to schedule a callback to run on next tick and check if the
-         * callback has been called. If not we close the span because we assume being
-         * in the case 2) or 4).
+         * The workaround is to schedule a callback to run on next tick which
+         * would ensure that the layer was async, and then add a listener on the
+         * 'end' event to end the span.
          */
         const result = original.apply(this, arguments);
         setImmediate(() => {
           if (callbackHasBeenCalled === false) {
-            span.end();
+            res.once('end', function() {
+              // its possible that the layer was async but called its callback
+              // between the setImmediate and the response was sent to the client
+              if (callbackHasBeenCalled === true) return;
+              span.end();
+            });
           }
-        });
+        }).unref();
         return result;
       };
     });