fix(instr-express): keep hidden properties in layer handlers

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

@@ -152,8 +152,7 @@
       ) {
         const route = original.apply(this, args);
         const layer = this._router.stack[this._router.stack.length - 1];
-        instrumentation._applyPatch.call(
-          instrumentation,
+        instrumentation._applyPatch(
           layer,
           getLayerPath(args)
         );
@@ -176,7 +175,8 @@
     this._wrap(layer, 'handle', (original: Function) => {
       // TODO: instrument error handlers
       if (original.length === 4) return original;
-      return function (
+
+      const patched = function (
         this: ExpressLayer,
         req: PatchedRequest,
         res: express.Response
@@ -313,6 +313,25 @@
           }
         }
       };
+
+      // `handle` isn't just a regular function in some cases. It also contains
+      // some properties holding metadata and state so we need to proxy them
+      // through through patched function
+      // ref: https://github.com/open-telemetry/opentelemetry-js-contrib/issues/1950
+      Object.keys(original).forEach(key => {
+        Object.defineProperty(patched, key, {
+          get() {
+            // @ts-expect-error -- the original function has hidden props indexed by strings
+            return original[key];
+          },
+          set(value) {
+            // @ts-expect-error -- the original function has hidden props indexed by strings
+            original[key] = value;
+          },
+        });
+      });
+
+      return patched;
     });
   }
 

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

@@ -488,6 +488,35 @@ describe('ExpressInstrumentation', () => {
         }
       );
     });
+
+    it('should keep stack in the router layer handle', async () => {
+      const rootSpan = tracer.startSpan('rootSpan');
+      let routerLayer: { name: string; handle: { stack: any[] } };
+      const httpServer = await serverWithMiddleware(tracer, rootSpan, app => {
+        app.use(express.json());
+        app.get('/bare_route', (req, res) => {
+          const stack = req.app._router.stack as any[];
+          routerLayer = stack.find(layer => layer.name === 'router');
+          return res.status(200).end('test');
+        });
+      });
+      server = httpServer.server;
+      port = httpServer.port;
+      await context.with(
+        trace.setSpan(context.active(), rootSpan),
+        async () => {
+          const response = await httpRequest.get(
+            `http://localhost:${port}/bare_route`
+          );
+          assert.strictEqual(response, 'test');
+          rootSpan.end();
+          assert.ok(
+            routerLayer?.handle?.stack?.length === 1,
+            'router layer stack is accessible'
+          );
+        }
+      );
+    });
   });
 
   describe('Disabling plugin', () => {
@@ -527,6 +556,7 @@ describe('ExpressInstrumentation', () => {
       );
     });
   });
+
   it('should work with ESM usage', async () => {
     await testUtils.runTestFixture({
       cwd: __dirname,