fix: support addRequest from custom http agents

src/interceptors/ClientRequest/agents.ts

@@ -10,6 +10,7 @@ import {
 declare module 'node:http' {
   interface Agent {
     createConnection(options: any, callback: any): net.Socket
+    addRequest(request: http.ClientRequest, options: http.RequestOptions): void
   }
 }
 
@@ -50,6 +51,18 @@ export class MockAgent extends http.Agent {
 
     return socket
   }
+
+  addRequest(request: http.ClientRequest, options: http.RequestOptions) {
+    // First, execute the base class method.
+    super.addRequest.apply(this, [request, options])
+
+    // If there's a custom HTTP agent, call its `addRequest` method.
+    // This way, if the agent has side effects that affect the request,
+    // those will be applied to the intercepted request instance as well.
+    if (this.customAgent instanceof http.Agent) {
+      this.customAgent.addRequest(request, options)
+    }
+  }
 }
 
 export class MockHttpsAgent extends https.Agent {

test/modules/http/regressions/http-agent-add-request.test.ts

@@ -0,0 +1,56 @@
+/**
+ * @see https://github.com/mswjs/msw/issues/2338
+ */
+// @vitest-environment node
+import http from 'node:http'
+import { it, expect, beforeAll, afterEach, afterAll } from 'vitest'
+import { ClientRequestInterceptor } from '../../../../src/interceptors/ClientRequest'
+import { waitForClientRequest } from '../../../helpers'
+import { DeferredPromise } from '@open-draft/deferred-promise'
+
+const interceptor = new ClientRequestInterceptor()
+
+beforeAll(() => {
+  interceptor.apply()
+})
+
+afterEach(() => {
+  interceptor.removeAllListeners()
+})
+
+afterAll(() => {
+  interceptor.dispose()
+})
+
+/**
+ * A custom HTTP agent that adds a "cookie" header to
+ * any outgoing request.
+ */
+class CustomAgent extends http.Agent {
+  addRequest(request: http.ClientRequest) {
+    request.setHeader('cookie', 'key=value')
+  }
+}
+
+it('respects a custom "addRequest" method on the http agent', async () => {
+  const interceptedRequestPromise = new DeferredPromise<Request>()
+
+  interceptor.on('request', ({ request, controller }) => {
+    interceptedRequestPromise.resolve(request)
+    controller.respondWith(new Response())
+  })
+
+  const request = http.get('http://localhost/resource', {
+    agent: new CustomAgent(),
+  })
+  await waitForClientRequest(request)
+
+  const interceptedRequest = await interceptedRequestPromise
+
+  // Must have the cookie header set by the custom agent.
+  expect(Object.fromEntries(interceptedRequest.headers)).toEqual(
+    expect.objectContaining({
+      cookie: 'key=value',
+    })
+  )
+})

test/modules/http/regressions/http-concurrent-different-response-source.test.ts

@@ -1,6 +1,4 @@
-/**
- * @vitest-environment node
- */
+// @vitest-environment node
 import { it, expect, beforeAll, afterEach, afterAll } from 'vitest'
 import { HttpServer } from '@open-draft/test-server/http'
 import { ClientRequestInterceptor } from '../../../../src/interceptors/ClientRequest'