fix(mock): expose aborted on the mock controller

lib/mock/mock-utils.js

@@ -386,6 +386,12 @@ function dispatchMockReply (mockDispatches, mockDispatch, key, opts, handler, re
     paused: false,
     rawHeaders: null,
     rawTrailers: null,
+    // Mirrors the `aborted` property of a real Request, which fetch consults to
+    // decide whether an error on the current connection was caused by itself
+    // aborting the connection (e.g. during a redirect)
+    get aborted () {
+      return aborted
+    },
     pause () {
       this.paused = true
     },

test/fetch/issue-5963.js

@@ -0,0 +1,22 @@
+'use strict'
+
+const { test } = require('node:test')
+const { fetch, MockAgent } = require('../..')
+
+// https://github.com/nodejs/undici/issues/5963
+test('MockAgent follows a redirect', async (t) => {
+  const mockAgent = new MockAgent()
+  mockAgent.disableNetConnect()
+  t.after(() => mockAgent.close())
+
+  const mockPool = mockAgent.get('http://localhost')
+  mockPool.intercept({ path: '/redirect', method: 'GET' })
+    .reply(307, '', { headers: { location: 'http://localhost/final' } })
+  mockPool.intercept({ path: '/final', method: 'GET' }).reply(200, 'ok')
+
+  const response = await fetch('http://localhost/redirect', { dispatcher: mockAgent })
+
+  t.assert.strictEqual(response.status, 200)
+  t.assert.strictEqual(await response.text(), 'ok')
+  t.assert.ok(response.redirected)
+})