feat: Implement UserRefreshClient#fetchIdToken (#1811)

* feat: Implement `UserRefreshClient#fetchIdToken`

* test: UserRefreshClient client for getIdTokenClient

* refactor: Use `target_audience` > `audience`

* refactor: Use `transporter`

Removes redundant calls

* test: Improve tests

src/auth/refreshclient.ts

@@ -13,12 +13,13 @@
 // limitations under the License.
 
 import * as stream from 'stream';
-import {JWTInput} from './credentials';
+import {CredentialRequest, JWTInput} from './credentials';
 import {
   GetTokenResponse,
   OAuth2Client,
   OAuth2ClientOptions,
 } from './oauth2client';
+import {stringify} from 'querystring';
 
 export const USER_REFRESH_ACCOUNT_TYPE = 'authorized_user';
 
@@ -78,6 +79,26 @@ export class UserRefreshClient extends OAuth2Client {
     return super.refreshTokenNoCache(this._refreshToken);
   }
 
+  async fetchIdToken(targetAudience: string): Promise<string> {
+    const res = await this.transporter.request<CredentialRequest>({
+      ...UserRefreshClient.RETRY_CONFIG,
+      url: this.endpoints.oauth2TokenUrl,
+      headers: {
+        'Content-Type': 'application/x-www-form-urlencoded',
+      },
+      method: 'POST',
+      data: stringify({
+        client_id: this._clientId,
+        client_secret: this._clientSecret,
+        grant_type: 'refresh_token',
+        refresh_token: this._refreshToken,
+        target_audience: targetAudience,
+      }),
+    });
+
+    return res.data.id_token!;
+  }
+
   /**
    * Create a UserRefreshClient credentials instance using the given input
    * options.

test/test.googleauth.ts

@@ -55,6 +55,7 @@ import {
 import {BaseExternalAccountClient} from '../src/auth/baseexternalclient';
 import {AuthClient, DEFAULT_UNIVERSE} from '../src/auth/authclient';
 import {ExternalAccountAuthorizedUserClient} from '../src/auth/externalAccountAuthorizedUserClient';
+import {stringify} from 'querystring';
 
 nock.disableNetConnect();
 
@@ -1520,37 +1521,80 @@ describe('googleauth', () => {
       assert(client.idTokenProvider instanceof JWT);
     });
 
-    it('should call getClient for getIdTokenClient', async () => {
+    it('should return a UserRefreshClient client for getIdTokenClient', async () => {
       // Set up a mock to return path to a valid credentials file.
       mockEnvVar(
         'GOOGLE_APPLICATION_CREDENTIALS',
-        './test/fixtures/private.json'
+        './test/fixtures/refresh.json'
       );
+      mockEnvVar('GOOGLE_CLOUD_PROJECT', 'some-project-id');
 
-      const spy = sinon.spy(auth, 'getClient');
       const client = await auth.getIdTokenClient('a-target-audience');
       assert(client instanceof IdTokenClient);
-      assert(spy.calledOnce);
+      assert(client.idTokenProvider instanceof UserRefreshClient);
     });
 
-    it('should fail when using UserRefreshClient', async () => {
+    it('should properly use `UserRefreshClient` client for `getIdTokenClient`', async () => {
       // Set up a mock to return path to a valid credentials file.
       mockEnvVar(
         'GOOGLE_APPLICATION_CREDENTIALS',
         './test/fixtures/refresh.json'
       );
       mockEnvVar('GOOGLE_CLOUD_PROJECT', 'some-project-id');
 
-      try {
-        await auth.getIdTokenClient('a-target-audience');
-      } catch (e) {
-        assert(e instanceof Error);
-        assert(
-          e.message.startsWith('Cannot fetch ID token in this environment')
-        );
-        return;
-      }
-      assert.fail('failed to throw');
+      // Assert `UserRefreshClient`
+      const baseClient = await auth.getClient();
+      assert(baseClient instanceof UserRefreshClient);
+
+      // Setup variables
+      const idTokenPayload = Buffer.from(JSON.stringify({exp: 100})).toString(
+        'base64'
+      );
+      const testIdToken = `TEST.${idTokenPayload}.TOKEN`;
+      const targetAudience = 'a-target-audience';
+      const tokenEndpoint = new URL(baseClient.endpoints.oauth2TokenUrl);
+      const expectedTokenRequestBody = stringify({
+        client_id: baseClient._clientId,
+        client_secret: baseClient._clientSecret,
+        grant_type: 'refresh_token',
+        refresh_token: baseClient._refreshToken,
+        target_audience: targetAudience,
+      });
+      const url = new URL('https://my-protected-endpoint.a.app');
+      const expectedRes = {hello: true};
+
+      // Setup mock endpoints
+      nock(tokenEndpoint.origin)
+        .post(tokenEndpoint.pathname, expectedTokenRequestBody)
+        .reply(200, {id_token: testIdToken});
+      nock(url.origin, {
+        reqheaders: {
+          authorization: `Bearer ${testIdToken}`,
+        },
+      })
+        .get(url.pathname)
+        .reply(200, expectedRes);
+
+      // Make assertions
+      const client = await auth.getIdTokenClient(targetAudience);
+      assert(client instanceof IdTokenClient);
+      assert(client.idTokenProvider instanceof UserRefreshClient);
+
+      const res = await client.request({url});
+      assert.deepStrictEqual(res.data, expectedRes);
+    });
+
+    it('should call getClient for getIdTokenClient', async () => {
+      // Set up a mock to return path to a valid credentials file.
+      mockEnvVar(
+        'GOOGLE_APPLICATION_CREDENTIALS',
+        './test/fixtures/private.json'
+      );
+
+      const spy = sinon.spy(auth, 'getClient');
+      const client = await auth.getIdTokenClient('a-target-audience');
+      assert(client instanceof IdTokenClient);
+      assert(spy.calledOnce);
     });
 
     describe('getUniverseDomain', () => {