fix(cli): "cdk flags" message writes to stderr unconditionally (#790)

This ignores our rules for avoiding writing to `stderr` in CI
environments. Use the IoHost instead.

Also update the message to include the use of the unstable flag.

---
By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license

Author: rix0rrr

packages/aws-cdk/lib/cli/cdk-toolkit.ts

@@ -1065,7 +1065,7 @@ export class CdkToolkit {
         await printSerializedObject(this.ioHost.asIoHelper(), obscureTemplate(stacks.firstStack.template), json ?? false);
       }
 
-      await displayFlagsMessage(this.toolkit, this.props.cloudExecutable);
+      await displayFlagsMessage(this.ioHost.asIoHelper(), this.toolkit, this.props.cloudExecutable);
       return undefined;
     }
 
@@ -1075,7 +1075,7 @@ export class CdkToolkit {
       `Supply a stack id (${stacks.stackArtifacts.map((s) => chalk.green(s.hierarchicalId)).join(', ')}) to display its template.`,
     );
 
-    await displayFlagsMessage(this.toolkit, this.props.cloudExecutable);
+    await displayFlagsMessage(this.ioHost.asIoHelper(), this.toolkit, this.props.cloudExecutable);
     return undefined;
   }
 
@@ -2112,13 +2112,14 @@ async function askUserConfirmation(
     }
   });
 }
-export async function displayFlagsMessage(toolkit: InternalToolkit, cloudExecutable: CloudExecutable): Promise<void> {
+
+export async function displayFlagsMessage(ioHost: IoHelper, toolkit: InternalToolkit, cloudExecutable: CloudExecutable): Promise<void> {
   let numUnconfigured = (await toolkit.flags(cloudExecutable))
     .filter(flag => !OBSOLETE_FLAGS.includes(flag.name))
     .filter(flag => flag.userValue === undefined).length;
 
   if (numUnconfigured > 0) {
-    process.stderr.write(`You currently have ${numUnconfigured} unconfigured feature flag(s) that may require attention to keep your application up-to-date. Run 'cdk flags' to learn more.`);
+    await ioHost.defaults.warn(`${numUnconfigured} feature flags are not configured. Run 'cdk --unstable=flags flags' to learn more.`);
   }
 }
 

packages/aws-cdk/test/cli/display-flags-message.test.ts

@@ -1,20 +1,21 @@
 import { Toolkit } from '@aws-cdk/toolkit-lib';
 import { displayFlagsMessage } from '../../lib/cli/cdk-toolkit';
+import { TestIoHost } from '../_helpers/io-host';
 
 describe('displayFlagsMessage', () => {
   let mockToolkit: jest.Mocked<Toolkit>;
   let mockCloudExecutable: any;
-  let stderrWriteSpy: jest.SpyInstance;
+  let ioHost: TestIoHost;
 
   beforeEach(() => {
     mockCloudExecutable = {};
+    ioHost = new TestIoHost();
 
     mockToolkit = {
       flags: jest.fn(),
     } as any;
 
     jest.spyOn(Toolkit.prototype, 'flags').mockImplementation(mockToolkit.flags);
-    stderrWriteSpy = jest.spyOn(process.stderr, 'write').mockImplementation(() => true);
   });
 
   afterEach(() => {
@@ -48,12 +49,12 @@ describe('displayFlagsMessage', () => {
 
     mockToolkit.flags.mockResolvedValue(mockFlagsData);
 
-    await displayFlagsMessage(mockToolkit as any, mockCloudExecutable);
+    await displayFlagsMessage(ioHost.asHelper(), mockToolkit as any, mockCloudExecutable);
 
     expect(mockToolkit.flags).toHaveBeenCalledWith(mockCloudExecutable);
-    expect(stderrWriteSpy).toHaveBeenCalledWith(
-      'You currently have 1 unconfigured feature flag(s) that may require attention to keep your application up-to-date. Run \'cdk flags\' to learn more.',
-    );
+    expect(ioHost.notifySpy).toHaveBeenCalledWith(expect.objectContaining({
+      message: expect.stringContaining('1 feature flags are not configured'),
+    }));
   });
   test('does not display a message when user has no unconfigured flags', async () => {
     const mockFlagsData = [
@@ -67,10 +68,10 @@ describe('displayFlagsMessage', () => {
     ];
     mockToolkit.flags.mockResolvedValue(mockFlagsData);
 
-    await displayFlagsMessage(mockToolkit as any, mockCloudExecutable);
+    await displayFlagsMessage(ioHost.asHelper(), mockToolkit as any, mockCloudExecutable);
 
     expect(mockToolkit.flags).toHaveBeenCalledWith(mockCloudExecutable);
-    expect(stderrWriteSpy).not.toHaveBeenCalled();
+    expect(ioHost.notifySpy).not.toHaveBeenCalled();
   });
 });