fix(cli): diff against multiple stacks do not always fail if any have…

… a diff (aws#7690)

### Commit Message
Currently, the diff command only remembers if there are any diffs
with the last stack that was diffed. So, if all of your stacks have
a diff except for the last one, the diff command would exit
successfully even if told to fail on diff.

This fix corrects the behavior so that if any of the stacks have
a diff, then the exit code would be non-zero if configured to do so.

fixes aws#7492
### End Commit Message

----

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

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

@@ -101,7 +101,7 @@ export class CdkToolkit {
       for (const stack of stacks.stackArtifacts) {
         stream.write(format('Stack %s\n', colors.bold(stack.displayName)));
         const currentTemplate = await this.props.cloudFormation.readCurrentTemplate(stack);
-        diffs = printStackDiff(currentTemplate, stack, strict, contextLines, stream);
+        diffs += printStackDiff(currentTemplate, stack, strict, contextLines, stream);
       }
     }
 

packages/aws-cdk/test/diff.test.ts

@@ -1,6 +1,7 @@
 import { Writable } from 'stream';
 import { NodeStringDecoder, StringDecoder  } from 'string_decoder';
 import * as cxschema from '@aws-cdk/cloud-assembly-schema';
+import { CloudFormationStackArtifact } from '@aws-cdk/cx-api';
 import { CloudFormationDeployments } from '../lib/api/cloudformation-deployments';
 import { CdkToolkit } from '../lib/cdk-toolkit';
 import { classMockOf, MockCloudExecutable } from './util';
@@ -31,6 +32,10 @@ beforeEach(() => {
           },
         ],
       },
+    },
+    {
+      stackName: 'D',
+      template: { resource: 'D' },
     }],
   });
 
@@ -44,7 +49,12 @@ beforeEach(() => {
   });
 
   // Default implementations
-  cloudFormation.readCurrentTemplate.mockResolvedValue({});
+  cloudFormation.readCurrentTemplate.mockImplementation((stackArtifact: CloudFormationStackArtifact) => {
+    if (stackArtifact.stackName === 'D') {
+      return Promise.resolve({ resource: 'D' });
+    }
+    return Promise.resolve({});
+  });
   cloudFormation.deployStack.mockImplementation((options) => Promise.resolve({
     noOp: true,
     outputs: {},
@@ -86,6 +96,21 @@ test('exits with 1 with diffs and fail set to true', async () => {
   expect(exitCode).toBe(1);
 });
 
+test('exits with 1 with diff in first stack, but not in second stack and fail set to true', async () => {
+  // GIVEN
+  const buffer = new StringWritable();
+
+  // WHEN
+  const exitCode = await toolkit.diff({
+    stackNames: ['A', 'D'],
+    stream: buffer,
+    fail: true,
+  });
+
+  // THEN
+  expect(exitCode).toBe(1);
+});
+
 test('throws an error during diffs on stack with error metadata', async () => {
   const buffer = new StringWritable();
 

packages/aws-cdk/test/integ/cli/cli.integtest.ts

@@ -362,6 +362,32 @@ integTest('cdk diff', async () => {
     .rejects.toThrow('exited with error');
 });
 
+integTest('cdk diff --fail on multiple stacks exits with error if any of the stacks contains a diff', async () => {
+  // GIVEN
+  const diff1 = await cdk(['diff', fullStackName('test-1')]);
+  expect(diff1).toContain('AWS::SNS::Topic');
+
+  await cdkDeploy('test-2');
+  const diff2 = await cdk(['diff', fullStackName('test-2')]);
+  expect(diff2).toContain('There were no differences');
+
+  // WHEN / THEN
+  await expect(cdk(['diff', '--fail', fullStackName('test-1'), fullStackName('test-2')])).rejects.toThrow('exited with error');
+});
+
+integTest('cdk diff --fail with multiple stack exits with if any of the stacks contains a diff', async () => {
+  // GIVEN
+  await cdkDeploy('test-1');
+  const diff1 = await cdk(['diff', fullStackName('test-1')]);
+  expect(diff1).toContain('There were no differences');
+
+  const diff2 = await cdk(['diff', fullStackName('test-2')]);
+  expect(diff2).toContain('AWS::SNS::Topic');
+
+  // WHEN / THEN
+  await expect(cdk(['diff', '--fail', fullStackName('test-1'), fullStackName('test-2')])).rejects.toThrow('exited with error');
+});
+
 integTest('deploy stack with docker asset', async () => {
   await cdkDeploy('docker');
 });