fix(cli): always exit with 0 on cdk diff

packages/aws-cdk/bin/cdk.ts

@@ -67,6 +67,7 @@ async function parseCommandLineArguments() {
       .option('context-lines', { type: 'number', desc: 'Number of context lines to include in arbitrary JSON diff rendering', default: 3, requiresArg: true })
       .option('template', { type: 'string', desc: 'The path to the CloudFormation template to compare with', requiresArg: true })
       .option('strict', { type: 'boolean', desc: 'Do not filter out AWS::CDK::Metadata resources', default: false }))
+      .option('fail', { type: 'boolean', desc: 'Fail with exit code 1 in case of diff', default: false })
     .command('metadata [STACK]', 'Returns all metadata associated with this stack')
     .command('init [TEMPLATE]', 'Create a new, empty CDK project from a template. Invoked without TEMPLATE, the app template will be used.', yargs => yargs
       .option('language', { type: 'string', alias: 'l', desc: 'The language to be used for the new project (default can be configured in ~/.cdk.json)', choices: initTemplateLanuages })
@@ -183,7 +184,8 @@ async function initCommandLine() {
           exclusively: args.exclusively,
           templatePath: args.template,
           strict: args.strict,
-          contextLines: args.contextLines
+          contextLines: args.contextLines,
+          fail: args.fail
         });
 
       case 'bootstrap':

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

@@ -51,7 +51,7 @@ export class CdkToolkit {
     const contextLines = options.contextLines || 3;
     const stream = options.stream || process.stderr;
 
-    let ret = 0;
+    let diffs = 0;
     if (options.templatePath !== undefined) {
       // Compare single stack against fixed template
       if (stacks.length !== 1) {
@@ -62,19 +62,17 @@ export class CdkToolkit {
         throw new Error(`There is no file at ${options.templatePath}`);
       }
       const template = deserializeStructure(await fs.readFile(options.templatePath, { encoding: 'UTF-8' }));
-      ret = printStackDiff(template, stacks[0], strict, contextLines, options.stream);
+      diffs = printStackDiff(template, stacks[0], strict, contextLines, stream);
     } else {
       // Compare N stacks against deployed templates
       for (const stack of stacks) {
         stream.write(format('Stack %s\n', colors.bold(stack.name)));
         const currentTemplate = await this.provisioner.readCurrentTemplate(stack);
-        if (printStackDiff(currentTemplate, stack, !!options.strict, options.contextLines || 3, stream) !== 0) {
-          ret = 1;
-        }
+        diffs = printStackDiff(currentTemplate, stack, strict, contextLines, stream);
       }
     }
 
-    return ret;
+    return diffs && options.fail ? 1 : 0;
   }
 
   public async deploy(options: DeployOptions) {
@@ -247,6 +245,13 @@ export interface DiffOptions {
    * @default stderr
    */
   stream?: NodeJS.WritableStream;
+
+  /**
+   * Whether to fail with exit code 1 in case of diff
+   *
+   * @default false
+   */
+  fail?: boolean;
 }
 
 export interface DeployOptions {

packages/aws-cdk/test/integ/cli/test-cdk-diff.sh

@@ -6,11 +6,14 @@ source ${scriptdir}/common.bash
 
 setup
 
-function cdk_diff() {
-    cdk diff $1 2>&1 || true
-}
+cdk diff ${STACK_NAME_PREFIX}-test-1 2>&1 | grep "AWS::SNS::Topic"
+cdk diff ${STACK_NAME_PREFIX}-test-2 2>&1 | grep "AWS::SNS::Topic"
 
-cdk_diff ${STACK_NAME_PREFIX}-test-1 | grep "AWS::SNS::Topic"
-cdk_diff ${STACK_NAME_PREFIX}-test-2 | grep "AWS::SNS::Topic"
+fail=0
+cdk diff --fail ${STACK_NAME_PREFIX}-test-1 2>&1 || fail=1
+
+if [ $fail -ne 1 ]; then
+  fail 'cdk diff with --fail does not fail'
+fi
 
 echo "✅  success"

packages/aws-cdk/test/integ/cli/test-cdk-iam-diff.sh

@@ -7,7 +7,7 @@ source ${scriptdir}/common.bash
 setup
 
 function nonfailing_diff() {
-    ( cdk diff $1 2>&1 || true ) | strip_color_codes
+    cdk diff $1 2>&1 | strip_color_codes
 }
 
 assert "nonfailing_diff ${STACK_NAME_PREFIX}-iam-test" <<HERE

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

@@ -53,6 +53,32 @@ export = {
     test.ok(plainTextOutput.indexOf('Stack A') > -1, `Did not contain "Stack A": ${plainTextOutput}`);
     test.ok(plainTextOutput.indexOf('Stack B') > -1, `Did not contain "Stack B": ${plainTextOutput}`);
 
+    test.equals(0, exitCode);
+
+    test.done();
+  },
+
+  async 'exits with 1 with diffs and fail set to true'(test: Test) {
+    // GIVEN
+    const provisioner: IDeploymentTarget = {
+      async readCurrentTemplate(_stack: cxapi.CloudFormationStackArtifact): Promise<Template> {
+        return {};
+      },
+      async deployStack(_options: DeployStackOptions): Promise<DeployStackResult> {
+        return { noOp: true, outputs: {}, stackArn: ''};
+      }
+    };
+    const toolkit = new CdkToolkit({ appStacks, provisioner });
+    const buffer = new StringWritable();
+
+    // WHEN
+    const exitCode = await toolkit.diff({
+      stackNames: ['A'],
+      stream: buffer,
+      fail: true
+    });
+
+    // THEN
     test.equals(1, exitCode);
 
     test.done();