feat(amplify-alpha): support custom response headers in monorepo structures

packages/@aws-cdk/aws-amplify-alpha/lib/app.ts

@@ -519,6 +519,12 @@ export class CustomRule {
  * Custom response header of an Amplify App.
  */
 export interface CustomResponseHeader {
+  /**
+   * If the app uses a monorepo structure, the appRoot from the build spec to apply the custom headers to.
+   * @default - The appRoot is omitted in the custom headers output.
+   */
+  readonly appRoot?: string;
+
   /**
    * These custom headers will be applied to all URL file paths that match this pattern.
    */
@@ -531,16 +537,24 @@ export interface CustomResponseHeader {
 }
 
 function renderCustomResponseHeaders(customHeaders: CustomResponseHeader[]): string {
-  const yaml = [
-    'customHeaders:',
-  ];
+  const hasAppRoot = customHeaders[0].appRoot !== undefined;
+  const yaml = [hasAppRoot ? 'applications' : 'customHeaders:'];
 
   for (const customHeader of customHeaders) {
-    yaml.push(`  - pattern: "${customHeader.pattern}"`);
-    yaml.push('    headers:');
+    if ((customHeader.appRoot !== undefined) !== hasAppRoot) {
+      throw new Error('appRoot must be either be present or absent across all custom response headers');
+    }
+
+    const baseIndentation = ' '.repeat(customHeader.appRoot ? 6 : 2);
+    if (hasAppRoot) {
+      yaml.push(`  - appRoot: ${customHeader.appRoot}`);
+      yaml.push('    customHeaders:');
+    }
+    yaml.push(`${baseIndentation}- pattern: "${customHeader.pattern}"`);
+    yaml.push(`${baseIndentation}  headers:`);
     for (const [key, value] of Object.entries(customHeader.headers)) {
-      yaml.push(`      - key: "${key}"`);
-      yaml.push(`        value: "${value}"`);
+      yaml.push(`${baseIndentation}    - key: "${key}"`);
+      yaml.push(`${baseIndentation}      value: "${value}"`);
     }
   }
 

packages/@aws-cdk/aws-amplify-alpha/test/app.test.ts

@@ -443,6 +443,86 @@ test('with custom headers', () => {
   });
 });
 
+test('with custom headers in a monorepo structure', () => {
+  // WHEN
+  new amplify.App(stack, 'App', {
+    sourceCodeProvider: new amplify.GitHubSourceCodeProvider({
+      owner: 'aws',
+      repository: 'aws-cdk',
+      oauthToken: SecretValue.unsafePlainText('secret'),
+    }),
+    customResponseHeaders: [
+      {
+        appRoot: 'frontend',
+        pattern: '*.json',
+        headers: {
+          'custom-header-name-1': 'custom-header-value-1',
+          'custom-header-name-2': 'custom-header-value-2',
+        },
+      },
+      {
+        appRoot: 'backend',
+        pattern: '/path/*',
+        headers: {
+          'custom-header-name-1': 'custom-header-value-2',
+        },
+      },
+      {
+        appRoot: 'other',
+        pattern: '/with-tokens/*',
+        headers: {
+          'x-custom': `${'hello'.repeat(10)}${Stack.of(stack).urlSuffix} `,
+        },
+      },
+    ],
+  });
+
+  // THEN
+  Template.fromStack(stack).hasResourceProperties('AWS::Amplify::App', {
+    CustomHeaders: {
+      'Fn::Join': [
+        '',
+        [
+          'applications\n  - appRoot: frontend\n    customHeaders:\n      - pattern: \"*.json\"\n        headers:\n          - key: \"custom-header-name-1\"\n            value: \"custom-header-value-1\"\n          - key: \"custom-header-name-2\"\n            value: \"custom-header-value-2\"\n  - appRoot: backend\n    customHeaders:\n      - pattern: \"/path/*\"\n        headers:\n          - key: \"custom-header-name-1\"\n            value: \"custom-header-value-2\"\n  - appRoot: other\n    customHeaders:\n      - pattern: \"/with-tokens/*\"\n        headers:\n          - key: \"x-custom\"\n            value: \"hellohellohellohellohellohellohellohellohellohello',
+          {
+            Ref: 'AWS::URLSuffix',
+          },
+          ' "\n',
+        ],
+      ],
+    },
+  });
+});
+
+test('error with inconsistent appRoot in custom headers', () => {
+  // WHEN
+  expect(() => {
+    new amplify.App(stack, 'App', {
+      sourceCodeProvider: new amplify.GitHubSourceCodeProvider({
+        owner: 'aws',
+        repository: 'aws-cdk',
+        oauthToken: SecretValue.unsafePlainText('secret'),
+      }),
+      customResponseHeaders: [
+        {
+          pattern: '*.json',
+          headers: {
+            'custom-header-name-1': 'custom-header-value-1',
+            'custom-header-name-2': 'custom-header-value-2',
+          },
+        },
+        {
+          appRoot: 'backend',
+          pattern: '/path/*',
+          headers: {
+            'custom-header-name-1': 'custom-header-value-2',
+          },
+        },
+      ],
+    });
+  }).toThrow('appRoot must be either be present or absent across all custom response headers');
+});
+
 test('create a statically hosted app by default', () => {
   // WHEN
   new amplify.App(stack, 'App', {});