fix: add-to-group PostConfirmation Lambda should not throw

/**
 * For some reason, naively returning a value / Promise or throwing an error
 * will not work as it is specified in the Lambda docs:
 * @see https://docs.aws.amazon.com/lambda/latest/dg/nodejs-handler.html
 *
 * This causes an "Invalid JSON" error which can, so far, only be worked
 * around by resolving with deprecated non-async `callback`:
 * @see aws-amplify#2735
 * @see aws-amplify#4341
 * @see aws-amplify#7179
 */

packages/amplify-category-auth/provider-utils/awscloudformation/triggers/PostConfirmation/add-to-group.js

@@ -1,6 +1,6 @@
 /* eslint-disable-line */ const aws = require('aws-sdk');
 
-exports.handler = async (event, context) => {
+exports.handler = async (event, context, callback) => {
   const cognitoidentityserviceprovider = new aws.CognitoIdentityServiceProvider({ apiVersion: '2016-04-18' });
   const groupParams = {
     GroupName: process.env.GROUP,
@@ -19,16 +19,21 @@ exports.handler = async (event, context) => {
     await cognitoidentityserviceprovider.createGroup(groupParams).promise();
   }
 
+  /**
+   * For some reason, naively returning a value / Promise or throwing an error
+   * will not work as it is specified in the Lambda docs:
+   * @see https://docs.aws.amazon.com/lambda/latest/dg/nodejs-handler.html
+   *
+   * This causes an "Invalid JSON" error which can, so far, only be worked
+   * around by resolving with deprecated non-async `callback`:
+   * @see https://github.com/aws-amplify/amplify-cli/issues/2735
+   * @see https://github.com/aws-amplify/amplify-cli/issues/4341
+   * @see https://github.com/aws-amplify/amplify-cli/issues/7179
+   */
   try {
     await cognitoidentityserviceprovider.adminAddUserToGroup(addUserParams).promise();
-    return {
-      statusCode: 200,
-      body: JSON.stringify(event)
-    }
+    callback(null, event);
   } catch (error) {
-    return {
-      statusCode: 500,
-      body: JSON.stringify(error)
-    }
+    callback(new Error(error));
   }
 };