Fix Post login trigger according to recommendations in https://github…

….com/aws-amplify/amplify-cli/pull/7219\#issuecomment-881697658

web/amplify/backend/function/pettracking426fb16cPostConfirmation/src/index.js

@@ -1,14 +1,40 @@
-/*
-  this file will loop through all js modules which are uploaded to the lambda resource,
-  provided that the file names (without extension) are included in the "MODULES" env variable.
-  "MODULES" is a comma-delimmited string.
-*/
-const moduleNames = process.env.MODULES.split(',');
-const modules = moduleNames.map(name => require(`./${name}`));
+/**
+ * @fileoverview
+ *
+ * This CloudFormation Trigger creates a handler which awaits the other handlers
+ * specified in the `MODULES` env var, located at `./${MODULE}`.
+ */
 
-exports.handler = (event, context, callback) => {
-  for (let i = 0; i < modules.length; i += 1) {
-    const { handler } = modules[i];
-    handler(event, context, callback);
-  }
-};
+/**
+ * The names of modules to load are stored as a comma-delimited string in the
+ * `MODULES` env var.
+ */
+ const moduleNames = process.env.MODULES.split(',');
+ /**
+  * The array of imported modules.
+  */
+ const modules = moduleNames.map(name => require(`./${name}`));
+
+ /**
+  * This async handler iterates over the given modules and awaits them.
+  *
+  * @see https://docs.aws.amazon.com/lambda/latest/dg/nodejs-handler.html#nodejs-handler-async
+  *
+  * @param {object} event
+  *
+  * The event that triggered this Lambda.
+  *
+  * @returns
+  *
+  * The handler response.
+  */
+ exports.handler = async event => {
+   /**
+    * Instead of naively iterating over all handlers, run them concurrently with
+    * `await Promise.all(...)`. This would otherwise just be determined by the
+    * order of names in the `MODULES` var.
+    */
+   await Promise.all(modules.map(module => module.handler(event)));
+   return event;
+ };
+