fix(toolkit): scrutiny dialog should fail with no tty (#1382)

If STDIN is not connected to a TTY (terminal), and scrutiny is enabled,
we expect the program to fail (exit with non-zero exit code).

This is especially important for CI/CD scenarios where you wouldn't want
to accidentally deploy changes that didn't pass a scrutiny check.

Added integration test.

Fixes #1380

packages/aws-cdk/bin/cdk.ts

@@ -306,6 +306,14 @@ async function initCommandLine() {
       if (requireApproval !== RequireApproval.Never) {
         const currentTemplate = await readCurrentTemplate(stack);
         if (printSecurityDiff(currentTemplate, stack, requireApproval)) {
+
+          // only talk to user if we STDIN is a terminal (otherwise, fail)
+          if (!process.stdin.isTTY) {
+            throw new Error(
+              '"--require-approval" is enabled and stack includes security-sensitive updates, ' +
+              'but terminal (TTY) is not attached so we are unable to get a confirmation from the user');
+          }
+
           const confirmed = await confirm(`Do you wish to deploy these changes (y/n)?`);
           if (!confirmed) { throw new Error('Aborted by user'); }
         }

packages/aws-cdk/integ-tests/common.bash

@@ -21,6 +21,7 @@ function cleanup_stack() {
 function cleanup() {
   cleanup_stack cdk-toolkit-integration-test-1
   cleanup_stack cdk-toolkit-integration-test-2
+  cleanup_stack cdk-toolkit-integration-iam-test
 }
 
 function setup() {

packages/aws-cdk/integ-tests/test-cdk-deploy-no-tty.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -euo pipefail
+scriptdir=$(cd $(dirname $0) && pwd)
+source ${scriptdir}/common.bash
+# ----------------------------------------------------------
+
+setup
+
+# redirect /dev/null to stdin, which means there will not be tty attached
+# since this stack includes security-related changes, the deployment should
+# immediately fail because we can't confirm the changes
+if cdk deploy cdk-toolkit-integration-iam-test < /dev/null; then
+    fail "test failed. we expect 'cdk deploy' to fail if there are security-related changes and no tty"
+fi
+
+echo "✅  success"