fix(toolkit): 'cdk deploy' updates to Outputs only

If only stack Outputs are changed, CloudFormation generates a ChangeSet
that is executable but has 0 changes.

Before, we looked at the amount of changes to say there was nothing to
do, but now we look at the actual change set status to determine
whether it's an empty change set or not.

The effect is that we can now deploy updates even if only Outputs
changed. This becomes very important when the only thing changed
to a stack is an Output got added because a cross-stack reference
was taken by a downstream stack.

Fixes #778.

packages/aws-cdk/lib/api/deploy-stack.ts

@@ -7,7 +7,7 @@ import { debug, error, print } from '../logging';
 import { toYAML } from '../serialize';
 import { Mode } from './aws-auth/credentials';
 import { ToolkitInfo } from './toolkit-info';
-import { describeStack, stackExists, stackFailedCreating, waitForChangeSet, waitForStack } from './util/cloudformation';
+import { changeSetHasNoChanges, describeStack, stackExists, stackFailedCreating, waitForChangeSet, waitForStack  } from './util/cloudformation';
 import { StackActivityMonitor } from './util/cloudformation/stack-activity-monitor';
 import { StackStatus } from './util/cloudformation/stack-status';
 import { SDK } from './util/sdk';
@@ -77,16 +77,17 @@ export async function deployStack(options: DeployStackOptions): Promise<DeploySt
   }).promise();
   debug('Initiated creation of changeset: %s; waiting for it to finish creating...', changeSet.Id);
   const changeSetDescription = await waitForChangeSet(cfn, deployName, changeSetName);
-  if (!changeSetDescription || !changeSetDescription.Changes || changeSetDescription.Changes.length === 0) {
-    debug('No changes are to be performed on %s, assuming success.', deployName);
+
+  if (changeSetHasNoChanges(changeSetDescription)) {
+    debug('No changes are to be performed on %s.', deployName);
     await cfn.deleteChangeSet({ StackName: deployName, ChangeSetName: changeSetName }).promise();
     return { noOp: true, outputs: await getStackOutputs(cfn, deployName), stackArn: changeSet.StackId! };
   }
 
   debug('Initiating execution of changeset %s on stack %s', changeSetName, deployName);
   await cfn.executeChangeSet({ StackName: deployName, ChangeSetName: changeSetName }).promise();
   // tslint:disable-next-line:max-line-length
-  const monitor = options.quiet ? undefined : new StackActivityMonitor(cfn, deployName, options.stack, changeSetDescription.Changes.length).start();
+  const monitor = options.quiet ? undefined : new StackActivityMonitor(cfn, deployName, options.stack, (changeSetDescription.Changes || []).length).start();
   debug('Execution of changeset %s on stack %s has started; waiting for the update to complete...', changeSetName, deployName);
   await waitForStack(cfn, deployName);
   if (monitor) { await monitor.stop(); }

packages/aws-cdk/lib/api/util/cloudformation.ts

@@ -86,32 +86,53 @@ async function waitFor<T>(valueProvider: () => Promise<T | null | undefined>, ti
 /**
  * Waits for a ChangeSet to be available for triggering a StackUpdate.
  *
+ * Will return a changeset that is either ready to be executed or has no changes.
+ * Will throw in other cases.
+ *
  * @param cfn       a CloudFormation client
  * @param stackName   the name of the Stack that the ChangeSet belongs to
  * @param changeSetName the name of the ChangeSet
  *
  * @returns       the CloudFormation description of the ChangeSet
  */
 // tslint:disable-next-line:max-line-length
-export async function waitForChangeSet(cfn: CloudFormation, stackName: string, changeSetName: string): Promise<CloudFormation.DescribeChangeSetOutput | undefined> {
+export async function waitForChangeSet(cfn: CloudFormation, stackName: string, changeSetName: string): Promise<CloudFormation.DescribeChangeSetOutput> {
   debug('Waiting for changeset %s on stack %s to finish creating...', changeSetName, stackName);
-  return waitFor(async () => {
+  const ret = await waitFor(async () => {
     const description = await describeChangeSet(cfn, stackName, changeSetName);
     // The following doesn't use a switch because tsc will not allow fall-through, UNLESS it is allows
     // EVERYWHERE that uses this library directly or indirectly, which is undesirable.
     if (description.Status === 'CREATE_PENDING' || description.Status === 'CREATE_IN_PROGRESS') {
       debug('Changeset %s on stack %s is still creating', changeSetName, stackName);
       return undefined;
-    } else if (description.Status === 'CREATE_COMPLETE') {
+    }
+
+    if (description.Status === 'CREATE_COMPLETE' || changeSetHasNoChanges(description)) {
       return description;
-    } else if (description.Status === 'FAILED') {
-      if (description.StatusReason && description.StatusReason.startsWith('The submitted information didn\'t contain changes.')) {
-        return description;
-      }
     }
+
     // tslint:disable-next-line:max-line-length
     throw new Error(`Failed to create ChangeSet ${changeSetName} on ${stackName}: ${description.Status || 'NO_STATUS'}, ${description.StatusReason || 'no reason provided'}`);
   });
+
+  if (!ret) {
+    throw new Error('Change set took too long to be created; aborting');
+  }
+
+  return ret;
+}
+
+/**
+ * Return true if the given change set has no changes
+ *
+ * This must be determined from the status, not the 'Changes' array on the
+ * object; the latter can be empty because no resources were changed, but if
+ * there are changes to Outputs, the change set can still be executed.
+ */
+export function changeSetHasNoChanges(description: CloudFormation.DescribeChangeSetOutput) {
+    return description.Status === 'FAILED'
+    && description.StatusReason
+    && description.StatusReason.startsWith('The submitted information didn\'t contain changes.');
 }
 
 /**