fix(cli): hotswapping is slow for many resources deployed at once (#19081)

In the `LazyListStackResources` class,
we were incorrectly caching the results of the ListStackResources CloudFormation API call.
This resulted in executing redundant calls,
and very visibly impacted the performance of hotswapping when the Stack included many resources.

In manual testing, this change brought down deployment time of a large Stack from 80 to 7 seconds.

Closes #19021

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: skinny85

packages/aws-cdk/lib/api/evaluate-cloudformation-template.ts

@@ -7,30 +7,30 @@ export interface ListStackResources {
 }
 
 export class LazyListStackResources implements ListStackResources {
-  private stackResources: AWS.CloudFormation.StackResourceSummary[] | undefined;
+  private stackResources: Promise<AWS.CloudFormation.StackResourceSummary[]> | undefined;
 
   constructor(private readonly sdk: ISDK, private readonly stackName: string) {
   }
 
   public async listStackResources(): Promise<AWS.CloudFormation.StackResourceSummary[]> {
     if (this.stackResources === undefined) {
-      this.stackResources = await this.getStackResources();
+      this.stackResources = this.getStackResources(undefined);
     }
     return this.stackResources;
   }
 
-  private async getStackResources(): Promise<AWS.CloudFormation.StackResourceSummary[]> {
+  private async getStackResources(nextToken: string | undefined): Promise<AWS.CloudFormation.StackResourceSummary[]> {
     const ret = new Array<AWS.CloudFormation.StackResourceSummary>();
-    let nextToken: string | undefined;
-    do {
-      const stackResourcesResponse = await this.sdk.cloudFormation().listStackResources({
-        StackName: this.stackName,
-        NextToken: nextToken,
-      }).promise();
+    return this.sdk.cloudFormation().listStackResources({
+      StackName: this.stackName,
+      NextToken: nextToken,
+    }).promise().then(async stackResourcesResponse => {
       ret.push(...(stackResourcesResponse.StackResourceSummaries ?? []));
-      nextToken = stackResourcesResponse.NextToken;
-    } while (nextToken);
-    return ret;
+      if (stackResourcesResponse.NextToken) {
+        ret.push(...await this.getStackResources(stackResourcesResponse.NextToken));
+      }
+      return ret;
+    });
   }
 }
 

packages/aws-cdk/test/api/lazy-list-stack-resources.test.ts

@@ -0,0 +1,29 @@
+import * as AWS from 'aws-sdk';
+import { LazyListStackResources } from '../../lib/api/evaluate-cloudformation-template';
+import { MockSdk } from '../util/mock-sdk';
+
+describe('Lazy ListStackResources', () => {
+  test('correctly caches calls to the CloudFormation API', async () => {
+    // GIVEN
+    const listStackResMock: jest.Mock<AWS.CloudFormation.ListStackResourcesOutput, AWS.CloudFormation.ListStackResourcesInput[]> = jest.fn();
+    const mockSdk = new MockSdk();
+    mockSdk.stubCloudFormation({
+      listStackResources: listStackResMock,
+    });
+    listStackResMock.mockReturnValue({
+      StackResourceSummaries: [],
+      NextToken: undefined,
+    });
+    const res = new LazyListStackResources(mockSdk, 'StackName');
+
+    // WHEN
+    void res.listStackResources();
+    void res.listStackResources();
+    void res.listStackResources();
+    const result = await res.listStackResources();
+
+    // THEN
+    expect(result.length).toBe(0);
+    expect(listStackResMock).toHaveBeenCalledTimes(1);
+  });
+});