fix(cli): hotswap deploy fails on multiple CfnEvaluationException (#22339)

closes #22323 

To avoid unhandled rejections, we run promises just before we call `Promise.all`.

The concern of this fix is that hotswap process may take longer time because now async tasks run lazily. However I don't think it will be a big problem since those tasks are not I/O bound, so most of them are already running sequentially, not in parallel.

----

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### Adding new Unconventional Dependencies:

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### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: tmokmss

packages/aws-cdk/lib/api/hotswap-deployments.ts

@@ -76,7 +76,7 @@ async function findAllHotswappableChanges(
   const resourceDifferences = getStackResourceDifferences(stackChanges);
 
   let foundNonHotswappableChange = false;
-  const promises: Array<Array<Promise<ChangeHotswapResult>>> = [];
+  const promises: Array<() => Array<Promise<ChangeHotswapResult>>> = [];
   const hotswappableResources = new Array<HotswapOperation>();
 
   // gather the results of the detector functions
@@ -97,7 +97,8 @@ async function findAllHotswappableChanges(
     } else if (resourceHotswapEvaluation === ChangeHotswapImpact.IRRELEVANT) {
       // empty 'if' just for flow-aware typing to kick in...
     } else {
-      promises.push([
+      // run isHotswappable* functions lazily to prevent unhandled rejections
+      promises.push(() => [
         isHotswappableLambdaFunctionChange(logicalId, resourceHotswapEvaluation, evaluateCfnTemplate),
         isHotswappableStateMachineChange(logicalId, resourceHotswapEvaluation, evaluateCfnTemplate),
         isHotswappableEcsServiceChange(logicalId, resourceHotswapEvaluation, evaluateCfnTemplate),
@@ -111,7 +112,7 @@ async function findAllHotswappableChanges(
   // resolve all detector results
   const changesDetectionResults: Array<Array<ChangeHotswapResult>> = [];
   for (const detectorResultPromises of promises) {
-    const hotswapDetectionResults = await Promise.all(detectorResultPromises);
+    const hotswapDetectionResults = await Promise.all(detectorResultPromises());
     changesDetectionResults.push(hotswapDetectionResults);
   }
 

packages/aws-cdk/test/api/hotswap/hotswap-deployments.test.ts

@@ -1,4 +1,5 @@
 import { Lambda, StepFunctions } from 'aws-sdk';
+import { CfnEvaluationException } from '../../../lib/api/evaluate-cloudformation-template';
 import * as setup from './hotswap-test-setup';
 
 let hotswapMockSdkProvider: setup.HotswapMockSdkProvider;
@@ -414,3 +415,89 @@ test('A change to both a hotswappable resource and a stack output results in a f
   expect(mockUpdateMachineDefinition).not.toHaveBeenCalled();
   expect(mockUpdateLambdaCode).not.toHaveBeenCalled();
 });
+
+test('Multiple CfnEvaluationException will not cause unhandled rejections', async () => {
+  // GIVEN
+  setup.setCurrentCfnStackTemplate({
+    Resources: {
+      Func1: {
+        Type: 'AWS::Lambda::Function',
+        Properties: {
+          Code: {
+            S3Bucket: 'current-bucket',
+            S3Key: 'current-key',
+          },
+          Environment: {
+            key: 'old',
+          },
+          FunctionName: 'my-function',
+        },
+        Metadata: {
+          'aws:asset:path': 'old-path',
+        },
+      },
+      Func2: {
+        Type: 'AWS::Lambda::Function',
+        Properties: {
+          Code: {
+            S3Bucket: 'current-bucket',
+            S3Key: 'current-key',
+          },
+          Environment: {
+            key: 'old',
+          },
+          FunctionName: 'my-function',
+        },
+        Metadata: {
+          'aws:asset:path': 'old-path',
+        },
+      },
+    },
+  });
+  const cdkStackArtifact = setup.cdkStackArtifactOf({
+    template: {
+      Resources: {
+        Func1: {
+          Type: 'AWS::Lambda::Function',
+          Properties: {
+            Code: {
+              S3Bucket: 'current-bucket',
+              S3Key: 'current-key',
+            },
+            Environment: {
+              key: { Ref: 'ErrorResource' },
+            },
+            FunctionName: 'my-function',
+          },
+          Metadata: {
+            'aws:asset:path': 'new-path',
+          },
+        },
+        Func2: {
+          Type: 'AWS::Lambda::Function',
+          Properties: {
+            Code: {
+              S3Bucket: 'current-bucket',
+              S3Key: 'current-key',
+            },
+            Environment: {
+              key: { Ref: 'ErrorResource' },
+            },
+            FunctionName: 'my-function',
+          },
+          Metadata: {
+            'aws:asset:path': 'new-path',
+          },
+        },
+      },
+    },
+  });
+
+  // WHEN
+  const deployStackResult = hotswapMockSdkProvider.tryHotswapDeployment(cdkStackArtifact);
+
+  // THEN
+  await expect(deployStackResult).rejects.toThrowError(CfnEvaluationException);
+  expect(mockUpdateMachineDefinition).not.toHaveBeenCalled();
+  expect(mockUpdateLambdaCode).not.toHaveBeenCalled();
+});