expose bucket prefix api

packages/@aws-cdk/app-staging-synthesizer/README.md

@@ -198,3 +198,40 @@ const app = new App({
   }),
 });
 ```
+
+### Ephemeral Assets
+
+Some assets that get put into the staging S3 Bucket are ephemeral - they are only necessary
+during CloudFormation deployment and not after. As long as you know what assets are ephemeral,
+you can tag them as such and they will be marked with an `eph-` prefix when they are staged.
+This allows configuration of a lifecycle rule specifically for ephemeral assets.
+
+A good example is a Lambda Function asset. The asset is only useful in the S3 Bucket at deploy
+time, because the source code gets copied into Lambda itself. So we can mark Lambda assets
+as ephemeral:
+
+```ts
+new lambda.Function(stack, 'lambda', {
+  code: lambda.AssetCode.fromAsset(path.join(__dirname, 'assets'), {
+    ephemeral: true,
+  }),
+  handler: 'index.handler',
+  runtime: lambda.Runtime.PYTHON_3_9,
+});
+```
+
+This means that the asset will go into the S3 Bucket with the prefix `eph-`. It will also be 
+subject to the lifecycle rule set on ephemeral assets. By default, the rule is `expiration: Duration.days(10)`.
+You can specify your own rule like this:
+
+```ts
+const app = new App({
+  defaultStackSynthesizer: TestAppScopedStagingSynthesizer.stackPerEnv({
+    ephemeralFileAssetLifecycleRule: {
+      prefix: 'eph-', // required
+      objectSizeGreaterThan: 10000,
+      expiration: Duration.days(1),
+    },
+  }),
+});
+```

packages/@aws-cdk/app-staging-synthesizer/lib/app-staging-synthesizer.ts

@@ -55,6 +55,8 @@ export interface StackPerEnvProps {
    */
   readonly qualifier?: string;
 
+  readonly bucketPrefix?: string;
+
   /**
    * Retain all assets in the s3 bucket, even the ones that have been
    * marked ephemeral.
@@ -128,6 +130,8 @@ interface AppStagingSynthesizerProps {
    * @default - Value of context key '@aws-cdk/core:bootstrapQualifier' if set, otherwise `DEFAULT_QUALIFIER`
    */
   readonly qualifier?: string;
+
+  readonly bucketPrefix?: string;
 }
 
 /**
@@ -160,6 +164,7 @@ export class AppStagingSynthesizer extends StackSynthesizer implements IReusable
     return new AppStagingSynthesizer({
       qualifier: props.qualifier,
       bootstrapRoles: props.bootstrapRoles,
+      bucketPrefix: props.bucketPrefix,
       stagingStackFactory: {
         stagingStackFactory(boundStack: Stack) {
           const app = App.of(boundStack);
@@ -317,11 +322,13 @@ class BoundAppStagingSynthesizer extends StackSynthesizer implements IBoundAppSt
   private readonly cloudFormationExecutionRoleArn?: string;
   private readonly deploymentActionRoleArn?: string;
   private readonly qualifier: string;
+  private readonly bucketPrefix?: string;
 
   constructor(stack: Stack, props: AppStagingSynthesizerProps) {
     super();
     super.bind(stack);
 
+    this.bucketPrefix = props.bucketPrefix;
     this.qualifier = props.qualifier ?? stack.node.tryGetContext(BOOTSTRAP_QUALIFIER_CONTEXT) ?? BoundAppStagingSynthesizer.DEFAULT_QUALIFIER;
     const spec = new StringSpecializer(stack, this.qualifier);
 
@@ -371,7 +378,7 @@ class BoundAppStagingSynthesizer extends StackSynthesizer implements IBoundAppSt
     const { bucketName, assumeRoleArn } = this.stagingStack.addFile(asset);
     const location = this.assetManifest.defaultAddFileAsset(this.boundStack, asset, {
       bucketName: bucketName,
-      bucketPrefix: asset.ephemeral ? EPHEMERAL_PREFIX : undefined,
+      bucketPrefix: asset.ephemeral ? EPHEMERAL_PREFIX : this.bucketPrefix,
       role: {
         assumeRoleArn,
       },

packages/@aws-cdk/app-staging-synthesizer/test/app-staging-synthesizer.test.ts

@@ -154,8 +154,59 @@ describe(AppStagingSynthesizer, () => {
     expect(evalCFN(location1.bucketName)).toEqual(evalCFN(location2.bucketName));
   });
 
+  test('can configure bucket prefix', () => {
+    // GIVEN
+    app = new App({
+      defaultStackSynthesizer: TestAppScopedStagingSynthesizer.stackPerEnv({
+        bucketPrefix: 'prefix',
+      }),
+    });
+    stack = new Stack(app, 'Stack', {
+      env: {
+        account: '000000000000',
+        region: 'us-west-2',
+      },
+    });
+
+    // WHEN
+    const location = stack.synthesizer.addFileAsset({
+      fileName: __filename,
+      packaging: FileAssetPackaging.FILE,
+      sourceHash: 'abcdef',
+    });
+
+    // THEN - assets have the same location
+    expect(evalCFN(location.objectKey)).toEqual('prefixabcdef.js');
+  });
+
   describe('ephemeral assets', () => {
-    test('ephemeral assets have the \'eph\' prefix', () => {
+    test('ephemeral assets have the \'eph-\' prefix', () => {
+      // WHEN
+      const location = stack.synthesizer.addFileAsset({
+        fileName: __filename,
+        packaging: FileAssetPackaging.FILE,
+        sourceHash: 'abcdef',
+        ephemeral: true,
+      });
+
+      // THEN - asset has bucket prefix
+      expect(evalCFN(location.objectKey)).toEqual('eph-abcdef.js');
+    });
+
+    test('ephemeral assets do not get specified bucketPrefix', () => {
+      // GIVEN
+      app = new App({
+        defaultStackSynthesizer: TestAppScopedStagingSynthesizer.stackPerEnv({
+          bucketPrefix: 'prefix',
+        }),
+      });
+      stack = new Stack(app, 'Stack', {
+        env: {
+          account: '000000000000',
+          region: 'us-west-2',
+        },
+      });
+
       // WHEN
       const location = stack.synthesizer.addFileAsset({
         fileName: __filename,

packages/aws-cdk-lib/aws-s3-assets/lib/asset.ts

@@ -34,6 +34,20 @@ export interface AssetOptions extends CopyOptions, cdk.FileCopyOptions, cdk.Asse
    * @deprecated see `assetHash` and `assetHashType`
    */
   readonly sourceHash?: string;
+
+  /**
+   * Whether or not the asset is ephemeral; i.e. only used during deployment
+   * and not needed afterwards. Setting this property to true has an impact
+   * on the lifecycle of the asset, because we will assume that it is safe to
+   * delete after the CloudFormation deployment succeeds.
+   *
+   * For example, Lambda Function assets are copied over to Lambda during
+   * deployment. Therefore, it is not necessary to store the asset in S3, so
+   * we consider those assets ephemeral.
+   *
+   * @default false
+   */
+  readonly ephemeral?: boolean;
 }
 
 export interface AssetProps extends AssetOptions {
@@ -147,6 +161,7 @@ export class Asset extends Construct implements cdk.IAsset {
       packaging: staging.packaging,
       sourceHash: this.sourceHash,
       fileName: this.assetPath,
+      ephemeral: props.ephemeral,
     });
 
     this.s3BucketName = location.bucketName;