fix(synthetics): synth-time failure for canary assets in nested stages (

#27167)

Fixes #27089.

Canaries inside stages would fail at synth time after #26291 was introduced. This was due to a misunderstanding of `assetOutdir`. It turns out that `this.asset.assetPath` is a relative path of `Stage.of(scope).outdir`, not of `Stage.of(scope).assetOutdir`. These two out directories would be equivalent at the root level, but if there were nested stages, `assetOutdir` would equal `../${outdir}`.

This diagram may help:

```
CLOUD ASSEMBLY ROOT
  +-- asset.12345abcdef/
  +-- assembly-Stage
    +-- MyStack.template.json
    +-- MyStack.assets.json <- will contain { "path": "../asset.12345abcdef" }
```


----

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

packages/@aws-cdk/aws-synthetics-alpha/lib/code.ts

@@ -132,8 +132,9 @@ export class AssetCode extends Code {
     }
 
     // Get the staged (or copied) asset path.
-    const assetOutDir = Stage.of(scope)?.assetOutdir;
-    const assetPath = assetOutDir ? path.join(assetOutDir, this.asset.assetPath): this.assetPath;
+    // `this.asset.assetPath` is relative to the `outdir`, not the `assetOutDir`.
+    const asmManifestDir = Stage.of(scope)?.outdir;
+    const assetPath = asmManifestDir ? path.join(asmManifestDir, this.asset.assetPath): this.assetPath;
 
     if (path.extname(assetPath) !== '.zip') {
       if (!fs.lstatSync(assetPath).isDirectory()) {

packages/@aws-cdk/aws-synthetics-alpha/test/code.test.ts

@@ -2,7 +2,7 @@ import * as fs from 'fs';
 import * as path from 'path';
 import { Template } from 'aws-cdk-lib/assertions';
 import * as s3 from 'aws-cdk-lib/aws-s3';
-import { App, Stack, DockerImage } from 'aws-cdk-lib';
+import { App, Stage, Stack, DockerImage } from 'aws-cdk-lib';
 import * as cxapi from 'aws-cdk-lib/cx-api';
 import * as synthetics from '../lib';
 import { RuntimeFamily } from '../lib';
@@ -123,6 +123,24 @@ describe(synthetics.Code.fromAsset, () => {
     expect(synthesized.assets.length).toEqual(1);
   });
 
+  test('works when stack is a part of a stage', () => {
+    // GIVEN
+    const app = new App();
+    const stage1 = new Stage(app, 'Stage1');
+    const stage2 = new Stage(stage1, 'Stage2');
+    const stack = new Stack(stage2);
+
+    // WHEN
+    const directoryAsset = synthetics.Code.fromAsset(path.join(__dirname, 'canaries'));
+    new synthetics.Canary(stack, 'Canary1', {
+      test: synthetics.Test.custom({
+        handler: 'canary.handler',
+        code: directoryAsset,
+      }),
+      runtime: synthetics.Runtime.SYNTHETICS_NODEJS_PUPPETEER_3_8,
+    });
+  });
+
   test('fails if path does not exist', () => {
     const assetPath = path.join(__dirname, 'does-not-exist');
     expect(() => synthetics.Code.fromAsset(assetPath))

packages/aws-cdk-lib/core/lib/stage.ts

@@ -197,7 +197,6 @@ export class Stage extends Construct {
    * will return an empty string.
    *
    * Derived from the construct path.
-   *
    */
   public get artifactId() {
     if (!this.node.path) { return ''; }