feat(pipelines): add static PipelineBase.isPipeline method (#21075)

This change adds a new `isPipeline` method to `PipelineBase`. This method works the same way as `Stack.isStack` (checking if a specific `Symbol` property is defined on the provided object), and serves the same purpose: to check if a provided object extends `PipelineBase` (e.g., `CodePipeline` would return true, while `Stack` or `s3.Bucket` would return false).

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### ~Adding new Unconventional Dependencies:~

~* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)~

### 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*

packages/@aws-cdk/pipelines/lib/main/pipeline-base.ts

@@ -2,6 +2,8 @@ import { Aspects, Stage } from '@aws-cdk/core';
 import { Construct } from 'constructs';
 import { AddStageOpts as StageOptions, WaveOptions, Wave, IFileSetProducer, ShellStep, FileSet } from '../blueprint';
 
+const PIPELINE_SYMBOL = Symbol.for('@aws-cdk/pipelines.PipelineBase');
+
 /**
  * Properties for a `Pipeline`
  */
@@ -32,6 +34,15 @@ export interface PipelineBaseProps {
  * happens first).
  */
 export abstract class PipelineBase extends Construct {
+  /**
+   * Return whether the given object extends {@link PipelineBase}.
+   *
+   * We do attribute detection since we can't reliably use 'instanceof'.
+   */
+  public static isPipeline(x: any): x is PipelineBase {
+    return x !== null && typeof (x) === 'object' && PIPELINE_SYMBOL in x;
+  }
+
   /**
    * The build step that produces the CDK Cloud Assembly
    */
@@ -54,6 +65,8 @@ export abstract class PipelineBase extends Construct {
   constructor(scope: Construct, id: string, props: PipelineBaseProps) {
     super(scope, id);
 
+    Object.defineProperty(this, PIPELINE_SYMBOL, { value: true });
+
     if (props.synth instanceof ShellStep && !props.synth.primaryOutput) {
       props.synth.primaryOutputDirectory('cdk.out');
     }

packages/@aws-cdk/pipelines/test/main/pipeline-base.test.ts

@@ -0,0 +1,29 @@
+import * as cdk from '@aws-cdk/core';
+import { Construct } from 'constructs';
+import { App } from '../../../core/lib';
+import { PipelineBase, IFileSetProducer, FileSet } from '../../lib';
+import { PIPELINE_ENV } from '../testhelpers';
+
+describe('pipeline base', () => {
+
+  test('PipelineBase.isPipeline indicates that a construct extends PipelineBase', () => {
+    const app = new App();
+    const pipelineStack = new cdk.Stack(app, 'PipelineStack', { env: PIPELINE_ENV });
+    const construct = new Construct(pipelineStack, 'Construct');
+    const pipeline = new class extends PipelineBase {
+      constructor() {
+        super(pipelineStack, 'Pipeline', {
+          synth: new class implements IFileSetProducer {
+            public readonly primaryOutput = new FileSet('PrimaryOutput');
+          }(),
+        });
+      }
+      protected doBuildPipeline(): void { }
+    }();
+
+    expect(PipelineBase.isPipeline(pipelineStack)).toStrictEqual(false);
+    expect(PipelineBase.isPipeline(construct)).toStrictEqual(false);
+    expect(PipelineBase.isPipeline(pipeline)).toStrictEqual(true);
+  });
+
+});