(pipelines): add a way to add steps that aren't attached to stages

We receive an error “The given Stage construct (‘foo/bar’) should contain at least one Stack” when migrating our code to the modern pipelines.

Reproduction Steps

pipeline.addStage(new cdk.Stage(scope, 'MyStage'));

What did you expect to happen?

We would like to continue adding custom stages containing only CodeBuild projects and/or Lambda Invoke actions. Not all of our stages have stacks.

Environment

• CDK CLI Version : 1.117.0 (build 0047c98)
• Framework Version: 1.117.0 (build 0047c98)
• Node.js Version: v14.15.0
• OS : macOS
• Language (Version): TypeScript

Workaround

  const stage = new cdk.Stage(scope, 'MyStage');
  new cdk.Stack(stage, 'EmptyStackRemoveThis', {
    stackName: 'EmptyStackRemoveThis',
  });

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This is 🐛 Bug Report

[markusl]

@rix0rrr thanks, addWave works indeed.

So basically, you cannot do this:

pipeline.addStage(createInfra('dev', ...));
pipeline.addStage(createInfra('test', ...));
pipeline.addStage(createInfra('prod', ...));

// Does not deploy stacks but only runs some post-production actions (CodeBuild/Lambdas)

const postproductionStage = new cdk.Stage(scope, 'PostProduction');
// ... add actions

pipeline.addStage(postproductionStage);

Instead users can do this:

pipeline.addStage(createInfra('dev', ...));
pipeline.addStage(createInfra('test', ...));
pipeline.addStage(createInfra('prod', ...));

// Does not deploy stacks but only runs some post-production actions (CodeBuild/Lambdas)
const postproductionWave = pipeline.addWave('PostProduction');
postproductionWave.addPost(...);

I have to admit that it was not very intuitive to use addWave to add a CodePipeline stage instead of using addStage for adding a stage 😃 In general, the new API seems quite intuitive and easy to use but maybe more of these use cases could be clarified in the docs?

Br, Markus

[nnbosko]

Still occurring in version 119.