> For some reason publishAssetsInParallel must be set true though (default). Otherwiese got "This BuildSpec contains CloudFormation references and is supported by publishInParallel=false" and the build failed.

> For some reason publishAssetsInParallel must be set true though (default). Otherwise got "This BuildSpec contains CloudFormation references and is supported by publishInParallel=false" and the build failed.

Just banged my head on this for ~4~ 7 hours today... -_- This has to be a bug right?

_Originally posted by @cheruvian in https://github.com/aws/aws-cdk/issues/10999#issuecomment-1933638576_

CDK Pipelines (`CodePipeline`) fails if you add CloudFormation references (for example with `dockerCredentials`) and `publishAssetsInParallel=false`.

It looks like this is [caused here:](https://github.com/aws/aws-cdk/blob/431df3b67bc2dd2d0e833642a67cff890fc6be29/packages/aws-cdk-lib/pipelines/lib/codepipeline/codepipeline.ts#L858-L860)
```
// If we use a single publisher, pass buildspec via file otherwise it'll
// grow too big.
passBuildSpecViaCloudAssembly: this.singlePublisherPerAssetType,
```

This is then used in the [`CodePipelineFactory`](https://github.com/aws/aws-cdk/blob/431df3b67bc2dd2d0e833642a67cff890fc6be29/packages/aws-cdk-lib/pipelines/lib/codepipeline/private/codebuild-factory.ts#L263-L277)

```
if (this.props.passBuildSpecViaCloudAssembly) {
...
if (typeof fileContents !== 'string') {
throw new Error(`This BuildSpec contains CloudFormation references and is supported by publishInParallel=false: ${JSON.stringify(fileContents, undefined, 2)}`);
}
...
} else {
projectBuildSpec = actualBuildSpec;
}
```

So tl;dr; seems as though

1. Deploying BuildSpec via CloudAssembly is not possible (this makes sense)
2. BuildSpecs are passed via CloudAssembly only if `singlePublisherPerAssetType` (this maybe makes sense?)
3. `singlePublisherPerAssetType` is only set if `publishAssetsInParallel` is `true` (this doesn't make sense necessarily?)

FWIW setting `publishAssetsInParallel=false` is the recommendation from the `[aws-deployment-pipeline-reference-architecture](https://github.com/aws-samples/aws-deployment-pipeline-reference-architecture/blob/8e5989a467834fa3a6c84bbe79295641bc2361ed/examples/cdk-application-pipeline/infrastructure/src/pipeline.ts#L112)

Start in packages/aws-cdk-lib/pipelines/lib/codepipeline/codepipeline.ts and follow passBuildSpecViaCloudAssembly into private/codebuild-factory.ts. Reproduce a CodePipeline with dockerCredentials or another CloudFormation reference and publishAssetsInParallel=false, then trace the singlePublisherPerAssetType decision. Done means that configuration no longer fails because the BuildSpec is passed through the Cloud Assembly.

Written by the indexing model from the issue text.