(pipelines): --output Argument leaks into SimpleSynthAction when deploying the Pipeline

[aax]

I ran into this issue when synthesizing an app containing a CdkPipeline with a SimpleSynthAction. For my needs, I have to specify a custom --output folder when deploying the app. But this output folder path somehow leaks into the CdkPipeline itself. The SimpleSynthAction expects the cloud assembly artifact to be at the same location within the CodeBuild project.

Reproduction Steps

Define a CdkPipeline with a SimpleSynthAction:

cloud_assembly_artifact = codepipeline.Artifact()

CdkPipeline(stack, "Pipeline",
    cloud_assembly_artifact=cloud_assembly_artifact,
    source_action=...,
    synth_action=SimpleSynthAction(
        source_artifact=...,
        cloud_assembly_artifact=cloud_assembly_artifact,
        install_command='npm install -g aws-cdk@1.91.0',
        synth_command='cdk synth'
    )
)

Deploy the app with a custom output argument:

cdk deploy --output /tmp/cdk

What did you expect to happen?

I would expect the pipeline to be deployed and executed successfully.

What actually happened?

The synth action in the pipeline fails. The build spec generated for the synth action looks as follows. Note the primary artifact's base-directory is /tmp/cdk, but the synth command writes its output to cdk.out by default. Therefore the action has an empty output artifact and the build fails.

{
  "version": "0.2",
  "phases": {
    "pre_build": {
      "commands": [
        "npm install -g aws-cdk@1.91.0"
      ]
    },
    "build": {
      "commands": [
        "pipenv run cdk synth"
      ]
    }
  },
  "artifacts": {
    "base-directory": "/tmp/cdk",
    "files": "**/*"
  }
}

Environment

• CDK CLI Version : 1.91.0
• Framework Version: 1.91.0
• Node.js Version: v15.5.0
• OS : Windows
• Language (Version): Python 3.7.9

Other

I can't see how this behaviour is intended. But if it is, I would need to change it for my case. Can I somehow specify the location of the cloud assembly artifact within the SimpleSynthAction?

---

This is 🐛 Bug Report

[aax]

I was able to find a workaround:

SimpleSynthAction(
    source_artifact=...,
    cloud_assembly_artifact=cloud_assembly_artifact,
    install_command='npm install -g aws-cdk@1.91.0',
    synth_command=f'cdk synth --output {scope.node.root.outdir}'
)

This sets the output directory of the synth call within CodeBuild to the same path that is assumed in for the build artifact.

I'm lucky this works for me. But if you need to use an exotic output directory when you deploy your CDK Pipeline or if you are on a different system than within your CodeBuild project (Windows vs Linux) it might not be possible to use the same output dir like this. Hence, I still consider this behaviour as undesirable.

[rix0rrr]

The relative paths we use in the ACTUAL synth dir are still correct, but the base output directory should ALWAYS be cdk.out, and not depend on deploy settings.

[github-actions]

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