(core): BundlingDockerImage.cp() needs to be explained more in the README

[dmlook-pillpack]

When using lambda.Code.fromAsset and cdk.BundlingDockerImage.fromAsset together, synth fails to find anything in \asset-output

Reproduction Steps

1. Create a Dockerfile that compiles and copies files to an /asset-output directory

FROM python:3.7-slim
COPY . /asset-input
COPY . /asset-output
WORKDIR /asset-input
RUN apt-get update && apt-get -y install curl make automake gcc g++ subversion python3-dev
RUN curl -sSL https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py | python3 -
ENV PATH "/root/.poetry/bin:/opt/venv/bin:${PATH}"
RUN poetry export -f requirements.txt -o requirements.txt
RUN pip3 install -r requirements.txt -t /asset-output

2. Use the following snippet when creating the lambda using cdk:

code: lambda.Code.fromAsset(PROJECT_DIR, {
        bundling: {
          image: cdk.BundlingDockerImage.fromAsset(PROJECT_DIR)
        }
      }),

3. Run tsc && cdk synth -o cdk.out

What did you expect to happen?

Docker should find the compiled assets in /asset-output

What actually happened?

Error: Bundling did not produce any output. Check that content is written to /asset-output.

Environment

• CDK CLI Version : 1.75.0
• Framework Version: 1.75.0
• Node.js Version: v15.3.0
• OS : Mac Catalina 10.15.7
• Language (Version): TypeScript 4.1.2

Other

If I use an implementation of ILocalBundling that is mostly copied from asset-staging.ts but calls both run and cp the synth works but I don't believe that should be necessary:

class LocalBundling implements ILocalBundling {
  tryBundle(outputDir: string, options: BundlingOptions): boolean {

    let user: string;
    if (options.user) {
      user = options.user;
    } else {
      // Default to current user
      const userInfo = os.userInfo();
      user =
        userInfo.uid !== -1 // uid is -1 on Windows
          ? `${userInfo.uid}:${userInfo.gid}`
          : "1000:1000";
    }

    // Always mount input and output dir
    const volumes = [
      {
        hostPath: PROJECT_DIR, // this.sourcePath
        containerPath: AssetStaging.BUNDLING_INPUT_DIR,
      },
      {
        hostPath: outputDir, // bundleDir
        containerPath: AssetStaging.BUNDLING_OUTPUT_DIR ?? outputDir,
      },
      ...(options.volumes ?? []),
    ];

    options.image.run({
      command: options.command,
      user,
      volumes,
      environment: options.environment,
      workingDirectory:
        options.workingDirectory ?? AssetStaging.BUNDLING_INPUT_DIR,
    });

    options.image.cp(AssetStaging.BUNDLING_OUTPUT_DIR ?? outputDir, outputDir);

    return true;
  }
}

---

This is 🐛 Bug Report

[jogold]

I think that what you are looking for is BundlingDockerImage.cp() (introduced in #9728).

After building their own Docker images, users can more easily run the image or copy files out of the image to create their own assets without using the bundling mechanism.

aws-cdk/packages/@aws-cdk/core/lib/bundling.ts

Lines 178 to 181 in cbe7a10

	/**
	* Copies a file or directory out of the Docker image to the local filesystem
	*/
	public cp(imagePath: string, outputPath: string) {

/asset-input and /asset-output (with mounted volumes) works when running the container not when building it.

[dmlook-pillpack]

So what is the appropriate use of the default bundling behavior? I would assume the snippet in step 2 above would work without needing to implement ILocalBundling to use the cp method.

[jogold]

So what is the appropriate use of the default bundling behavior?

The default behavior is to run a command in a container where the source path is mounted at /asset-input and during the execution it should put content at /asset-output. You can see examples here:

• https://github.com/aws/aws-cdk/blob/v1.77.0/packages/%40aws-cdk/aws-s3-assets/test/integ.assets.bundling.lit.ts
• https://github.com/aws/aws-cdk/blob/v1.77.0/packages/%40aws-cdk/aws-lambda-python/lib/bundling.ts
• https://github.com/aws/aws-cdk/blob/v1.77.0/packages/%40aws-cdk/aws-lambda-nodejs/lib/bundling.ts