0072-image-workdir.md

Meta

• Name: Set Image Working Directory to value of CNB_APP_DIR
• Start Date: 2021-01-13
• Author(s): @josegonzalez
• Status: Implemented
• RFC Pull Request: rfcs#134
• CNB Pull Request: (leave blank)
• CNB Issue: buildpacks/spec#183, buildpacks/lifecycle#516, buildpacks/docs#290
• Supersedes: N/A

Summary

This RFC proposes setting the Working Directory property of run images to the value of CNB_APP_DIR.

Definitions

N/A

Motivation

This change will enable platforms to more easily introspect on the correct working directory for exec'ing into a container without being tied to CNBs as the foundation for image building. Without this change, platforms will need to special-case CNB images to ensure entering new or existing containers is performed in the correct path.

By way of example, it is very easy to forget to specify the launcher as the entrypoint. Assuming a user sets an entrypoint of bash:

# sample app setup
$ git clone https://github.com/buildpacks/samples
$ pack build sample-app --path apps/java-maven --builder cnbs/sample-builder:bionic

# bash as entrypoint, docker run
$ docker run --entrypoint bash -it sample-app
cnb@3e8152452126:/$ echo $PWD
/

# launcher as entrypoint, docker run
$ docker run --entrypoint launcher -it sample-app bash
cnb@d58a2c0003d8:/workspace$ echo $PWD
/workspace

# creating a launcher container
$ docker run --entrypoint launcher -d -it sample-app bash
e2b84e2e0182e7ec17b4b3f9054fcf5094d2fa7501e63a19fb433db5eb81ea4b

# entering the launcher container
$ docker exec -it e2b84e2e0182e7ec17b4b3f9054fcf5094d2fa7501e63a19fb433db5eb81ea4b bash
cnb@e2b84e2e0182:/$ echo $PWD
/

# creating a launcher container with the workdir option
$ docker run --entrypoint launcher -d --workdir /workspace -it sample-app bash
4cbdafe02f8af69cd790451e016d13ee51a70aff82fdefec9b16bdc0a4717f0b

# entering the launcher container
$ docker exec -it 4cbdafe02f8af69cd790451e016d13ee51a70aff82fdefec9b16bdc0a4717f0b bash
cnb@4cbdafe02f8a:/workspace$ echo $PWD
/workspace

Additionally, copying files from an image may be incorrect if the value is not introspected properly for CNB images.

What it is

The proposes one change

• Injection of /workspace - the value of CNB_APP_DIR - as the WorkingDir of built images

How it Works

The Exporter.Export() would need to be modified to call Image.SetWorkingDIr() with the correct value.

Drawbacks

It is less code to do so.

Alternatives

N/A

Prior Art

N/A

Unresolved Questions

N/A

Spec. Changes (OPTIONAL)

• There would be a new property - WorkingDir - on exported images that was previously an empty string and will now contain the value of CNB_APP_DIR.