`CNB_TARGET_ARCH` and `CNB_TARGET_OS` not set correctly when using Platform API 0.9

[edmorley]

Summary

The Buildpack API v0.10 spec says that the CNB_TARGET_ARCH and CNB_TARGET_OS env vars will always be set: https://github.com/buildpacks/spec/blob/buildpack/v0.10/buildpack.md#targets

However, this is not the case iff the Platform API is 0.9 or below, even if the buildpack is using Buildpack API 0.10, and the builder is using latest lifecycle (which has all the previous fixes for the targets related bugs).

This scenario can occur when a user uses an outdated Pack CLI version that does not support Platform API 0.10, such as Pack CLI v0.27.0 which only supports Platform API <= 0.9.

This came up in: https://github.com/heroku/buildpacks-php/issues/121

If Buildpack API 0.10 isn't compatible with older Platform APIs, then lifecycle should exit with a suitable error message. Otherwise the Buildpack API 0.10 spec should be honoured.

---

Reproduction

Steps

1. Download Pack CLI 0.27.0 (which only supports Platform API <= 0.9), eg : curl -fL https://github.com/buildpacks/pack/releases/download/v0.27.0/pack-v0.27.0-macos-arm64.tgz | tar -xz && mv ./pack ./pack-0.27.0
2. mkdir -p testcase/bin
3. echo -e 'api = "0.10"\n\n[buildpack]\nid = "testcase"\nversion = "0.0.1"\n\n[[stacks]]\nid = "*"' > testcase/buildpack.toml
4. echo -e '#!/usr/bin/env bash\n\nprintenv | grep CNB_TARGET | sort && exit 1' > testcase/bin/detect
5. ./pack-0.27.0 build --builder heroku/builder:24 --trust-builder --buildpack testcase/ --path testcase/ testapp --verbose

Current behavior

When using Pack CLI 0.27.0 (which only supports Platform API <= 0.9), the CNB_TARGET_ARCH and CNB_TARGET_OS env vars are not set correctly (they are set to the empty string, instead of arm64 and linux respectively):

$ ./pack-0.27.0 build --builder heroku/builder:24 --trust-builder --buildpack testcase/ --path testcase/ testapp --verbose
Builder heroku/builder:24 is trusted
Pulling image index.docker.io/heroku/builder:24
...
Running the creator on OS linux with:
Container Settings:
  Args: /cnb/lifecycle/creator -daemon -launch-cache /launch-cache -log-level debug -app /workspace -cache-dir /cache -run-image heroku/heroku:24 testapp
  System Envs: CNB_PLATFORM_API=0.9
  Image: pack.local/builder/697477756b6764727773:latest
  User: root
  Labels: map[author:pack]
Host Settings:
  Binds: pack-cache-library_testapp_latest-64516644bb6c.build:/cache /var/run/docker.sock:/var/run/docker.sock pack-cache-library_testapp_latest-64516644bb6c.launch:/launch-cache pack-layers-vtzzazmhya:/layers pack-app-ogpwkrqjst:/workspace
  Network Mode:
Starting creator...
Parsing inputs...
Ensuring privileges...
Executing command...
===> ANALYZING
Timer: Analyzer started at 2024-07-04T13:51:51Z
Image with name "testapp" not found
Found image with identifier "a35bef8f087d6c0804bc83511d01b01b2d203389168e705d2f61921f7b256bab"
Timer: Analyzer ran for 18.375µs and ended at 2024-07-04T13:51:51Z
Run image info in analyzed metadata is:
{"Reference":"a35bef8f087d6c0804bc83511d01b01b2d203389168e705d2f61921f7b256bab","Image":"","Extend":false}
===> DETECTING
Timer: Detector started at 2024-07-04T13:51:51Z
target distro name/version labels not found, reading /etc/os-release file
======== Output: [email protected] ========
CNB_TARGET_ARCH=
CNB_TARGET_ARCH_VARIANT=
CNB_TARGET_DISTRO_NAME=ubuntu
CNB_TARGET_DISTRO_VERSION=24.04
CNB_TARGET_OS=
...

Compare this to when latest Pack CLI is used, which supports Platform API 0.10:

$ pack build --builder heroku/builder:24 --trust-builder --buildpack testcase/ --path testcase/ testapp --verbose
Builder heroku/builder:24 is trusted
Pulling image index.docker.io/heroku/builder:24
...
Running the creator on OS linux from image pack.local/builder/76726b63726261776e7a:latest with:
Container Settings:
  Args: /cnb/lifecycle/creator -daemon -launch-cache /launch-cache -log-level debug -app /workspace -cache-dir /cache -run-image heroku/heroku:24 testapp
  System Envs: CNB_PLATFORM_API=0.13
  Image: pack.local/builder/76726b63726261776e7a:latest
  User: root
  Labels: map[author:pack]
Host Settings:
  Binds: pack-cache-library_testapp_latest-64516644bb6c.build:/cache /var/run/docker.sock:/var/run/docker.sock pack-cache-library_testapp_latest-64516644bb6c.launch:/launch-cache pack-layers-vzajudrqdn:/layers pack-app-wsqlwyuuoc:/workspace
  Network Mode:
Starting creator...
Parsing inputs...
Ensuring privileges...
Executing command...
===> ANALYZING
Timer: Analyzer started at 2024-07-04T13:51:30Z
Image with name "testapp" not found
Found image with identifier "a35bef8f087d6c0804bc83511d01b01b2d203389168e705d2f61921f7b256bab"
Timer: Analyzer ran for 21.084µs and ended at 2024-07-04T13:51:30Z
Run image info in analyzed metadata is:
{"Reference":"a35bef8f087d6c0804bc83511d01b01b2d203389168e705d2f61921f7b256bab","Image":"heroku/heroku:24","Extend":false,"target":{"os":"linux","arch":"arm64","distro":{"name":"ubuntu","version":"24.04"}}}
===> DETECTING
Timer: Detector started at 2024-07-04T13:51:30Z
Checking for match against descriptor: {   []}
======== Output: [email protected] ========
CNB_TARGET_ARCH=arm64
CNB_TARGET_ARCH_VARIANT=
CNB_TARGET_DISTRO_NAME=ubuntu
CNB_TARGET_DISTRO_VERSION=24.04
CNB_TARGET_OS=linux
...

Expected behavior

Either:

1. Lifecycle should honour the Buildpack API 0.10 specification regardless of the Platform API version - and set CNB_TARGET_ARCH and CNB_TARGET_OS correctly.
2. Or, lifecycle should fail the build with a clear error message that explains the current combination of Platform API and Buildpack API versions are not compatible, and the user should try upgrading their tools.

---

Context

lifecycle version

$ pack builder inspect heroku/builder:24
...
Lifecycle:
  Version: 0.19.7
  Buildpack APIs:
    Deprecated: (none)
    Supported: 0.7, 0.8, 0.9, 0.10, 0.11
  Platform APIs:
    Deprecated: (none)
    Supported: 0.7, 0.8, 0.9, 0.10, 0.11, 0.12, 0.13

platform version(s)

$ ./pack-0.27.0 report
Pack:
  Version:  0.27.0+git-f4f5be1.build-3382
  OS/Arch:  darwin/arm64

Default Lifecycle Version:  0.14.1

Supported Platform APIs:  0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9
...