CNB_TARGET_ environment variables are not set when expected

[joshwlewis]

Summary

In Buildpack API 0.10, it says:

The following environment variables MUST be set by the lifecycle during the detect and build phases to describe the target runtime image, if inputs are provided.

But lifecycle fails to provide these CNB_TARGET_* environment variables if the Platform API is less than 0.12:

lifecycle/phase/analyzer.go

Lines 84 to 93 in be881f2

	if a.PlatformAPI.AtLeast("0.12") {
	runImageName = a.RunImage.Name()
	atm, err = platform.GetTargetMetadata(a.RunImage)
	if err != nil {
	return files.Analyzed{}, errors.Wrap(err, "unpacking metadata from image")
	}
	if atm.OS == "" {
	platform.GetTargetOSFromFileSystem(&fsutil.Detect{}, atm, a.Logger)
	}
	}

This means that a buildpack using Buildpack API 0.10 running on a platform using Platform API 0.9 won't have CNB_TARGET_OS and CNB_TARGET_ARCH set, even though these should be set according to the Buildpack API.

---

Reproduction

Steps

1. Use an older version of pack, like v0.27.0, that uses a Platform API version < 0.12
2. Run pack build myapp --builder heroku/builder@sha256:5856c2e1f6c55ec5e289453b872f57f4ebe1a32a6da31f053cd27eddccbf21c5
3. Observe that the build fails (some of the included buildpacks try to read CNB_TARGET_OS, which is not present)
4. Use a newer version of pack that uses Platform API version >= 0.12
5. Run pack build myapp --builder heroku/builder@sha256:5856c2e1f6c55ec5e289453b872f57f4ebe1a32a6da31f053cd27eddccbf21c5 again.
6. Observe that the build doesn't error on missing environment variables.

Current behavior

Buildpacks using Buildpack API 0.10 may not receive CNB_TARGET_OS / CNB_TARGET_PLATFORM env vars when an older Platform API is in use.

Expected behavior

Buildpacks using Buildpack API 0.10 always receive CNB_TARGET_OS / CNB_TARGET_PLATFORM (regardless of Platform API version).

---

Context

lifecycle version

0.18.5 in my testing, but I think older versions are affected as well.

platform version(s)

Anything < 0.12.

anything else?

[natalieparellano]

Related Slack conversation: https://cloud-native.slack.com/archives/C0333LG7C9J/p1709323194121239

We can fix this in 0.19