Differing behavior between libcnb read_env on a layer and the lifecycle

[schneems]

Lifecycle and libcnb.rs both auto-add a <layer-name>/bin to the PATH

Expected

Writing to a layer and then calling read_env should return the same environment modification as lifecycle will produce

Actual

Writing a layer and reading it in Rust produces places the <layer-name/bin> on the front of a manual path prepend:

[builder] read_env: PATH="/layers/libcnb_first/0001_first/bin:/manually_set_path_prepend/bin"

Lifecycle places <layer-name>/bin at the end (of a path prepend):

[builder] PATH=/manually_set_path_prepend/bin:/layers/libcnb_first/0001_first/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

Reproduction

• Bash buildpacks https://gist.github.com/schneems/72a67cd8e9267f4a3ffce2798ad18db0
• Rust buildpack https://gist.github.com/schneems/185f4a64440d832baa0c42631a825a77

Build output calling the rust buildpack followed by the bash "print PATH" buildpack:

$ pack build debugging-app --path . --buildpack /private/tmp/ba6c8b19618d86f52e85c8c8896d5a9e/first-path-buildpack-rust/packaged/x86_64-unknown-linux-musl/debug/libcnb_first --buildpack ./echo-path-buildpack --platform linux/amd64
24: Pulling from heroku/builder
Digest: sha256:770cbacea87ae3e06c3773723015b03706f98f78b31be4fd1b963c9ba861eb04
Status: Image is up to date for heroku/builder:24
24: Pulling from heroku/heroku
Digest: sha256:9483dc324ebd0615f936808da8da14c401c790f96b13a331208c4513a57c2051
Status: Image is up to date for heroku/heroku:24
Warning: Builder is trusted but additional modules were added; using the untrusted (5 phases) build flow
0.20.5: Pulling from buildpacksio/lifecycle
Digest: sha256:3274fe28594f484240fc3f17e46b7b45e304830e544dcdd82ce19472d159ef71
Status: Image is up to date for buildpacksio/lifecycle:0.20.5
===> ANALYZING
[analyzer] Restoring data for SBOM from previous image
===> DETECTING
[detector] libcnb/first       0.1.0
[detector] examples/echo-path 0.0.1
===> RESTORING
===> BUILDING
[builder] Hello World!
[builder] The build is running on: linux (amd64)!
[builder] read_env: PATH="/layers/libcnb_first/0001_first/bin:/manually_set_path_prepend/bin"
[builder] PATH=/manually_set_path_prepend/bin:/layers/libcnb_first/0001_first/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
===> EXPORTING
[exporter] Reusing layer 'libcnb/first:0001_first'
[exporter] Reusing layer 'buildpacksio/lifecycle:launch.sbom'
[exporter] Reused 1/1 app layer(s)
[exporter] Reusing layer 'buildpacksio/lifecycle:launcher'
[exporter] Reusing layer 'buildpacksio/lifecycle:config'
[exporter] Adding label 'io.buildpacks.lifecycle.metadata'
[exporter] Adding label 'io.buildpacks.build.metadata'
[exporter] Adding label 'io.buildpacks.project.metadata'
[exporter] no default process type
[exporter] Saving debugging-app...
[exporter] *** Images (9270e5dbb797):
[exporter]       debugging-app
Successfully built image debugging-app

Beyond fixing this in libcnb, it's unclear if this behavior is in the Spec or not https://github.com/buildpacks/spec/blob/main/buildpack.md#layer-paths. If not, we should document it.