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0100-buildpack-input-vars.md

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Summary

This is a proposal to replace the positional arguments of Buildpack executables with named environment variables.

Definitions

  • positional arguments - values passed into an executable command such that they are accessible as $1, $2, etc

Motivation

Postional arguments to Buildpack executables have been the way for the buildpack execution engine to provide inputs for buildpacks since buildpacks v1 was created. However, positional arguments are limiting, and what they represent is not obvious without reading the spec.

In the spec today they are defined as:

  • /bin/detect <platform[AR]> <plan[E]>
  • /bin/build <layers[EIC]> <platform[AR]> <plan[ER]>

Using env vars helps make these inputs more easily accesible from language bindings like libcnb.bash.

What it is

We will deprecate the positional arguments to bin/detect and bin/build with the following environment variables:

bin/detect

  • CNB_PLATFORM_DIR - replaces the first positional argument to bin/detect. Uses the same env var name as the Platform spec.
  • CNB_BUILD_PLAN_PATH - replaces the second positional argument to bin/detect. Uses the same env var name as the Platform spec.

bin/build

  • CNB_LAYERS_DIR - replaces the first positional argument to bin/build. Note: Uses the same env var name as the Platform spec, but refers to a different location.
  • CNB_PLATFORM_DIR - replaces the second positional argument to bin/build. Uses the same env var name as the Platform spec.
  • CNB_BP_PLAN_PATH - replaces the third positional argument to bin/build.

How it Works

Provide the environment variables with the same mechanism used to provide CNB_BUILDPACK_DIR. For example in lifecycle's build.go:

cmd.Env = append(cmd.Env, EnvPlatformDir+"="+b.Dir)

The positional arguments will be deprecated, but no warnings will be emitted if they are consumed. The lifecycle will continue to provide them to buildpack executable indefinitely, with no plan to remove them.

Renaming Platform Env Vars

Because CNB_LAYERS_DIR in the new input vars conflicts with a platform env var name, we intend to rename the platform env var as part of a larger effort to refactor the platform spec. That will be convered in a future RFC.

The conflict of env var names is strictly an experience problem. Because the env vars are used in different contexts, there is little risk that they will be reused by the internals of lifecycle or pack.

Drawbacks

  • People have been using positional arguments to buildpacks for literally a decade
  • New env var names conflict with platform env vars

Alternatives

  • Do this but don't deprecate the positional arguments (support both)

Prior Art

  • Buildpack v1, v2a, & v2b

Unresolved Questions

N/A

Spec. Changes (OPTIONAL)

Buildpack spec

Detection

Executable: /bin/detect, Working Dir: <app[AR]>

Input Attributes Description
$0 Absolute path of /bin/detect executable
$CNB_BUILD_PLAN_PATH E Absolute path to the build plan
$CNB_PLATFORM_DIR AR Absolute path to the platform directory
$CNB_PLATFORM_DIR/env/ User-provided environment variables for build
$CNB_PLATFORM_DIR/# Platform-specific extensions
Output Description
[exit status] Pass (0), fail (100), or error (1-99, 101+)
Standard output Logs (info)
Standard error Logs (warnings, errors)
$CNB_BUILD_PLAN_PATH Contributions to the the Build Plan (TOML)

Build

Executable: /bin/build, Working Dir: <app[AI]>

Input Attributes Description
$0 Absolute path of /bin/build executable
$CNB_LAYERS_DIR EIC Absolute path to the buildpack layers directory
$CNB_BP_PLAN_PATH ER Relevant Buildpack Plan entries from detection (TOML)
$CNB_PLATFORM_DIR AR Absolute path to the platform directory
$CNB_PLATFORM_DIR/env/ User-provided environment variables for build
$CNB_PLATFORM_DIR/# Platform-specific extensions
Output Description
[exit status] Success (0) or failure (1+)
Standard output Logs (info)
Standard error Logs (warnings, errors)
$CNB_LAYERS_DIR/launch.toml App metadata (see launch.toml)
$CNB_LAYERS_DIR/launch.sbom.<ext> Launch Software Bill of Materials (see Software-Bill-of-Materials)
$CNB_LAYERS_DIR/build.toml Build metadata (see build.toml)
$CNB_LAYERS_DIR/build.sbom.<ext> Build Software Bill of Materials (see Software-Bill-of-Materials)
$CNB_LAYERS_DIR/store.toml Persistent metadata (see store.toml)
$CNB_LAYERS_DIR/<layer>.toml Layer metadata (see Layer Content Metadata)
$CNB_LAYERS_DIR/<layer>.sbom.<ext> Layer Software Bill of Materials (see Software-Bill-of-Materials)
$CNB_LAYERS_DIR/<layer>/bin/ Binaries for launch and/or subsequent buildpacks
$CNB_LAYERS_DIR/<layer>/lib/ Shared libraries for launch and/or subsequent buildpacks
$CNB_LAYERS_DIR/<layer>/profile.d/ Scripts sourced by Bash before launch
$CNB_LAYERS_DIR/<layer>/profile.d/<process>/ Scripts sourced by Bash before launch for a particular process type
$CNB_LAYERS_DIR/<layer>/exec.d/ Executables that provide env vars via the Exec.d Interface before launch
$CNB_LAYERS_DIR/<layer>/exec.d/<process>/ Executables that provide env vars for a particular process type via the Exec.d Interface before launch
$CNB_LAYERS_DIR/<layer>/include/ C/C++ headers for subsequent buildpacks
$CNB_LAYERS_DIR/<layer>/pkgconfig/ Search path for pkg-config for subsequent buildpacks
$CNB_LAYERS_DIR/<layer>/env/ Env vars for launch and/or subsequent buildpacks
$CNB_LAYERS_DIR/<layer>/env.launch/ Env vars for launch (after env, before profile.d)
$CNB_LAYERS_DIR/<layer>/env.launch/<process>/ Env vars for launch (after env, before profile.d) for the launched process
$CNB_LAYERS_DIR/<layer>/env.build/ Env vars for subsequent buildpacks (after env)
$CNB_LAYERS_DIR/<layer>/* Other content for launch and/or subsequent buildpacks