Improve usability of Target (#821)

* Makes the distro name/version fields non-optional (`String` instead of
  `Option<String>`), since (a) they will never be `None` in practice
  (see #820), and (b) it otherwise causes a lot of unnecessary friction
  for languages that have distro-version-specific binaries and need to
  use the metadata in multiple places in the buildpack.
* Implements `Clone` and `Debug` on `Target`, which unblocks a number
  of use-cases, such as including the target as part of a buildpack
  Error enum variant.

Fixes #820.
GUS-W-15639138.

CHANGELOG.md

@@ -9,6 +9,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `libcnb`:
+  - `Target` now implements `Clone` and `Debug`. ([#821](https://github.com/heroku/libcnb.rs/pull/821))
+
+### Changed
+
+- `libcnb`:
+  - Changed the type of `Target`'s `distro_name` and `distro_version` fields from `Option<String>` to `String`. ([#821](https://github.com/heroku/libcnb.rs/pull/821))
+  - The libcnb runtime now enforces that the `CNB_TARGET_DISTRO_NAME` and `CNB_TARGET_DISTRO_VERSION` env vars have been set by `lifecycle`. ([#821](https://github.com/heroku/libcnb.rs/pull/821))
 
 ## [0.20.0] - 2024-04-12
 

libcnb/src/error.rs

@@ -29,6 +29,12 @@ pub enum Error<E> {
     #[error("Couldn't determine target arch: {0}")]
     CannotDetermineTargetArch(std::env::VarError),
 
+    #[error("Couldn't determine target distro name: {0}. Ensure the `io.buildpacks.base.distro.*` Docker labels are set on the base image.")]
+    CannotDetermineTargetDistroName(std::env::VarError),
+
+    #[error("Couldn't determine target distro version: {0}. Ensure the `io.buildpacks.base.distro.*` Docker labels are set on the base image.")]
+    CannotDetermineTargetDistroVersion(std::env::VarError),
+
     #[error("Couldn't create platform from platform path: {0}")]
     CannotCreatePlatformFromPath(std::io::Error),
 

libcnb/src/layer/tests.rs

@@ -904,8 +904,8 @@ fn build_context(temp_dir: &TempDir) -> BuildContext<TestBuildpack> {
             os: String::from("linux"),
             arch: String::from("amd64"),
             arch_variant: None,
-            distro_name: Some(String::from("ubuntu")),
-            distro_version: Some(String::from("22.04")),
+            distro_name: String::from("ubuntu"),
+            distro_version: String::from("22.04"),
         },
         platform: GenericPlatform::new(Env::new()),
         buildpack_plan: BuildpackPlan {

libcnb/src/runtime.rs

@@ -364,8 +364,10 @@ where
     let os = env::var("CNB_TARGET_OS").map_err(Error::CannotDetermineTargetOs)?;
     let arch = env::var("CNB_TARGET_ARCH").map_err(Error::CannotDetermineTargetArch)?;
     let arch_variant = env::var("CNB_TARGET_ARCH_VARIANT").ok();
-    let distro_name = env::var("CNB_TARGET_DISTRO_NAME").ok();
-    let distro_version = env::var("CNB_TARGET_DISTRO_VERSION").ok();
+    let distro_name =
+        env::var("CNB_TARGET_DISTRO_NAME").map_err(Error::CannotDetermineTargetDistroName)?;
+    let distro_version =
+        env::var("CNB_TARGET_DISTRO_VERSION").map_err(Error::CannotDetermineTargetDistroVersion)?;
 
     Ok(Target {
         os,

libcnb/src/target.rs

@@ -1,35 +1,40 @@
+#[derive(Clone, Debug)]
 pub struct Target {
     /// The name of the target operating system.
     ///
     /// The value should conform to [Go's `$GOOS`](https://golang.org/doc/install/source#environment), for example
     /// `linux` or `windows`.
     ///
-    /// CNB `lifecycle` sources this value from the build OCI image's [`os` property](https://github.com/opencontainers/image-spec/blob/main/config.md#properties).
+    /// CNB `lifecycle` sources this value from the run OCI image's [`os` property](https://github.com/opencontainers/image-spec/blob/main/config.md#properties).
     pub os: String,
     /// The name of the target CPU architecture.
     ///
     /// The value should conform to [Go's $GOARCH](https://golang.org/doc/install/source#environment), for example
     /// `amd64` or `arm64`.
     ///
-    /// CNB `lifecycle` sources this value from the build OCI image's [`architecture` property](https://github.com/opencontainers/image-spec/blob/main/config.md#properties).
+    /// CNB `lifecycle` sources this value from the run OCI image's [`architecture` property](https://github.com/opencontainers/image-spec/blob/main/config.md#properties).
     pub arch: String,
     /// The variant of the specified CPU architecture.
     ///
     /// The value should conform to [OCI image spec platform variants](https://github.com/opencontainers/image-spec/blob/main/image-index.md#platform-variants), for example
     /// `v7` or `v8`.
     ///
-    /// CNB `lifecycle` sources this value from the build OCI image's [`variant` property](https://github.com/opencontainers/image-spec/blob/main/config.md#properties).
+    /// CNB `lifecycle` sources this value from the run OCI image's [`variant` property](https://github.com/opencontainers/image-spec/blob/main/config.md#properties).
     pub arch_variant: Option<String>,
     /// The name of the operating system distribution. Should be empty for Windows.
     ///
     /// For example: `ubuntu` or `alpine`.
     ///
-    /// CNB `lifecycle` sources this value from the build OCI image's `io.buildpacks.base.distro.name` label.
-    pub distro_name: Option<String>,
+    /// CNB `lifecycle` sources this value from either:
+    /// 1. The `io.buildpacks.base.distro.name` OCI image label, if set on the run image.
+    /// 2. Or else, the `ID` field of the `/etc/os-release` file in the build image.
+    pub distro_name: String,
     /// The version of the operating system distribution.
     ///
     /// For example: `22.04` or `3.19`.
     ///
-    /// CNB `lifecycle` sources this value from the build OCI image's `io.buildpacks.base.distro.version` label.
-    pub distro_version: Option<String>,
+    /// CNB `lifecycle` sources this value from either:
+    /// 1. The `io.buildpacks.base.distro.version` OCI image label, if set on the run image.
+    /// 2. Or else, the `VERSION_ID` field of the `/etc/os-release` file in the build image.
+    pub distro_version: String,
 }