Only os and arch are truly required

Signed-off-by: Natalie Arellano <narellano@vmware.com>

platform/target_data.go

@@ -1,6 +1,7 @@
 package platform
 
 import (
+	"fmt"
 	"runtime"
 
 	"github.com/buildpacks/imgutil"
@@ -120,21 +121,30 @@ func EnvVarsFor(d fsutil.Detector, tm files.TargetMetadata, logger log.Logger) [
 		logger.Info("target distro name/version labels not found, reading /etc/os-release file")
 		GetTargetOSFromFileSystem(d, &tm, logger)
 	}
+	// required
 	ret := []string{
 		"CNB_TARGET_OS=" + tm.OS,
 		"CNB_TARGET_ARCH=" + tm.Arch,
-		"CNB_TARGET_ARCH_VARIANT=" + tm.ArchVariant,
 	}
+	// optional
 	var distName, distVersion string
 	if tm.Distro != nil {
 		distName = tm.Distro.Name
 		distVersion = tm.Distro.Version
 	}
-	ret = append(ret, "CNB_TARGET_DISTRO_NAME="+distName)
-	ret = append(ret, "CNB_TARGET_DISTRO_VERSION="+distVersion)
+	ret = appendIfNotEmpty(ret, "CNB_TARGET_ARCH_VARIANT", tm.ArchVariant)
+	ret = appendIfNotEmpty(ret, "CNB_TARGET_DISTRO_NAME", distName)
+	ret = appendIfNotEmpty(ret, "CNB_TARGET_DISTRO_VERSION", distVersion)
 	return ret
 }
 
+func appendIfNotEmpty(env []string, key, val string) []string {
+	if val == "" {
+		return env
+	}
+	return append(env, fmt.Sprintf("%s=%s", key, val))
+}
+
 // TargetSatisfiedForRebase treats optional fields (ArchVariant and Distribution fields) as wildcards if empty, returns true if all populated fields match
 func TargetSatisfiedForRebase(t files.TargetMetadata, appTargetMetadata files.TargetMetadata) bool {
 	if t.OS != appTargetMetadata.OS || t.Arch != appTargetMetadata.Arch {

platform/target_data_test.go

@@ -202,12 +202,28 @@ func testTargetData(t *testing.T, when spec.G, it spec.S) {
 			observed := platform.EnvVarsFor(d, tm, &log.Logger{Handler: memory.New()})
 			h.AssertContains(t, observed, "CNB_TARGET_ARCH="+tm.Arch)
 			h.AssertContains(t, observed, "CNB_TARGET_OS="+tm.OS)
-			// note: per the spec only the ID field is optional, so I guess the others should always be set: https://github.com/buildpacks/rfcs/blob/main/text/0096-remove-stacks-mixins.md#runtime-metadata
-			// the empty ones:
-			h.AssertContains(t, observed, "CNB_TARGET_ARCH_VARIANT=")
-			h.AssertContains(t, observed, "CNB_TARGET_DISTRO_NAME=")
-			h.AssertContains(t, observed, "CNB_TARGET_DISTRO_VERSION=")
-			h.AssertEq(t, len(observed), 5)
+			h.AssertEq(t, len(observed), 2)
+		})
+
+		when("optional vars are empty", func() {
+			it("omits them", func() {
+				tm := files.TargetMetadata{
+					// required
+					OS:   "linux",
+					Arch: "pentium",
+					// optional
+					ArchVariant: "",
+					Distro:      &files.OSDistro{Name: "nix", Version: ""},
+					ID:          "",
+				}
+				d := &mockDetector{
+					contents: "this is just test contents really",
+					t:        t,
+					HasFile:  false,
+				}
+				observed := platform.EnvVarsFor(d, tm, &log.Logger{Handler: memory.New()})
+				h.AssertEq(t, len(observed), 3)
+			})
 		})
 	})