feat: add Stemcell method on BOSHReleaseManifest

internal/builder/release_manifest_reader.go

@@ -3,13 +3,11 @@ package builder
 import (
 	"crypto/sha1"
 	"fmt"
-	"io"
-	"path/filepath"
-	"strings"
-
 	"github.com/go-git/go-billy/v5"
 	"github.com/go-git/go-billy/v5/osfs"
 	"github.com/pivotal-cf/kiln/pkg/tile"
+	"io"
+	"path/filepath"
 )
 
 type ReleaseManifest struct {
@@ -49,14 +47,9 @@ func (r ReleaseManifestReader) Read(releaseTarball string) (Part, error) {
 		return Part{}, err
 	}
 
-	var stemcellOS,
-		stemcellVersion string
-	if len(inputReleaseManifest.CompiledPackages) > 0 {
-		var found bool
-		stemcellOS, stemcellVersion, found = strings.Cut(inputReleaseManifest.CompiledPackages[0].Stemcell, "/")
-		if !found {
-			return Part{}, fmt.Errorf("%s/%s has invalid stemcell: %q", inputReleaseManifest.Name, inputReleaseManifest.Version, inputReleaseManifest.CompiledPackages[0].Stemcell)
-		}
+	stemcellOS, stemcellVersion, stemcelOK := inputReleaseManifest.Stemcell()
+	if !stemcelOK && len(inputReleaseManifest.CompiledPackages) > 0 {
+		return Part{}, fmt.Errorf("%s/%s has invalid stemcell: %q", inputReleaseManifest.Name, inputReleaseManifest.Version, inputReleaseManifest.CompiledPackages[0].Stemcell)
 	}
 
 	outputReleaseManifest := ReleaseManifest{

pkg/tile/bosh_release.go

@@ -12,6 +12,7 @@ import (
 	"io/fs"
 	"os"
 	"path"
+	"strings"
 
 	"golang.org/x/exp/slices"
 	"gopkg.in/yaml.v2"
@@ -104,6 +105,13 @@ type BOSHReleaseManifest struct {
 	Packages         []BOSHReleasePackage         `yaml:"packages"`
 }
 
+func (mf BOSHReleaseManifest) Stemcell() (string, string, bool) {
+	if len(mf.CompiledPackages) == 0 {
+		return "", "", false
+	}
+	return strings.Cut(mf.CompiledPackages[0].Stemcell, "/")
+}
+
 type BOSHReleaseTarball struct {
 	Manifest BOSHReleaseManifest