fallback to run image mirrors if unavailable

Signed-off-by: Pavel Busko <pavel.busko@sap.com>

Co-authored-by: Ralf Pannemans <ralf.pannemans@sap.com>
Co-authored-by: Johannes Dillmann <j.dillmann@sap.com>

platform/files.go

@@ -10,7 +10,10 @@ import (
 	"github.com/google/go-containerregistry/pkg/name"
 	"github.com/pkg/errors"
 
+	"github.com/buildpacks/imgutil/remote"
+
 	"github.com/buildpacks/lifecycle/api"
+	"github.com/buildpacks/lifecycle/auth"
 	"github.com/buildpacks/lifecycle/buildpack"
 	"github.com/buildpacks/lifecycle/launch"
 	"github.com/buildpacks/lifecycle/layers"
@@ -283,34 +286,50 @@ func (rm *RunImageMetadata) BestRunImageMirror(registry string) (string, error)
 	if rm.Image == "" {
 		return "", errors.New("missing run-image metadata")
 	}
+
 	runImageMirrors := []string{rm.Image}
 	runImageMirrors = append(runImageMirrors, rm.Mirrors...)
-	runImageRef, err := byRegistry(registry, runImageMirrors)
+	runImageRef := byRegistry(registry, runImageMirrors)
+	if runImageRef != "" {
+		// Found run image in the registry of the target image
+		return runImageRef, nil
+	}
+
+	keychain, err := auth.DefaultKeychain(runImageMirrors...)
 	if err != nil {
-		return "", errors.Wrap(err, "failed to find run image")
+		return "", errors.Wrap(err, "failed to get registry credentials")
+	}
+
+	for _, image := range runImageMirrors {
+		img, err := remote.NewImage(image, keychain)
+		if err != nil {
+			return "", errors.Wrap(err, "failed to parse image reference")
+		}
+
+		if img.CheckReadAccess() {
+			return image, nil
+		}
 	}
-	return runImageRef, nil
+
+	return "", errors.Wrap(err, "failed to find accessible run image")
 }
 
 func (sm *StackMetadata) BestRunImageMirror(registry string) (string, error) {
 	return sm.RunImage.BestRunImageMirror(registry)
 }
 
-func byRegistry(reg string, imgs []string) (string, error) {
-	if len(imgs) < 1 {
-		return "", errors.New("no images provided to search")
-	}
-
+func byRegistry(reg string, imgs []string) string {
 	for _, img := range imgs {
 		ref, err := name.ParseReference(img, name.WeakValidation)
 		if err != nil {
 			continue
 		}
 		if reg == ref.Context().RegistryStr() {
-			return img, nil
+			return img
 		}
 	}
-	return imgs[0], nil
+
+	return ""
 }
 
 func ReadStack(stackPath string, logger log.Logger) (StackMetadata, error) {