fix: handle ancient empty docker layers (#522)

Earlier versions of docker images had empty layers of 1024 zero-valued
octets.

moby/moby#20917 (comment)

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

Author: rchincha

pkg/log/log.go

@@ -46,6 +46,10 @@ func Infof(msg string, v ...interface{}) {
 	addStackerLogSentinel(log.NewEntry(log.Log.(*log.Logger))).Infof(msg, v...)
 }
 
+func Warnf(msg string, v ...interface{}) {
+	addStackerLogSentinel(log.NewEntry(log.Log.(*log.Logger))).Warnf(msg, v...)
+}
+
 func Errorf(msg string, v ...interface{}) {
 	addStackerLogSentinel(log.NewEntry(log.Log.(*log.Logger))).Errorf(msg, v...)
 }

pkg/overlay/metadata.go

@@ -3,6 +3,7 @@ package overlay
 import (
 	"bytes"
 	"encoding/json"
+	"io/fs"
 	"os"
 	"path"
 
@@ -117,7 +118,15 @@ func (ovl overlayMetadata) lxcRootfsString(config types.StackerConfig, tag strin
 	for _, layer := range manifest.Layers {
 		contents := overlayPath(config.RootFSDir, layer.Digest, "overlay")
 		if _, err := os.Stat(contents); err != nil {
-			return "", errors.Wrapf(err, "%s does not exist", contents)
+			if errors.Is(err, fs.ErrNotExist) {
+				// some docker layers may be empty tars, so ignore these
+				// https://github.com/moby/moby/issues/20917#issuecomment-191901912
+				log.Warnf("%s skipping empty tar layer", layer.Digest)
+
+				continue
+			}
+
+			return "", errors.Wrapf(err, "%s unable to stat", contents)
 		}
 		lowerdirs = append(lowerdirs, contents)
 	}

pkg/stacker/base.go

@@ -87,7 +87,11 @@ func SetupRootfs(o BaseLayerOpts) error {
 	case types.OCILayer:
 		fallthrough
 	case types.DockerLayer:
-		return setupContainersImageRootfs(o)
+		err := setupContainersImageRootfs(o)
+		if err != nil && errors.Is(err, types.ErrEmptyLayers) {
+			return o.Storage.SetupEmptyRootfs(o.Name)
+		}
+		return err
 	default:
 		return errors.Errorf("unknown layer type: %v", o.Layer.From.Type)
 	}

pkg/types/layer_type.go

@@ -10,6 +10,8 @@ import (
 	"stackerbuild.io/stacker/pkg/squashfs"
 )
 
+var ErrEmptyLayers = errors.New("empty layers")
+
 type LayerType struct {
 	Type   string
 	Verity squashfs.VerityMetadata
@@ -60,7 +62,7 @@ func NewLayerType(lt string, verity squashfs.VerityMetadata) (LayerType, error)
 
 func NewLayerTypeManifest(manifest ispec.Manifest) (LayerType, error) {
 	if len(manifest.Layers) == 0 {
-		return LayerType{}, errors.Errorf("no existing layers to determine layer type")
+		return NewLayerType("tar", squashfs.VerityMetadataMissing)
 	}
 
 	switch manifest.Layers[0].MediaType {

test/empty-layers.bats

@@ -61,3 +61,28 @@ EOF
 
     [ "$layers0" = "$layers1" ]
 }
+
+@test "an image with empty layers" {
+  umoci init --layout oci
+  umoci new --image oci:emptylayer
+  chmod -R a+rw oci
+
+  cat > stacker.yaml <<EOF
+test_empty_layer:
+    from:
+        type: oci
+        url: oci:emptylayer
+EOF
+    stacker build
+}
+
+@test "a real-world docker image with empty/filler layer" {
+    cat > stacker.yaml <<EOF
+image:
+    from:
+        type: docker
+        url: docker://ghcr.io/project-stacker/grafana-oss:10.1.2-ubuntu
+EOF
+    stacker build
+}
+