fix: handle ancient empty docker layers

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

moby/moby#20917 (comment)

Signed-off-by: Ramkumar Chinchani <[email protected]>

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)
 	}

test/docker-base.bats

@@ -31,3 +31,13 @@ EOF
     umoci unpack --image oci:layer1 dest
     [ ! -f dest/rootfs/favicon.ico ]
 }
+
+@test "image with empty layer" {
+    cat > stacker.yaml <<EOF
+image:
+    from:
+        type: docker
+        url: docker://ghcr.io/project-stacker/grafana-oss:10.1.2-ubuntu
+EOF
+    stacker build
+}