fix: mount previously built squashfs layers if needed (#502)

When we run a stackerfile a second time, previously built tar layers
are assumed to already exist.  But if we are using squashfs layers,
then those layers are missing on a second run.  Make sure to mount
them when we find a built layer.

Closes #454

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>
Co-authored-by: Serge Hallyn <serge.hallyn@ubuntu.com>

pkg/overlay/overlay.go

@@ -11,8 +11,11 @@ import (
 	"path"
 	"syscall"
 
+	ispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
 	"golang.org/x/sys/unix"
+	"stackerbuild.io/stacker/pkg/mount"
+	"stackerbuild.io/stacker/pkg/squashfs"
 	"stackerbuild.io/stacker/pkg/types"
 )
 
@@ -141,6 +144,14 @@ func (o *overlay) SetupEmptyRootfs(name string) error {
 	return ovl.write(o.config, name)
 }
 
+func hasDirEntries(dir string) bool {
+	ents, err := os.ReadDir(dir)
+	if err != nil {
+		return false
+	}
+	return len(ents) != 0
+}
+
 func (o *overlay) snapshot(source string, target string) error {
 	err := o.Create(target)
 	if err != nil {
@@ -153,6 +164,31 @@ func (o *overlay) snapshot(source string, target string) error {
 		return err
 	}
 
+	var manifest ispec.Manifest
+	for _, m := range ovl.Manifests {
+		manifest = m
+	}
+	cacheDir := path.Join(o.config.StackerDir, "layer-bases", "oci")
+	for _, layer := range manifest.Layers {
+		if !squashfs.IsSquashfsMediaType(layer.MediaType) {
+			continue
+		}
+		digest := layer.Digest
+		contents := overlayPath(o.config.RootFSDir, digest, "overlay")
+		mounted, err := mount.IsMountpoint(contents)
+		if err == nil && mounted {
+			// We have already mounted this atom
+			continue
+		}
+		if hasDirEntries(contents) {
+			// We have done an unsquashfs of this atom
+			continue
+		}
+		if err := unpackOne(cacheDir, contents, digest, true); err != nil {
+			return errors.Wrapf(err, "Failed mounting %#v", layer)
+		}
+	}
+
 	ovl.BuiltLayers = append(ovl.BuiltLayers, source)
 
 	return ovl.write(o.config, target)