write molecule config to metadata path

The molecule config contains the OCI path and tag, which is important if
we want to track what container image is mounted at a particular path,
and thus what container might need to stop if a verity error is detected
in one of the atoms in the molecule it's using.

This commit writes that config to a JSON file in the metadata path.

Signed-off-by: Michael McCracken <[email protected]>

molecule.go

@@ -165,6 +165,11 @@ func (m Molecule) Mount(dest string) error {
 		return err
 	}
 
+	err = m.config.WriteToFile(filepath.Join(m.config.MetadataPath, "config.json"))
+	if err != nil {
+		return err
+	}
+
 	// now, do the actual overlay mount
 	err = unix.Mount("overlay", dest, "overlay", 0, mntOpts)
 	return errors.Wrapf(err, "couldn't do overlay mount to %s, opts: %s", dest, mntOpts)

oci.go

@@ -1,6 +1,8 @@
 package atomfs
 
 import (
+	"encoding/json"
+	"io/ioutil"
 	"path"
 
 	ispec "github.com/opencontainers/image-spec/specs-go/v1"
@@ -26,6 +28,18 @@ func (c MountOCIOpts) MountedAtomsPath(parts ...string) string {
 	return path.Join(append([]string{mounts}, parts...)...)
 }
 
+func (c MountOCIOpts) WriteToFile(filename string) error {
+	b, err := json.Marshal(c)
+	if err != nil {
+		return err
+	}
+	err = ioutil.WriteFile(filename, b, 0644)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
 func BuildMoleculeFromOCI(opts MountOCIOpts) (Molecule, error) {
 	oci, err := umoci.OpenLayout(opts.OCIDir)
 	if err != nil {