Adds versioning for out-of-process storage driver

The registry currently only accepts storage driver versions with the
same major version and an equal or lower minor version as its own
current storage driver api version, but this may be changed in the
future if we decide to implement specific version cross-compatibility.

storagedriver/storagedriver.go

@@ -3,8 +3,32 @@ package storagedriver
 import (
 	"fmt"
 	"io"
+	"strconv"
+	"strings"
 )
 
+// Version is a string representing the storage driver version, of the form Major.Minor.
+// The registry must accept storage drivers with equal major version and greater minor version,
+// but may not be compatible with older storage driver versions.
+type Version string
+
+// Major returns the major (primary) component of a version
+func (version Version) Major() uint {
+	majorPart := strings.Split(string(version), ".")[0]
+	major, _ := strconv.ParseUint(majorPart, 10, 0)
+	return uint(major)
+}
+
+// Minor returns the minor (secondary) component of a version
+func (version Version) Minor() uint {
+	minorPart := strings.Split(string(version), ".")[1]
+	minor, _ := strconv.ParseUint(minorPart, 10, 0)
+	return uint(minor)
+}
+
+// CurrentVersion is the current storage driver Version
+const CurrentVersion Version = "0.1"
+
 // StorageDriver defines methods that a Storage Driver must implement for a filesystem-like
 // key/value object storage
 type StorageDriver interface {