Add database migrations

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

metadata/migrations.go

@@ -0,0 +1,75 @@
+package metadata
+
+import "github.com/boltdb/bolt"
+
+type migration struct {
+	schema  string
+	version int
+	migrate func(*bolt.Tx) error
+}
+
+var migrations = []migration{
+	{
+		schema:  "v1",
+		version: 1,
+		migrate: addChildLinks,
+	},
+}
+
+// addChildLinks Adds children key to the snapshotters to enforce snapshot
+// entries cannot be removed which have children
+func addChildLinks(tx *bolt.Tx) error {
+	v1bkt := tx.Bucket(bucketKeyVersion)
+	if v1bkt == nil {
+		return nil
+	}
+
+	// iterate through each namespace
+	v1c := v1bkt.Cursor()
+
+	for k, v := v1c.First(); k != nil; k, v = v1c.Next() {
+		if v != nil {
+			continue
+		}
+		nbkt := v1bkt.Bucket(k)
+
+		sbkt := nbkt.Bucket(bucketKeyObjectSnapshots)
+		if sbkt != nil {
+			// Iterate through each snapshotter
+			if err := sbkt.ForEach(func(sk, sv []byte) error {
+				if sv != nil {
+					return nil
+				}
+				snbkt := sbkt.Bucket(sk)
+
+				// Iterate through each snapshot
+				return snbkt.ForEach(func(k, v []byte) error {
+					if v != nil {
+						return nil
+					}
+					parent := snbkt.Bucket(k).Get(bucketKeyParent)
+					if len(parent) > 0 {
+						pbkt := snbkt.Bucket(parent)
+						if pbkt == nil {
+							// Not enforcing consistency during migration, skip
+							return nil
+						}
+						cbkt, err := pbkt.CreateBucketIfNotExists(bucketKeyChildren)
+						if err != nil {
+							return err
+						}
+						if err := cbkt.Put(k, nil); err != nil {
+							return err
+						}
+					}
+
+					return nil
+				})
+			}); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}