Add database migrations

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

metadata/db.go

@@ -1,9 +1,30 @@
 package metadata
 
 import (
+	"context"
+	"encoding/binary"
+	"time"
+
 	"github.com/boltdb/bolt"
 	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/snapshot"
+	"github.com/pkg/errors"
+)
+
+const (
+	// schemaVersion represents the schema version of
+	// the database. This schema version represents the
+	// structure of the data in the database. The schema
+	// can envolve at any time but any backwards
+	// incompatible changes or structural changes require
+	// bumping the schema version.
+	schemaVersion = "v1"
+
+	// dbVersion represents updates to the schema
+	// version which are additions and compatible with
+	// prior version of the same schema.
+	dbVersion = 1
 )
 
 type DB struct {
@@ -20,6 +41,81 @@ func NewDB(db *bolt.DB, cs content.Store, ss map[string]snapshot.Snapshotter) *D
 	}
 }
 
+func (m *DB) Init(ctx context.Context) error {
+	// errSkip is used when no migration or version needs to be written
+	// to the database and the transaction can be immediately rolled
+	// back rather than performing a much slower and unnecessary commit.
+	var errSkip = errors.New("skip update")
+
+	err := m.db.Update(func(tx *bolt.Tx) error {
+		var (
+			// current schema and version
+			schema  = "v0"
+			version = 0
+		)
+
+		i := len(migrations)
+		for ; i > 0; i-- {
+			migration := migrations[i-1]
+
+			bkt := tx.Bucket([]byte(migration.schema))
+			if bkt == nil {
+				// Hasn't encountered another schema, go to next migration
+				if schema == "v0" {
+					continue
+				}
+				break
+			}
+			if schema == "v0" {
+				schema = migration.schema
+				vb := bkt.Get(bucketKeyDBVersion)
+				if vb != nil {
+					v, _ := binary.Varint(vb)
+					version = int(v)
+				}
+			}
+
+			if version >= migration.version {
+				break
+			}
+		}
+
+		// Previous version fo database found
+		if schema != "v0" {
+			updates := migrations[i:]
+
+			// No migration updates, return immediately
+			if len(updates) == 0 {
+				return errSkip
+			}
+
+			for _, m := range updates {
+				t0 := time.Now()
+				if err := m.migrate(tx); err != nil {
+					return errors.Wrapf(err, "failed to migrate to %s.%d", m.schema, m.version)
+				}
+				log.G(ctx).WithField("d", time.Now().Sub(t0)).Debugf("database migration to %s.%d finished", m.schema, m.version)
+			}
+		}
+
+		bkt, err := tx.CreateBucketIfNotExists(bucketKeyVersion)
+		if err != nil {
+			return err
+		}
+
+		versionEncoded, err := encodeInt(dbVersion)
+		if err != nil {
+			return err
+		}
+
+		return bkt.Put(bucketKeyDBVersion, versionEncoded)
+	})
+	if err == errSkip {
+		err = nil
+	}
+	return err
+}
+
 func (m *DB) ContentStore() content.Store {
 	if m.cs == nil {
 		return nil