channeldb: added channeldb versioning

In this commit the upgrade mechanism for database was added which makes he current schema rigid and upgradeable. Additional bucket 'metaBucket' was added which stores
    key that house meta-data related to the current/version state of the database. 'createChannelDB' was modified to create this new bucket+key during initializing. Also
    backup logic was added which makes a complete copy of the current database during migration process and restore the previous version of database if migration failed.

channeldb/db.go

@@ -15,10 +15,32 @@ import (
 )
 
 const (
-	dbName = "channel.db"
+	dbName           = "channel.db"
+	dbFilePermission = 0600
 )
 
+// Migration is a function which takes a prior outdated version of the database
+// instances and mutates the key/bucket structure to arrive at a more up-to-date
+// version of the database.
+type migration func(tx *bolt.Tx) error
+
+type version struct {
+	number    uint32
+	migration migration
+}
+
 var (
+	// DBVersions is storing all versions of database. If current version of
+	// database don't match with latest version this list will be used for
+	// retrieving all migration function that are need to apply to the
+	// current db.
+	DBVersions = []version{
+		{
+			number:    1,
+			migration: nil, // The base DB version requires no migration
+		},
+	}
+
 	// Big endian is the preferred byte order, due to cursor scans over integer
 	// keys iterating in order.
 	byteOrder = binary.BigEndian
@@ -32,9 +54,9 @@ var bufPool = &sync.Pool{
 // information related to nodes, routing data, open/closed channels, fee
 // schedules, and reputation data.
 type DB struct {
-	store *bolt.DB
-
+	store     *bolt.DB
 	netParams *chaincfg.Params
+	dbPath    string
 }
 
 // Open opens an existing channeldb created under the passed namespace with
@@ -49,12 +71,16 @@ func Open(dbPath string, netParams *chaincfg.Params) (*DB, error) {
 		}
 	}
 
-	bdb, err := bolt.Open(path, 0600, nil)
+	bdb, err := bolt.Open(path, dbFilePermission, nil)
 	if err != nil {
 		return nil, err
 	}
 
-	return &DB{store: bdb, netParams: netParams}, nil
+	return &DB{
+		store:     bdb,
+		netParams: netParams,
+		dbPath:    dbPath,
+	}, nil
 }
 
 // Wipe completely deletes all saved state within all used buckets within the
@@ -103,7 +129,7 @@ func createChannelDB(dbPath string) error {
 	}
 
 	path := filepath.Join(dbPath, dbName)
-	bdb, err := bolt.Open(path, 0600, nil)
+	bdb, err := bolt.Open(path, dbFilePermission, nil)
 	if err != nil {
 		return err
 	}
@@ -125,6 +151,10 @@ func createChannelDB(dbPath string) error {
 			return err
 		}
 
+		if _, err := tx.CreateBucket(metaBucket); err != nil {
+			return err
+		}
+
 		return nil
 	})
 	if err != nil {
@@ -268,3 +298,59 @@ func (d *DB) FetchAllChannels() ([]*OpenChannel, error) {
 
 	return channels, err
 }
+
+// SyncVersions function is used for safe db version synchronization. It applies
+// migration functions to the current database and recovers the previous
+// state of db if at least one error/panic appeared during migration.
+func (d *DB) SyncVersions(versions []version) error {
+	meta, err := d.FetchMeta(nil)
+	if err != nil {
+		return err
+	}
+
+	latestVersion := getLatestDBVersion(versions)
+
+	if meta.dbVersionNumber < latestVersion {
+		migrations := getMigrationsToApply(versions, meta.dbVersionNumber)
+
+		return d.store.Update(func(tx *bolt.Tx) error {
+			for _, migration := range migrations {
+				if migration == nil {
+					continue
+				}
+
+				if err := migration(tx); err != nil {
+					return err
+				}
+			}
+
+			meta.dbVersionNumber = latestVersion
+			if err := d.PutMeta(meta, tx); err != nil {
+				return err
+			}
+
+			return nil
+		})
+
+	}
+
+	return nil
+}
+
+func getLatestDBVersion(versions []version) uint32 {
+	return versions[len(versions)-1].number
+}
+
+// getMigrationsToApply retrieves the migration function that should be
+// applied to the database.
+func getMigrationsToApply(versions []version, version uint32) []migration {
+	migrations := make([]migration, 0, len(versions))
+
+	for _, v := range versions {
+		if v.number > version {
+			migrations = append(migrations, v.migration)
+		}
+	}
+
+	return migrations
+}