channeldb: move .SyncVersions() into .Open(), minor migration cleanups

This commit unexports the SyncVerions PR, in favor of making it private
and moving it into the .Open() method. With this change, callers no
longer need worry about ensuring the database version is up to sync
before usage, as it will happen automatically once the database is
opened.

This commit also unexports some additional variables within the package
that don’t need be consumed by the public, and exports the
DbVersionNumber attribute on the meta struct. Finally some minor
formatting fixes have neen carried out to ensure the new code more
closely matches the style of the rest of the codebase.

channeldb/meta.go

@@ -9,14 +9,15 @@ var (
 	// the database.
 	metaBucket = []byte("metadata")
 
-	// dbVersionKey is a boltdb key and it's used for storing/retrieveing
+	// dbVersionKey is a boltdb key and it's used for storing/retrieving
 	// current database version.
 	dbVersionKey = []byte("dbp")
 )
 
 // Meta structure holds the database meta information.
 type Meta struct {
-	dbVersionNumber uint32
+	// DbVersionNumber is the current schema version of the database.
+	DbVersionNumber uint32
 }
 
 // FetchMeta fetches the meta data from boltdb and returns filled meta
@@ -74,7 +75,7 @@ func (d *DB) PutMeta(meta *Meta, tx *bolt.Tx) error {
 
 func putDbVersion(metaBucket *bolt.Bucket, meta *Meta) error {
 	scratch := make([]byte, 4)
-	byteOrder.PutUint32(scratch, meta.dbVersionNumber)
+	byteOrder.PutUint32(scratch, meta.DbVersionNumber)
 	if err := metaBucket.Put(dbVersionKey, scratch); err != nil {
 		return err
 	}
@@ -83,8 +84,8 @@ func putDbVersion(metaBucket *bolt.Bucket, meta *Meta) error {
 
 func fetchDbVersion(metaBucket *bolt.Bucket, meta *Meta) {
 	if data := metaBucket.Get(dbVersionKey); data != nil {
-		meta.dbVersionNumber = byteOrder.Uint32(data)
+		meta.DbVersionNumber = byteOrder.Uint32(data)
 	} else {
-		meta.dbVersionNumber = getLatestDBVersion(DBVersions)
+		meta.DbVersionNumber = getLatestDBVersion(dbVersions)
 	}
 }