wtdb: export versions of wtclient.db

We now make it possible to get the current db version of the
wtclient.db. Moreover we can now fetch the latest available
migration version for the client db. This allows us to compare
whether the client.db has all the expected migrations applied.

Author: ziggie1984

watchtower/wtdb/version.go

@@ -78,6 +78,12 @@ func getLatestDBVersion(versions []version) uint32 {
 	return uint32(len(versions))
 }
 
+// LatestDBMigrationVersion returns the number of the latest existing database
+// migration version available.
+func LatestDBMigrationVersion() uint32 {
+	return getLatestDBVersion(clientDBVersions)
+}
+
 // getMigrations returns a slice of all updates with a greater number that
 // curVersion that need to be applied to sync up with the latest version.
 func getMigrations(versions []version, curVersion uint32) []version {
@@ -91,6 +97,27 @@ func getMigrations(versions []version, curVersion uint32) []version {
 	return updates
 }
 
+// CurrentDatabaseVersion reads the current database version from the database
+// and returns it.
+func CurrentDatabaseVersion(db kvdb.Backend) (uint32, error) {
+	var (
+		version uint32
+		err     error
+	)
+
+	err = kvdb.View(db, func(tx kvdb.RTx) error {
+		version, err = getDBVersion(tx)
+		return err
+	}, func() {
+		version = 0
+	})
+	if err != nil {
+		return 0, err
+	}
+
+	return version, nil
+}
+
 // getDBVersion retrieves the current database version from the metadata bucket
 // using the dbVersionKey.
 func getDBVersion(tx kvdb.RTx) (uint32, error) {