func (l *Log) SetLogDir(logDir string) {
	if logDir != "" {
		l.mu.Lock()
		defer l.mu.Unlock()
		l.logDirs = append([]string{logDir}, l.logDirs...)
	}
}
func (l *Log) SetLogDir(logDir string) {
	if logDir != "" {
		l.mu.Lock()
		defer l.mu.Unlock()
		l.logDirs = append([]string{logDir}, l.logDirs...)
	}
}
// logDir if non-empty, write log files to this directory.