func (l *Log) lockAndFlushAll() {
	l.mu.Lock()
	l.flushAll()
	l.mu.Unlock()
}
func (l *Log) lockAndFlushAll() {
	l.mu.Lock()
	l.flushAll()
	l.mu.Unlock()
}
// lockAndFlushAll is like flushAll but locks l.mu first.