log.go
func callsite(flag uint32) (string, int) {
	_, file, line, ok := runtime.Caller(calldepth)
	if !ok {
		return "???", 0
	}
	if flag&Lshortfile != 0 {
		short := file
		for i := len(file) - 1; i > 0; i-- {
			if os.IsPathSeparator(file[i]) {
				short = file[i+1:]
				break
			}
		}
		file = short
	}
	return file, line
}
func callsite(flag uint32) (string, int) {
	_, file, line, ok := runtime.Caller(calldepth)
	if !ok {
		return "???", 0
	}
	if flag&Lshortfile != 0 {
		short := file
		for i := len(file) - 1; i > 0; i-- {
			if os.IsPathSeparator(file[i]) {
				short = file[i+1:]
				break
			}
		}
		file = short
	}
	return file, line
}
// callsite returns the file name and line number of the callsite to the
// subsystem logger.