core -> filex-> file.go -> firstLine has error

[Owen-Zhang]

发现两个问题：

1. 如果内容为空(什么都没有,包括没有\n\r什么的)，程序会出现死 loop, 如果 if n== 0 { return string(first), nil }
2. 没有对前端有空行(\r\n, \n, \t）这些字节进行处理, 如果前面有空行，返回的第一行数据有问题
3. lastline也有同样的问题

firstline 下面这样怎么样

func FirstLine(filename string) (string, error) {
	file, err := os.Open(filename)
	if err != nil {
		return "", err
	}
	defer file.Close()

	firstLine := ""
	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		temp := scanner.Text()
		if len(temp) == 0 {
			continue
		}
		firstLine = temp
		break
	}
	if err := scanner.Err(); err != nil {
		return "", err
	}
        return firstLine, nil
}

[JiChenSSG]

The problem actually exist. Emprt content cause infinite loop in Firstline read, and raise panic in Lastline. I thing empty file should return "". Maybe I can try to fix this.

问题确实存在。空内容导致 Firstline 读取时出现无限循环，Lastline 读取时出现 panic。我认为空文件应该返回 ""。也许我可以尝试修复这个问题。

报错信息：
`panic: runtime error: index out of range [-1] [recovered]
panic: runtime error: index out of range [-1]

goroutine 6 [running]:
testing.tRunner.func1.2({0x5d0600, 0xc00001a258})
/home/jichen/libs/go/src/testing/testing.go:1631 +0x24a
testing.tRunner.func1()
/home/jichen/libs/go/src/testing/testing.go:1634 +0x377
panic({0x5d0600?, 0xc00001a258?})
/home/jichen/libs/go/src/runtime/panic.go:770 +0x132
github.com/zeromicro/go-zero/core/filex.lastLine({0xc00001e600?, 0x2f?}, 0xc0000760e8)
/home/jichen/code/go-zero/core/filex/file.go:77 +0x269
github.com/zeromicro/go-zero/core/filex.LastLine({0xc00001e600, 0x2f})
/home/jichen/code/go-zero/core/filex/file.go:29 +0xa6
github.com/zeromicro/go-zero/core/filex.TestLastLineEmptyFile(0xc0000d0820)
/home/jichen/code/go-zero/core/filex/file_test.go:143 +0xb5
testing.tRunner(0xc0000d0820, 0x612490)
/home/jichen/libs/go/src/testing/testing.go:1689 +0xfb
created by testing.(*T).Run in goroutine 1
/home/jichen/libs/go/src/testing/testing.go:1742 +0x390
FAIL github.com/zeromicro/go-zero/core/filex 0.005s`

[kevwan]

fixed by #4258