LOG MODULES:
### Author:品茶
> Go log实现了一个简单的日志包，定义了一个logger类型，并提供了一些日志格式化的方法，输出到标准输出，如果日志没有换行，log模块会自动加上换行。
> 品茶：简单理解就是：按特定方式如何输出并根据情况调用不同的方案
方法集列表：
~~~
Constants
致命错误，等于print()加上os.exit(1)
func Fatal(v ...interface{})
致命错误，等于printf()加上os.exit(1) 带格式化
func Fatalf(format string, v ...interface{})
Println
func Fatalln(v ...interface{})
func Flags() int
func Output(calldepth int, s string) error
print printf println 后调用panic()
func Panic(v ...interface{})
func Panicf(format string, v ...interface{})
func Panicln(v ...interface{})
加入日志前缀
func Prefix() string
func Print(v ...interface{})
func Printf(format string, v ...interface{})
func Println(v ...interface{})
func SetFlags(flag int)
func SetOutput(w io.Writer)
func SetPrefix(prefix string)
模块拥有的方法，类型拥有同样的方法，只是用到了OOP的一些概念。
type Logger
func New(out io.Writer, prefix string, flag int) *Logger
func (l *Logger) Fatal(v ...interface{})
func (l *Logger) Fatalf(format string, v ...interface{})
func (l *Logger) Fatalln(v ...interface{})
func (l *Logger) Flags() int
func (l *Logger) Output(calldepth int, s string) error
func (l *Logger) Panic(v ...interface{})
func (l *Logger) Panicf(format string, v ...interface{})
func (l *Logger) Panicln(v ...interface{})
func (l *Logger) Prefix() string
func (l *Logger) Print(v ...interface{})
func (l *Logger) Printf(format string, v ...interface{})
func (l *Logger) Println(v ...interface{})
func (l *Logger) SetFlags(flag int)
func (l *Logger) SetOutput(w io.Writer)
func (l *Logger) SetPrefix(prefix string)
~~~
Logger 类型：
> mu 锁用来保证原子性写入
> prefix 前缀字符串
> flat 标记
> out 输出的内容
> buf
~~~
// A Logger represents an active logging object that generates lines of
// output to an io.Writer. Each logging operation makes a single call to
// the Writer's Write method. A Logger can be used simultaneously from
// multiple goroutines; it guarantees to serialize access to the Writer.
type Logger struct {
mu sync.Mutex // ensures atomic writes; protects the following fields
prefix string // prefix to write at beginning of each line
flag int // properties
out io.Writer // destination for output
buf []byte // for accumulating text to write
}
~~~
