Golang官方log包源码分析

以下全是代码, 详解在注释中, 请从头到尾看

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package log implements a simple logging package. It defines a type, Logger,
// with methods for formatting output. It also has a predefined 'standard'
// Logger accessible through helper functions Print[f|ln], Fatal[f|ln], and
// Panic[f|ln], which are easier to use than creating a Logger manually.
// That logger writes to standard error and prints the date and time
// of each logged message.
// Every log message is output on a separate line: if the message being
// printed does not end in a newline, the logger will add one.
// The Fatal functions call os.Exit(1) after writing the log message.
// The Panic functions call panic after writing the log message.
// 官方包注释, 这个包让你自己不需要手动实现日志包, 但是还是我建议还是使用第三方包!
package golog

import (
    "fmt"
    "io"
    "os"
    "runtime"
    "sync"
    "time"
)

// 稍加注释!
// These flags define which text to prefix to each log entry generated by the Logger.
const (
    // Bits or'ed together to control what's printed.
    // There is no control over the order they appear (the order listed
    // here) or the format they present (as described in the comments).
    // The prefix is followed by a colon only when Llongfile or Lshortfile
    // is specified.
    // For example, flags Ldate | Ltime (or LstdFlags) produce,
    //  2009/01/23 01:23:23 message
    // while flags Ldate | Ltime | Lmicroseconds | Llongfile produce,
    //  2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message
    // 二进制或标志!
    Ldate         = 1 << iota     // the date in the local time zone: 2009/01/23
    Ltime                         // the time in the local time zone: 01:23:23
    Lmicroseconds                 // microsecond resolution: 01:23:23.123123.  assumes Ltime.
    Llongfile                     // full file name and line number: /a/b/c/d.go:23
    Lshortfile                    // final file name element and line number: d.go:23. overrides Llongfile
    LUTC                          // if Ldate or Ltime is set, use UTC rather than the local time zone
    // 默认不带文件名
    LstdFlags     = Ldate | Ltime // initial values for the standard logger
)

// 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.
// 这个可同时被多个协程使用
type Logger struct {
    // 加锁
    mu     sync.Mutex // ensures atomic writes; protects the following fields
    // 打日志的前缀
    prefix string     // prefix to write at beginning of each line
    // 日志格式的标志
    flag   int        // properties
    // 写入的目标
    out    io.Writer  // destination for output
    // 日志缓存, 待写入
    buf    []byte     // for accumulating text to write
}

// New creates a new Logger. The out variable sets the
// destination to which log data will be written.
// The prefix appears at the beginning of each generated log line.
// The flag argument defines the logging properties.
// 新一个日志记录, 这个对象(结构对象)承接了日志输出的责任.
func New(out io.Writer, prefix string, flag int) *Logger {
    return &Logger{out: out, prefix: prefix, flag: flag}
}

// SetOutput sets the output destination for the logger.
// 设置日志输出的地方
func (l *Logger) SetOutput(w io.Writer) {
    l.mu.Lock()
    defer l.mu.Unlock()
    l.out = w
}

// 默认记录对象, 输出为标准输出, 日志没前缀, 格式是标准: 2009/01/23 01:23:23 message
var std = New(os.Stderr, "", LstdFlags)

// Cheap integer to fixed-width decimal ASCII. Give a negative width to avoid zero-padding.
// 数字填充, 方便时间对齐
func itoa(buf *[]byte, i int, wid int) {
    // Assemble decimal in reverse order.
    // 填充后最长长度20!
    var b [20]byte
    bp := len(b) - 1

    // 类似辗转相除法取最大公约数, 此处是每次取数字的后一位, 等到数字取不到位了就按照wid开始填充
    // 比如 i=106 wid=5:
    //      wid q byte i
    //      4 10 6 106
    //      3 1 0 10
    //      2 0 1 1
    //      1 0 0 0  ===> 不符合i>=0 || wid>1, 跳出
    //      00106
    for i >= 10 || wid > 1 {
        wid--
        // 商, 如果第一次i是106, 那么就变成10
        // 填到q为0时
        q := i / 10
        // 余数, 第一次i-q*10就是6 ==> '0' + 6 ==> '6' 原封不动
        // 填到q为0时, i也为0了, 这时就按wid开始不断填'0'
        b[bp] = byte('0' + i - q*10)
        bp--
        //fmt.Println(wid,q,string(byte('0' + i - q*10)),i)
        i = q
    }
    // i < 10
    b[bp] = byte('0' + i)
    *buf = append(*buf, b[bp:]...)
}

// formatHeader writes log header to buf in following order:
//   * l.prefix (if it's not blank),
//   * date and/or time (if corresponding flags are provided),
//   * file and line number (if corresponding flags are provided).
// 日志输出时日志头部的格式化
func (l *Logger) formatHeader(buf *[]byte, t time.Time, file string, line int) {
    // 前缀增加, 如果前缀为空, 啥都没发生!
    *buf = append(*buf, l.prefix...)
    // 二进制或的魅力来了
    if l.flag&(Ldate|Ltime|Lmicroseconds) != 0 {
        // 以上表示有使用了时间格式
        // 如果有, 转换为UTC时间
        if l.flag&LUTC != 0 {
            t = t.UTC()
        }

        // 日期转换
        if l.flag&Ldate != 0 {
            year, month, day := t.Date()
            // 年份填充, 长度4位
            itoa(buf, year, 4)
            *buf = append(*buf, '/')
            itoa(buf, int(month), 2)
            *buf = append(*buf, '/')
            itoa(buf, day, 2)

            // 空一点后面接文件名或者日志消息
            *buf = append(*buf, ' ')
        }

        // 时间或微妙格式
        if l.flag&(Ltime|Lmicroseconds) != 0 {
            hour, min, sec := t.Clock()
            itoa(buf, hour, 2)
            *buf = append(*buf, ':')
            itoa(buf, min, 2)
            *buf = append(*buf, ':')
            itoa(buf, sec, 2)

            // 正常时间完, 是否有微秒?
            if l.flag&Lmicroseconds != 0 {
                *buf = append(*buf, '.')
                itoa(buf, t.Nanosecond()/1e3, 6)  // 纳秒转微秒, 长度6位
            }
            // 空一点后面接文件名或者日志消
            *buf = append(*buf, ' ')
        }
    }

    // 长文件名, 短文件名输出
    if l.flag&(Lshortfile|Llongfile) != 0 {
        // 短文件名那么截取最后一个, 如a/b/c ==> c
        if l.flag&Lshortfile != 0 {
            short := file
            for i := len(file) - 1; i > 0; i-- {
                if file[i] == '/' {
                    short = file[i+1:]
                    break
                }
            }
            file = short
        }

        // 写入缓冲
        *buf = append(*buf, file...)
        *buf = append(*buf, ':')

        // 行数, 代码打日志所在的地方, wid为-1表示不填冲
        itoa(buf, line, -1)

        // 后面开始接消息, :再空一点!
        *buf = append(*buf, ": "...)
    }
}

// Output writes the output for a logging event. The string s contains
// the text to print after the prefix specified by the flags of the
// Logger. A newline is appended if the last character of s is not
// already a newline. Calldepth is used to recover the PC and is
// provided for generality, although at the moment on all pre-defined
// paths it will be 2.
// 日志输出最重要的地方来了!
func (l *Logger) Output(calldepth int, s string) error {
    now := time.Now() // get this early.
    var file string
    var line int
    l.mu.Lock()
    defer l.mu.Unlock()

    // 使用文件标志格式, 可能会产生bug!
    // 此篇文章: https://www.cnblogs.com/zhangym/p/6709282.html
    // 重复加锁时协程会阻塞, 直到锁被解开, 只要不让主协程阻塞, 就不会死锁! 要及时解开锁!
    if l.flag&(Lshortfile|Llongfile) != 0 {
        // Release lock while getting caller info - it's expensive.
        // 先将锁释放, 方便其他协程能更快获取到调用的文件路径. 文件调用获取路径是一个昂贵的过程
        // 在这个解锁期间, 其他协程中的一个又会进来Output进行加锁, 加锁的这一个瞬间, 如果获取路径过程结束了, 之前的协程会再次加锁, 两次加锁会报错!
        // may BE BUG
        l.mu.Unlock()
        var ok bool
        // 0 表示获取自己的路径
        // 1 表示获取上一层调用的路径
        // 越往上那么逐次加一

        // 在此, 直接调用Output, 那么calldepth为1时可以知道是谁调用了Output, 但是Output被Print等又封装了一层, 所以是2!
        _, file, line, ok = runtime.Caller(calldepth)  // 昂贵!
        if !ok {
            file = "???"
            line = 0
        }

        // 路径获取完后, 在加锁之后, 其他协程可能也进入开头加锁! Maybe bug
        l.mu.Lock()
        // 下面这个测试, 可以Lock两次? 只要保证主进程不阻塞即可?
        //go func(){
        //  time.Sleep(time.Duration(2)*time.Second)
        //  l.mu.Unlock()   // 最终还是会解开锁, Golang的智能检测会发现死锁? 如果主协程在一定时间内还没有响应!
        //}()
        //l.mu.Lock()  // 加锁加不成功难度会阻塞?
    }

    // 以下这一段完全隔离的
    l.buf = l.buf[:0]  // 清空上次缓冲!

    // 格式化
    l.formatHeader(&l.buf, now, file, line)
    l.buf = append(l.buf, s...)

    // 加换行符号
    if len(s) == 0 || s[len(s)-1] != '\n' {
        l.buf = append(l.buf, '\n')
    }
    _, err := l.out.Write(l.buf)
    return err
}



// 下面的都是采用Output包装而来
// Printf calls l.Output to print to the logger.
// Arguments are handled in the manner of fmt.Printf.
func (l *Logger) Printf(format string, v ...interface{}) {
    l.Output(2, fmt.Sprintf(format, v...))
}

// Print calls l.Output to print to the logger.
// Arguments are handled in the manner of fmt.Print.
func (l *Logger) Print(v ...interface{}) { l.Output(2, fmt.Sprint(v...)) }

// Println calls l.Output to print to the logger.
// Arguments are handled in the manner of fmt.Println.
func (l *Logger) Println(v ...interface{}) { l.Output(2, fmt.Sprintln(v...)) }

// Fatal is equivalent to l.Print() followed by a call to os.Exit(1).
func (l *Logger) Fatal(v ...interface{}) {
    l.Output(2, fmt.Sprint(v...))
    os.Exit(1)
}

// Fatalf is equivalent to l.Printf() followed by a call to os.Exit(1).
func (l *Logger) Fatalf(format string, v ...interface{}) {
    l.Output(2, fmt.Sprintf(format, v...))
    os.Exit(1)
}

// Fatalln is equivalent to l.Println() followed by a call to os.Exit(1).
func (l *Logger) Fatalln(v ...interface{}) {
    l.Output(2, fmt.Sprintln(v...))
    os.Exit(1)
}

// Panic is equivalent to l.Print() followed by a call to panic().
func (l *Logger) Panic(v ...interface{}) {
    s := fmt.Sprint(v...)
    l.Output(2, s)
    panic(s)
}

// Panicf is equivalent to l.Printf() followed by a call to panic().
func (l *Logger) Panicf(format string, v ...interface{}) {
    s := fmt.Sprintf(format, v...)
    l.Output(2, s)
    panic(s)
}

// Panicln is equivalent to l.Println() followed by a call to panic().
func (l *Logger) Panicln(v ...interface{}) {
    s := fmt.Sprintln(v...)
    l.Output(2, s)
    panic(s)
}

// 以下都是原子更新
// Flags returns the output flags for the logger.
func (l *Logger) Flags() int {
    l.mu.Lock()
    defer l.mu.Unlock()
    return l.flag
}

// SetFlags sets the output flags for the logger.
func (l *Logger) SetFlags(flag int) {
    l.mu.Lock()
    defer l.mu.Unlock()
    l.flag = flag
}

// Prefix returns the output prefix for the logger.
func (l *Logger) Prefix() string {
    l.mu.Lock()
    defer l.mu.Unlock()
    return l.prefix
}

// SetPrefix sets the output prefix for the logger.
func (l *Logger) SetPrefix(prefix string) {
    l.mu.Lock()
    defer l.mu.Unlock()
    l.prefix = prefix
}

// SetOutput sets the output destination for the standard logger.
func SetOutput(w io.Writer) {
    std.mu.Lock()
    defer std.mu.Unlock()
    std.out = w
}

// Flags returns the output flags for the standard logger.
func Flags() int {
    return std.Flags()
}

// SetFlags sets the output flags for the standard logger.
func SetFlags(flag int) {
    std.SetFlags(flag)
}

// Prefix returns the output prefix for the standard logger.
func Prefix() string {
    return std.Prefix()
}

// SetPrefix sets the output prefix for the standard logger.
func SetPrefix(prefix string) {
    std.SetPrefix(prefix)
}

// These functions write to the standard logger.

// Print calls Output to print to the standard logger.
// Arguments are handled in the manner of fmt.Print.
func Print(v ...interface{}) {
    std.Output(2, fmt.Sprint(v...))
}

// Printf calls Output to print to the standard logger.
// Arguments are handled in the manner of fmt.Printf.
func Printf(format string, v ...interface{}) {
    std.Output(2, fmt.Sprintf(format, v...))
}

// Println calls Output to print to the standard logger.
// Arguments are handled in the manner of fmt.Println.
func Println(v ...interface{}) {
    std.Output(2, fmt.Sprintln(v...))
}

// Fatal is equivalent to Print() followed by a call to os.Exit(1).
func Fatal(v ...interface{}) {
    std.Output(2, fmt.Sprint(v...))
    os.Exit(1)
}

// Fatalf is equivalent to Printf() followed by a call to os.Exit(1).
func Fatalf(format string, v ...interface{}) {
    std.Output(2, fmt.Sprintf(format, v...))
    os.Exit(1)
}

// Fatalln is equivalent to Println() followed by a call to os.Exit(1).
func Fatalln(v ...interface{}) {
    std.Output(2, fmt.Sprintln(v...))
    os.Exit(1)
}

// Panic is equivalent to Print() followed by a call to panic().
func Panic(v ...interface{}) {
    s := fmt.Sprint(v...)
    std.Output(2, s)
    panic(s)
}

// Panicf is equivalent to Printf() followed by a call to panic().
func Panicf(format string, v ...interface{}) {
    s := fmt.Sprintf(format, v...)
    std.Output(2, s)
    panic(s)
}

// Panicln is equivalent to Println() followed by a call to panic().
func Panicln(v ...interface{}) {
    s := fmt.Sprintln(v...)
    std.Output(2, s)
    panic(s)
}

// Output writes the output for a logging event. The string s contains
// the text to print after the prefix specified by the flags of the
// Logger. A newline is appended if the last character of s is not
// already a newline. Calldepth is the count of the number of
// frames to skip when computing the file name and line number
// if Llongfile or Lshortfile is set; a value of 1 will print the details
// for the caller of Output.

// 默认的让你再包装一层的函数...
func Output(calldepth int, s string) error {
    return std.Output(calldepth+1, s) // +1 for this frame.
}