git上log日志组件
https://github.com/hpcloud/tail/blob/master/tail.go

我们写个test来测试下这个组件

package main

import (
    "fmt"
    "github.com/hpcloud/tail"
    "time"
)
func main() {
    filename := "E:\\develop\\kafka\\kafka_2.12-1.0.0\\config\\server.properties"
    tails,err := tail.TailFile(filename,tail.Config{
        ReOpen:    true,Follow:    true,// Location: &tail.SeekInfo{Offset: 0,Whence: 2},
        MustExist: false,Poll:      true,})
    if err != nil {
        fmt.Println("tail file err:",err)
        return
    }
    var msg *tail.Line
    var ok bool
    for true {
        msg,ok = <-tails.Lines
        if !ok {
            fmt.Printf("tail file close reopen,filename:%s\n",tails.Filename)
            time.Sleep(100 * time.Millisecond)
            continue
        }
        fmt.Println("msg:",msg)
    }
}

执行代码会在终端输出如下该文件的信息：

PS E:\golang\go_pro\src\safly> go run safly.go
msg: &{# Licensed to the Apache Software Foundation (ASF) under one or more 2017-11-18 19:22:49.194689
2 +0800 CST <nil>}
msg: &{# contributor license agreements. See the NOTICE file distributed with 2017-11-18 19:22:49.194
6892 +0800 CST <nil>}
msg: &{# this work for additional information regarding copyright ownership. 2017-11-18 19:22:49.20869
92 +0800 CST <nil>}
msg: &{# The ASF licenses this file to You under the Apache License,Version 2.0 2017-11-18 19:22:49.2
102004 +0800 CST <nil>}
。。。。。。省略

我们看看源码tailf.go的执行流程：

func TailFile(filename string,config Config) (*Tail,error) {
    if config.ReOpen && !config.Follow {
        util.Fatal("cannot set ReOpen without Follow.")
    }

    t := &Tail{
        Filename: filename,Lines:    make(chan *Line),Config:   config,}

    // when Logger was not specified in config,use default logger
    if t.Logger == nil {
        t.Logger = log.New(os.Stderr,"",log.LstdFlags)
    }

    if t.Poll {
        t.watcher = watch.NewPollingFileWatcher(filename)
    } else {
        t.watcher = watch.NewInotifyFileWatcher(filename)
    }

    if t.MustExist {
        var err error
        t.file,err = OpenFile(t.Filename)
        if err != nil {
            return nil,err
        }
    }

    go t.tailFileSync()

    return t,nil
}

在最后go t.tailFileSync()启动goroutine
在tailFileSync方法中

tail.openReader()

看看做了什么操作？

func (tail *Tail) openReader() {
    if tail.MaxLineSize > 0 {
        // add 2 to account for newline characters
        tail.reader = bufio.NewReaderSize(tail.file,tail.MaxLineSize+2)
    } else {
        tail.reader = bufio.NewReader(tail.file)
    }
}

创建reader,然后for读取line,err := tail.readLine()

然后写入chan 中

// Process `line` even if err is EOF.
        if err == nil {
            cooloff := !tail.sendLine(line)
            if cooloff {
                // Wait a second before seeking till the end of
                // file when rate limit is reached.
                msg := ("Too much log activity; waiting a second " +
                    "before resuming tailing")
                tail.Lines <- &Line{msg,time.Now(),errors.New(msg)}
                select {
                case <-time.After(time.Second):
                case <-tail.Dying():
                    return
                }
                if err := tail.seekEnd(); err != nil {
                    tail.Kill(err)
                    return
                }
            }
}

tail.Lines <- &Line{msg,time.Now(),errors.New(msg)}中的Lines是在Tail定义的chan

type Tail struct { Filename string Lines chan *Line Config file *os.File reader *bufio.Reader watcher watch.FileWatcher changes *watch.FileChanges tomb.Tomb // provides: Done,Kill,Dying lk sync.Mutex }