go context用法详解

转发自作者kingeasternsun /articles/10155?fr=sidebar

本文主要基于官方文档Go Concurrency Patterns: Context以及视频Advanced Go Concurrency Patterns的学习而得。

背景

在go服务器中，对于每个请求的request都是在单独的goroutine中进行的，处理一个request也可能设计多个goroutine之间的交互，使用context可以使开发者方便的在这些goroutine里传递request相关的数据、取消goroutine的signal或截止日期。

Context结构
// A Context carries a deadline, cancelation signal, and request-scoped values
// across API boundaries. Its methods are safe for simultaneous use by multiple
// goroutines.
type Context interface {
// Done returns a channel that is closed when this Context is canceled
// or times out.
Done() <-chan struct{}

// Err indicates why this context was canceled, after the Done channel
// is closed.
Err() error

// Deadline returns the time when this Context will be canceled, if any.
Deadline() (deadline , ok bool)

// Value returns the value associated with key or nil if none.
Value(key interface{}) interface{}

}

Done 方法在Context被取消或超时时返回一个close的channel,close的channel可以作为广播通知，告诉给context相关的函数要停止当前工作然后返回。

当一个父operation启动一个goroutine用于子operation，这些子operation不能够取消父operation。下面描述的WithCancel函数提供一种方式可以取消新创建的Context.

Context可以安全的被多个goroutine使用。开发者可以把一个Context传递给任意多个goroutine然后cancel这个context的时候就能够通知到所有的goroutine。

Err方法返回context为什么被取消。

Deadline返回context何时会超时。

Value返回context相关的数据。

继承的Context
BackGround
// Background returns an empty Context. It is never canceled, has no deadline,
// and has no values. Background is typically used in main, init, and tests,
// and as the top-level Context for incoming requests.
func Background() Context

BackGound是所有Context的root，不能够被cancel。

WithCancel
// WithCancel returns a copy of parent whose Done channel is closed as soon as
// is closed or cancel is called.
func WithCancel(parent Context) (ctx Context, cancel CancelFunc)

WithCancel返回一个继承的Context,这个Context在父Context的Done被关闭时关闭自己的Done通道，或者在自己被Cancel的时候关闭自己的Done。 WithCancel同时还返回一个取消函数cancel，这个cancel用于取消当前的Context。

视频Advanced Go Concurrency Patterns中的第一个关于WithCancel的样例代码，本人有所修改

package main

import (
"context"
"log"
"os"
"time"
)

var logg *

func someHandler() {
ctx, cancel := (())
go doStuff(ctx)

//10秒后取消doStuff
(10 * )
cancel()

}

//每1秒work一下，同时会判断ctx是否被取消了，如果是就退出
func doStuff(ctx ) {
for {
(1 * )
select {
case <-():
("done")
return
default:
("work")
}
}
}

func main() {
logg = (, "", )
someHandler()
("down")
}

结果

E:\wdy\goproject>go run context_learn.go
15:06:44 work
15:06:45 work
15:06:46 work
15:06:47 work
15:06:48 work
15:06:49 work
15:06:50 work
15:06:51 work
15:06:52 work
15:06:53 down
withDeadline withTimeout
WithTimeout func(parent Context, timeout ) (Context, CancelFunc)
WithTimeout returns WithDeadline(parent, ().Add(timeout)).

WithTimeout 等价于 WithDeadline(parent, ().Add(timeout)).

对上面的样例代码进行修改

func timeoutHandler() {
// ctx, cancel := ((), 5)
ctx, cancel := ((), ().Add(5))
// go doTimeOutStuff(ctx)
go doStuff(ctx)

(10 * )

cancel()

}

func main() {
logg = (, "", )
timeoutHandler()
("end")
}

输出

15:59:22 work
15:59:24 work
15:59:25 work
15:59:26 work
15:59:27 done
15:59:31 end

可以看到doStuff在context超时的时候被取消了，()被关闭。将替换为

func timeoutHandler() {
ctx, cancel := ((), 5)
// ctx, cancel := ((), ().Add(5))
// go doTimeOutStuff(ctx)
go doStuff(ctx)

(10 * )

cancel()

}

输出

16:02:47 work
16:02:49 work
16:02:50 work
16:02:51 work
16:02:52 done
16:02:56 end

根据视频Advanced Go Concurrency Patterns5分48秒处的代码编写doTimeOutStuff替换doStuff

func doTimeOutStuff(ctx ) {
for {
(1 * )

    if deadline, ok := (); ok { //设置了deadl
        ("deadline set")
        if ().After(deadline) {
            (().Error())
            return
        }

    }

    select {
    case <-():
        ("done")
        return
    default:
        ("work")
    }
}

}

func timeoutHandler() {
ctx, cancel := ((), 5)
// ctx, cancel := ((), ().Add(5))
go doTimeOutStuff(ctx)
// go doStuff(ctx)

(10 * )

cancel()

}

输出：

16:03:55 deadline set
16:03:55 work
16:03:56 deadline set
16:03:56 work
16:03:57 deadline set
16:03:57 work
16:03:58 deadline set
16:03:58 work
16:03:59 deadline set
16:03:59 context deadline exceeded
16:04:04 end

context deadline exceeded就是ctx超时的时候的错误消息。

搜索测试程序

完整代码参见官方文档Go Concurrency Patterns: Context，其中关键的地方在于函数httpDo

func httpDo(ctx , req , f func(, error) error) error {
// Run the HTTP request in a goroutine and pass the response to f.
tr := &{}
client := &{Transport: tr}
c := make(chan error, 1)
go func() { c <- f((req)) }()
select {
case <-():
(req)
<-c // Wait for f to return.
return ()
case err := <-c:
return err
}
}

httpDo关键的地方在于

select {
case <-():
    (req)
    <-c // Wait for f to return.
    return ()
case err := <-c:
    return err
}

要么ctx被取消，要么request请求出错。

WithValue
func WithValue(parent Context, key interface{}, val interface{}) Context

参见搜索程序userip中的代码关键的代码如下：

// NewContext returns a new Context carrying userIP.
func NewContext(ctx , userIP ) {
return (ctx, userIPKey, userIP)
}

// FromContext extracts the user IP address from ctx, if present.
func FromContext(ctx ) (, bool) {
// returns nil if ctx has no value for the key;
// the type assertion returns ok=false for nil.
userIP, ok := (userIPKey).()
return userIP, ok
}
go doc中的信息
The WithCancel, WithDeadline, and WithTimeout functions take a Context (the
parent) and return a derived Context (the child) and a CancelFunc. Calling
the CancelFunc cancels the child and its children, removes the parent's
reference to the child, and stops any associated timers.

里面需要注意的就是调用CancelFunc会取消child以及child生成的context，取出父context对这个child的引用，停止相关的计数器。