eMFtQj hIPNtw jQMBWr 所以在JS的世界里, 很需要一种侵入性不那么强而且易读的异步回调解决方案, ES7的规范里, ECMAScript又提出了async/await, 使用这个方案仍然需要使用Promise来改写异步过程, 但是需要用then来改写执行过程, 而是可以像同步代码一样将结果直接作为执行结果返回, 可阅读性也非常好. 这个方案的 hYcTaL hctuh jMfvbc gFwJbt [object Object],[object Object],[object Object],[object Object] ikkyVr bPjPUf cofShm lmYuZf euExUQ hDyWCg hUVKYO [object Object] uQPrX ekvxDO bpqJVJ iGaadT iGXldS hOxXax emJCYl ddGdPh iBSbYp dwejLW esHzLO"> /* sc-component-id: sc-bdVaJa */ .sc-bdVaJa {} .jTcPxY{height:100%;overflow:auto;} /* sc-component-id: sc-bwzfXH */ .sc-bwzfXH {} .iYgZUj{color:#BBB;margin:10px 0 20px 0;} /* sc-component-id: sc-htpNat */ .sc-htpNat {} .knYzzU{position:relative;padding:10px 0 10px 0;width:calc(100% - 10px);height:50px;-webkit-transition:-webkit-transform 0.6s;-webkit-transition:transform 0.6s;transition:transform 0.6s;} .knYzzU:hover{background-color:#EEE;-webkit-transform:translate(10px,0);-ms-transform:translate(10px,0);transform:translate(10px,0);}.null{position:relative;padding:10px 0 10px 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Life of xhu

Go Context 学习

Apr 09, 2018  |  Go  |  Context

使用场景

在使用 goroutine 的时候, 我们经常需要对 goroutine 进行超时控制, 一般是通过在 select 中加上超时条件来完成.

那么假设我们需要对一组 goroutine 来进行控制呢? 这时就可以使用 context 包.

几个常见用法:

  1. 需要对一组 goroutine 进行手动取消控制, 使用 WithCancel 返回的 cancelFunc;
  2. 需要对一组 goroutine 进行超时控制, 使用 WithTimeout 或者 WithDeadline, 其实前者的底层实现是基于后者的;
  3. 需要像下传值, 使用 WithValue.

使用的时候, 一般是吧 context 作为 goroutine 的第一个参数, 然后使用 select 监听 Done() 方法, 然后就可以在外部对 goroutine 进行同一控制:

package main

import (
    "context"
    "fmt"
    "time"
)

func main() {
    ctx1, cancel1 := context.WithCancel(context.Background())
    go func(ctx context.Context) {
        select {
        case <-ctx.Done():
            println("the goroutine is terminated by the context1")
        }
    }(ctx1)
    cancel1()
    time.Sleep(time.Second / 10)

    ctx2, _ := context.WithDeadline(context.Background(), time.Now().Add(time.Second))
    go func(ctx context.Context) {
        select {
        case <-ctx.Done():
            println("the goroutine is terminated by the context2")
        }
    }(ctx2)
    time.Sleep(time.Second * 2)

    ctx3, _ := context.WithTimeout(context.Background(), time.Second)
    go func(ctx context.Context) {
        select {
        case <-ctx.Done():
            println("the goroutine is terminated by the context3")
        }
    }(ctx3)
    time.Sleep(time.Second * 2)

    ctx4 := context.WithValue(context.Background(), "name", "xhu")
    go func(ctx context.Context) {
        fmt.Printf("the value of key %s is %s\n", "name", ctx.Value("name"))
    }(ctx4)
    time.Sleep(time.Second / 10)

    ctx5 := context.Background()
    ctx6, cancel2 := context.WithCancel(ctx5)
    ctx7 := ctx6
    ctx8, _ := context.WithCancel(ctx7)
    go func(ctx context.Context) {
        select {
        case <-ctx.Done():
            println("the goroutine is terminated by the context6")
        }
    }(ctx8)
    cancel2()
    time.Sleep(time.Second / 10)
}

源码阅读

context 包在 1.7 版本就被加入 Go 标准库, 源码在 go/src/context/context.go , 我们今天重点看一下 cancel context 的实现.

首先是 Context 的定义:

type Context interface {
    Deadline() (deadline time.Time, ok bool)
    Done() <-chan struct{}
    Err() error
    Value(key interface{}) interface{}
}

当我们来执行 context.Background() 的时候, 其实是创建了一个空的 context, 其定义如下:

type emptyCtx int

func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
    return
}

func (*emptyCtx) Done() <-chan struct{} {
    return nil
}

func (*emptyCtx) Err() error {
    return nil
}

func (*emptyCtx) Value(key interface{}) interface{} {
    return nil
}

至于这个地方为什么用 int 而不是一个空的 struct, 注释的解释是这样可以保证每个变量的地址不一样, 不过其实因为后面都是指针操作, 所以这块儿其实用 type emptyCtx struct{} 也是可以正常工作的.

然后我们可以看到 Done() 返回的是一个 nil, 通过上一篇我们可以知道, 使用 emptyCtx.Done() 做 select...case 分支的话, 是会一直阻塞下去的.

然后当我们用 WithCancel 来创造出一个可以 cancel 的 context 的时候, 调用的代码如下:

type cancelCtx struct {
    Context                         // 用来放父 context

    mu       sync.Mutex             // 给数据修改加锁
    done     chan struct{}          // 用来表示 Done 信号的 chnnel
    children map[canceler]struct{}  // 存储基于当前 context 的子 context
    err      error                  // context 已经 cancel, 或者其他出错情况, 置上这个字段, 否则为 nil
}

func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
    c := newCancelCtx(parent)
    propagateCancel(parent, &c)
    return &c, func() { c.cancel(true, Canceled) }
}

func newCancelCtx(parent Context) cancelCtx {
    return cancelCtx{Context: parent}
}

也就是说, 每次我们调用 withCancel 的时候, 都会创建出一个新的 cancelCtx 实例, 给这个实例嵌入了父 context, 这样一来的层层嵌套结构, 对于 cancelCtx/deadlineCtx 不需要的方法, 直接使用 emptyCtx 的默认实现就好了.

这段代码最重要的是通过 propagateCancel 来传递 parent 的 cancel 信号.

func propagateCancel(parent Context, child canceler) {
    if parent.Done() == nil {
        return // parent 无法被 cancel
    }
    if p, ok := parentCancelCtx(parent); ok {
        p.mu.Lock()
        if p.err != nil {
            // parent 已经被 cancel
            child.cancel(false, p.err)
        } else {
            if p.children == nil {
                p.children = make(map[canceler]struct{})
            }
            p.children[child] = struct{}{}
        }
        p.mu.Unlock()
    } else {
        go func() {
            select {
            case <-parent.Done():
                child.cancel(false, parent.Err())
            case <-child.Done():
            }
        }()
    }
}

func parentCancelCtx(parent Context) (*cancelCtx, bool) {
    for {
        switch c := parent.(type) {
        case *cancelCtx:
            return c, true
        case *timerCtx:
            return &c.cancelCtx, true
        case *valueCtx:
            parent = c.Context
        default:
            return nil, false
        }
    }
}

首先, 当 parent 无法被 cancel 的时候是不需要传递 cancel 信号的, 直接返回即可.

对于下面的条件语句, 最好结合 parentCancelCtx 来一起理解, 这个函数就是寻找 parent 所属的 cancelCtx. 对于 cancelCtx 实例, 最近的一个 cancelCtx 就是其本身, 对于 timerCtx 实例, 最近的是其成员变量 cancelCtx, 对于 valueCtx, 就通过 for 循环继续向上追溯. 如果都不是, 第二个返回值就是 false.

  1. 当我们可以找到这个 cancelCtx 时:

    首先给当前操作加锁.

    • 如果 parent 已经被 cancel, 直接 cancel 子 context 即可
    • 如果 parent 没有被 cancel, 将子 context 加入到 parent 的 children 成员变量里.
  2. 如果我们找不到 cancelCtx, 就起一个协程来监听 parent 的 Done(), 当有消息时直接 cancel 子 context.

那然后我们再看看 cancelCtx 上 cancel 这个函数:

func (c *cancelCtx) cancel(removeFromParent bool, err error) {
    if err == nil {
        panic("context: internal error: missing cancel error")
    }
    c.mu.Lock()
    if c.err != nil {
        c.mu.Unlock()
        return // 已经被 cancel 了
    }
    c.err = err
    if c.done == nil {
        c.done = closedchan
    } else {
        close(c.done)
    }
    for child := range c.children {
        child.cancel(false, err)
    }
    c.children = nil
    c.mu.Unlock()

    if removeFromParent {
        removeChild(c.Context, c)
    }
}

func removeChild(parent Context, child canceler) {
    p, ok := parentCancelCtx(parent)
    if !ok {
        return
    }
    p.mu.Lock()
    if p.children != nil {
        delete(p.children, child)
    }
    p.mu.Unlock()
}

cancel 函数的操作, 就是在我们 cancel 一个 context 的时候, 首先将其自身的 done 给关掉, 然后将 children 的 context 给 cancel 掉, 然后根据 removeFromParent 参数决定是否需要从 parent 的 children 中移除当前 context.

所以当我们手动去 cancel 一个 context 的时候, 会有一些额外的逻辑需要解释一下:

  1. 需要将其从 parent 的 children 中移除的, 因为 cancel 掉 parent 的时候会再次递归的 cancel 这个 context, 重复 close 一个 channel 会导致 panic;
  2. 当前 context 的 children 不用移除, 已经 cancel 的 context 就算保留着 children 也没问题, 反正是无法再次被 cancel, 这样也避免了多余的内存操作.

进行上述操作的时候, 也别忘了加锁以避免并发冲突.

对于 cancelCtx 的讲解就到此为止, context 包的主体部分应该就差不多了, 这个包的设计还是很有意思的, 比如对 valueCtx:

type valueCtx struct {
    Context
    key, val interface{}
}

func (c *valueCtx) Value(key interface{}) interface{} {
    if c.key == key {
        return c.val
    }
    return c.Context.Value(key)
}

并不是我想的会有一个 map[string]interface{}, 而也是通过一层层嵌套来构建, 在取值的时候用递归来查询, 不得不说这个包真的是把嵌套/递归这种数据组合和操作方式玩出花儿了.

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