context 使用背景

一个request可能对应启动多个goroutine去执行任务，我们可以通过context，实现对这些goroutine的生命周期的控制。同时，对于这些goroutine通用的常量变量，context可以提供存储。

使用demo

不断产生数字，直到程序退出

package main

import (
    "fmt"
    "context"
)

func main() {

    gen := func(ctx context.Context) <-chan int {
        dst := make(chan int)
        n := 1
        go func() {
            for {
                select {
                case <-ctx.Done():
                    fmt.Println("Done")
                    return
                    // dst 是否可补充
                case dst <- n:
                    fmt.Println("dst <- n",n)
                    n++
                }
            }
        }()
        return dst
    }

    ctx,cancel := context.WithCancel(context.Background())
    defer cancel()

    for n := range gen(ctx) {
        fmt.Println(n)
        if n == 5 {
            break
        }
    }
}

源码分析：

interface 定义：

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

background 定义：

var (
    background = new(emptyCtx)
    todo       = new(emptyCtx)
)
func Background() Context {
    return background
}
func TODO() Context {
    return todo
}

emptyCtx 定义：

// An emptyCtx is never canceled,has no values,and has no deadline. It is not
// struct{},since vars of this type must have distinct addresses.
// emptyCtx 不是ctx，而是int
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
}

WithCancel 定义：

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

// 新cancelCtx类型
func newCancelCtx(parent Context) cancelCtx {
    return cancelCtx{
        Context: parent,done:    make(chan struct{}),}
}

type cancelCtx struct {
    Context

    done chan struct{} // closed by the first cancel call.

    mu       sync.Mutex // 线程安全
    children map[canceler]struct{} // set to nil by the first cancel call
    err      error                 // set to non-nil by the first cancel call
}

func (c *cancelCtx) Done() <-chan struct{} {
    return c.done
}

func (c *cancelCtx) Err() error {
    c.mu.Lock()
    defer c.mu.Unlock()
    return c.err
}

// cancel closes c.done,cancels each of c's children,and,if
// removeFromParent is true,removes c from its parent's children.
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 // already canceled
    }
    c.err = err
    close(c.done)
    for child := range c.children {
        // NOTE: acquiring the child's lock while holding parent's lock.
        child.cancel(false,err)
    }
    c.children = nil
    c.mu.Unlock()

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

关联context：

// propagateCancel arranges for child to be canceled when parent is.
func propagateCancel(parent Context,child canceler) {
    if parent.Done() == nil {
        // 如果父ctx已经done，直接不关联
        return // parent is never canceled
    }
    // 强转为cancelctx，如果都为cancelctx
    if p,ok := parentCancelCtx(parent); ok {
        p.mu.Lock()
        if p.err != nil {
            // parent has already been canceled
            child.cancel(false,p.err)
        } else {
            // 加入到children节点中
            if p.children == nil {
                p.children = make(map[canceler]struct{})
            }
            p.children[child] = struct{}{}
        }
        p.mu.Unlock()
    } else {
        go func() {
            // 如果parent完成，child也完成；如果child完成，不做处理
            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
        }
    }
}