Golang context源码阅读与分析

https://jiajunhuang.com/articles/2020_04_22-golang_context.md.html

Golang中使用context作为goroutine之间的控制器，例如：

package main

import (
“context”
“log”
“time”
)

func UseContext(ctx context.Context) {
for {
select {
case <-ctx.Done():
log.Printf(“context is done with error %s”, ctx.Err())
return
default:
log.Printf(“nothing just loop…”)
time.Sleep(time.Second * time.Duration(1))
}
}
}

func main() {
ctx, cancel := context.WithCancel(context.Background())
go UseContext(ctx)

time.Sleep(time.Second * time.Duration(1))

cancel()

time.Sleep(time.Second * time.Duration(2)) } 这样就可以在 main 函数里告知 UseContext 所在的goroutine，主函数已经想要退出了。当然，并不是什么很神奇的方法， 也就是相当于main函数里传递一个变量，而UseContext里不断的去检查变量而已，只不过在Golang里，我们使用的是chanel来实现。

来看看 context 是啥，原来是个接口：

type Context interface {
Deadline() (deadline time.Time, ok bool)
Done() <-chan struct{}
Err() error
Value(key interface{}) interface{}
}
既然是接口，那么我们就得来找一个具体的实现来看看，比如 context.Background()：

func Background() Context {
return background
}

// but what is background? it’s:
var (
background = new(emptyCtx)
todo = new(emptyCtx)
)
原来是一个 emptyCtx 实例。那 WithCancel() 是咋实现的呢？

// WithCancel returns a copy of parent with a new Done channel. The returned
// context’s Done channel is closed when the returned cancel function is called
// or when the parent context’s Done channel is closed, whichever happens first.
//
// Canceling this context releases resources associated with it, so code should
// call cancel as soon as the operations running in this Context complete.
func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
c := newCancelCtx(parent)
propagateCancel(parent, &c)
return &c, func() { c.cancel(true, Canceled) }
}

// A cancelCtx can be canceled. When canceled, it also cancels any children
// that implement canceler.
type cancelCtx struct {
Context

mu       sync.Mutex            // protects following fields

done     chan struct{}         // created lazily, closed by first cancel call

children map[canceler]struct{} // set to nil by the first cancel call

err      error                 // set to non-nil by the first cancel call } 可以看出来，done是一个channel，我们在 UseContext 里，通过select来检测ctx是否已经关闭，其实就是看done这个channel 是否关闭了。不信可以看看 cancel 的实现：

// 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
if c.done == nil {
c.done = closedchan
} else {
close(c.done) // NOTE(jiajun): here it is :)
}
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)

} } 瞧，调用cancel的时候就是调用了 close(c.done)，然后下一次循环的时候，UseContext 就能检测到ctx已经关闭了。

总结
这篇博客里我们看到了context是怎么实现的，读者如果有兴趣的话，可以看看Context中的传值和取值是怎么实现的，以及思考一下 是否有更好的实现方案。