作用：用于在go协程中 传递上下文、超时、取消、传值

底层实现：是由互斥锁、channel、map来实现的
互斥锁：保护临界资源
channel: 用于信号通知，比如ctx.Done()
map: 保存父ctx下派生的所有子ctx, 父ctx关闭，子ctx都关闭

实现的接口

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

空ctx

type emptyCtx intfunc (*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
}

cancel ctx

使用map保存所有子ctx，确保父ctx cancel后，子ctx也cancel

type cancelCtx struct {Contextmu       sync.Mutex            // protects following fieldsdone     chan struct{}         // created lazily, closed by first cancel callchildren map[canceler]struct{} // set to nil by the first cancel callerr      error                 // set to non-nil by the first cancel call
}func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {if parent == nil {panic("cannot create context from nil parent")}c := newCancelCtx(parent)propagateCancel(parent, &c)return &c, func() { c.cancel(true, Canceled) }
}// newCancelCtx returns an initialized cancelCtx.
func newCancelCtx(parent Context) cancelCtx {return cancelCtx{Context: parent}
}func propagateCancel(parent Context, child canceler) {fmt.Println("propagateCancel")done := parent.Done()if done == nil {return // parent is never canceled}select {case <-done:// parent is already canceledchild.cancel(false, parent.Err())returndefault:}if p, ok := parentCancelCtx(parent); ok {p.mu.Lock()if p.err != nil {// parent has already been canceledchild.cancel(false, p.err)} else {if p.children == nil {p.children = make(map[canceler]struct{})}// 保存子ctxp.children[child] = struct{}{}}p.mu.Unlock()} else {atomic.AddInt32(&goroutines, +1)go func() {select {case <-parent.Done():child.cancel(false, parent.Err())case <-child.Done():}}()}
}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 = errif c.done == nil {c.done = closedchan} else {close(c.done) // 关闭channel, 用于通知ctx.Done()}// 关闭所有子ctxfor child := range c.children {// NOTE: acquiring the child's lock while holding parent's lock.child.cancel(false, err)}c.children = nilc.mu.Unlock()if removeFromParent {removeChild(c.Context, c)}
}

timeout ctx

在cancelctx的基础上实现，只是多了个定时器自动调用cancel

type timerCtx struct {cancelCtxtimer *time.Timer // Under cancelCtx.mu.deadline time.Time
}func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {if parent == nil {panic("cannot create context from nil parent")}if cur, ok := parent.Deadline(); ok && cur.Before(d) {// The current deadline is already sooner than the new one.return WithCancel(parent)}c := &timerCtx{cancelCtx: newCancelCtx(parent),deadline:  d,}propagateCancel(parent, c)dur := time.Until(d)if dur <= 0 {c.cancel(true, DeadlineExceeded) // deadline has already passedreturn c, func() { c.cancel(false, Canceled) }}c.mu.Lock()defer c.mu.Unlock()if c.err == nil {c.timer = time.AfterFunc(dur, func() {c.cancel(true, DeadlineExceeded)})}return c, func() { c.cancel(true, Canceled) }
}

value ctx

type valueCtx struct {Contextkey, val interface{}
}
func WithValue(parent Context, key, val interface{}) Context {if parent == nil {panic("cannot create context from nil parent")}if key == nil {panic("nil key")}//	if !reflectlite.TypeOf(key).Comparable() {//	panic("key is not comparable")//}return &valueCtx{parent, key, val}
}
func (c *valueCtx) Value(key interface{}) interface{} {if c.key == key {return c.val}return c.Context.Value(key)
}