go中context源码刨铣

type Context interface {
	// 返回context被取消的时间
	// 当没有设置Deadline时间，返回false
	Deadline() (deadline time.Time, ok bool)

	// 当context被关闭，返回一个被关闭的channel
	Done() <-chan struct{}

	// 在 channel Done 关闭后，返回 context 取消原因
	Err() error

	// 获取key对应的value
	Value(key interface{}) interface{}
}

// 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.
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
}

func (e *emptyCtx) String() string {
	switch e {
	case background:
		return "context.Background"
	case todo:
		return "context.TODO"
	}
	return "unknown empty Context"
}

var (
	background = new(emptyCtx)
	todo       = new(emptyCtx)
)

func Background() Context {
	return background
}

func TODO() Context {
	return todo
}

// Background returns a non-nil, empty Context. It is never canceled, has no
// values, and has no deadline. It is typically used by the main function,
// initialization, and tests, and as the top-level Context for incoming
// requests.

// TODO returns a non-nil, empty Context. Code should use context.TODO when
// it's unclear which Context to use or it is not yet available (because the
// surrounding function has not yet been extended to accept a Context
// parameter).

// 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
}

func (c *cancelCtx) Done() <-chan struct{} {
	// 加锁
	c.mu.Lock()
	// 如果done为空，创建make(chan struct{})
	if c.done == nil {
		c.done = make(chan struct{})
	}
	d := c.done
	c.mu.Unlock()
	return d
}

// 关闭 channel,c.done；递归地取消它的所有子节点；从父节点从删除自己。
func (c *cancelCtx) cancel(removeFromParent bool, err error) {
	if err == nil {
		panic("context: internal error: missing cancel error")
	}
	// 加锁
	c.mu.Lock()
	// 已经取消了
	if c.err != nil {
		c.mu.Unlock()
		return // already canceled
	}
	c.err = err
	// 关闭channel
	// channel没有初始化
	if c.done == nil {
		// 赋值一个关闭的channel,closedchan
		c.done = closedchan
	} else {
		// 初始化了channel，直接关闭
		close(c.done)
	}
	// 递归子节点，一层层取消
	for child := range c.children {
		// NOTE: acquiring the child's lock while holding parent's lock.
		child.cancel(false, err)
	}
	// 将子节点置空
	c.children = nil
	c.mu.Unlock()

	if removeFromParent {
		// 从父节点中移除自己 
		removeChild(c.Context, c)
	}
}

// 从父节点删除context
func removeChild(parent Context, child canceler) {
	p, ok := parentCancelCtx(parent)
	if !ok {
		return
	}
	p.mu.Lock()
	if p.children != nil {
		// 删除child
		delete(p.children, child)
	}
	p.mu.Unlock()
}

// closedchan is a reusable closed channel.
var closedchan = make(chan struct{})

func init() {
	close(closedchan)
}

// broadcastCancel安排在父级被取消时取消子级。
func propagateCancel(parent Context, child canceler) {
	if parent.Done() == nil {
		return // parent is never canceled
	}
	// 找到可以取消的父 context
	if p, ok := parentCancelCtx(parent); ok {
		p.mu.Lock()
		// 父节点取消了
		if p.err != nil {
			// 取消子节点
			child.cancel(false, p.err)
			// 父节点没有取消
		} else {
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			// 挂载
			p.children[child] = struct{}{}
		}
		p.mu.Unlock()
		// 没有找到父节点
	} else {
		// 启动一个新的节点监听父节点和子节点的取消信息
		go func() {
			select {
			// 如果父节点取消了
			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
		}
	}
}

func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
	// 将传入的上下文包装成私有结构体 context.cancelCtx
	c := newCancelCtx(parent)
	// 构建父子上下文之间的关联
	propagateCancel(parent, &c)
	return &c, func() { c.cancel(true, Canceled) }
}

// newCancelCtx returns an initialized cancelCtx.
func newCancelCtx(parent Context) cancelCtx {
	return cancelCtx{Context: parent}
}

type timerCtx struct {
	cancelCtx
	timer *time.Timer // Under cancelCtx.mu.

	deadline time.Time
}

func (c *timerCtx) cancel(removeFromParent bool, err error) {
	// 调用context.cancelCtx.cancel
	c.cancelCtx.cancel(false, err)
	if removeFromParent {
		// Remove this timerCtx from its parent cancelCtx's children.
		removeChild(c.cancelCtx.Context, c)
	}
	c.mu.Lock()
	// 关掉定时器，减少资源浪费
	if c.timer != nil {
		c.timer.Stop()
		c.timer = nil
	}
	c.mu.Unlock()
}

func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
	// 调用WithDeadline，传入时间
	return WithDeadline(parent, time.Now().Add(timeout))
}

func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
	// 判断结束时间，是否到了
	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
		// The current deadline is already sooner than the new one.
		return WithCancel(parent)
	}
	// 构建timerCtx
	c := &timerCtx{
		cancelCtx: newCancelCtx(parent),
		deadline:  d,
	}
	// 构建父子上下文之间的关联
	propagateCancel(parent, c)
	// 计算当前距离 deadline 的时间
	dur := time.Until(d)
	if dur <= 0 {
		c.cancel(true, DeadlineExceeded) // deadline has already passed
		return c, func() { c.cancel(false, Canceled) }
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.err == nil {
		// d 时间后，timer 会自动调用 cancel 函数。自动取消
		c.timer = time.AfterFunc(dur, func() {
			c.cancel(true, DeadlineExceeded)
		})
	}
	return c, func() { c.cancel(true, Canceled) }
}

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)
}

func WithValue(parent Context, key, val interface{}) Context {
	if key == nil {
		panic("nil key")
	}
	if !reflectlite.TypeOf(key).Comparable() {
		panic("key is not comparable")
	}
	return &valueCtx{parent, key, val}
}

func TimeoutCancelContext() {
	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(time.Millisecond*800))
	go func() {
		// 具体的业务逻辑
		// 取消超时
		defer cancel()
	}()

	select {
	case <-ctx.Done():
		fmt.Println("time out!!!")
		return
	}
}