go Context

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// +build go1.9
package context

import "context"

type Context = context.Context
type CancelFunc = context.CancelFunc

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

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

func Background() Context {
    return background
}

func TODO() Context {
    return todo
}

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

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ctx = context.TODO()
ctx = context.WithValue(ctx, "key1", "0001")
ctx = context.WithValue(ctx, "key2", "0001")
ctx = context.WithValue(ctx, "key3", "0001")
ctx = context.WithValue(ctx, "key4", "0004")

fmt.Println(ctx.Value("key1"))

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

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

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var Canceled = errors.New("context canceled")

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

    go func() {
        defer func() {
            fmt.Println("goroutine exit")
        }()

        for {
            select {
            case <-ctx.Done():
                return
            default:
                time.Sleep(time.Second)
            }
        }
    }()

    time.Sleep(time.Second)
    cancel()
    time.Sleep(2 * time.Second)
}

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func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
    // 当前时间+timeout就是deadline
    return WithDeadline(parent, time.Now().Add(timeout))
}

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func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
    // 如果parent的截止时间更早，直接返回一个cancelCtx即可
    if cur, ok := parent.Deadline(); ok && cur.Before(d) {
        return WithCancel(parent)
    }
    c := &timerCtx{
        cancelCtx: newCancelCtx(parent),
        deadline:  d,
    }
    propagateCancel(parent, c) // 同cancelCtx的处理逻辑
    dur := time.Until(d)
    if dur <= 0 { //当前时间已经超过了截止时间，直接cancel
        c.cancel(true, DeadlineExceeded)
        return c, func() { c.cancel(false, Canceled) }
    }
    c.mu.Lock()
    defer c.mu.Unlock()
    if c.err == nil {
        // 设置一个定时器，到截止时间后取消
        c.timer = time.AfterFunc(dur, func() {
            c.cancel(true, DeadlineExceeded)
        })
    }
    return c, func() { c.cancel(true, Canceled) }
}

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func slowOperationWithTimeout(ctx context.Context) (Result, error) {
  ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
  defer cancel() // 一旦慢操作完成就立马调用cancel
  return slowOperation(ctx)
}

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type emptyCtx int

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
}

type canceler interface {
	cancel(removeFromParent bool, err error)
	Done() <-chan struct{}
}

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

	deadline time.Time
}

type valueCtx struct {
	Context
	key, 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 WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
    c := newCancelCtx(parent)
    propagateCancel(parent, &c)// 把c朝上传播
    return &c, func() { c.cancel(true, Canceled) }
}

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

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func (c *valueCtx) Value(key interface{}) interface{} {
	if c.key == key {
		return c.val
	}
    // 除了 emptyCtx 所有类型的 Value() 方法都会调用 c.Context.Value 向上查找，所以会递归向上查找
	return c.Context.Value(key)
}

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var cancelCtxKey int
var closedchan = make(chan struct{})

func (c *cancelCtx) Value(key interface{}) interface{} {
	if key == &cancelCtxKey {
		return c
	}
	return c.Context.Value(key)
}

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

func propagateCancel(parent Context, child canceler) {
    // 1. 
    // emptyCtx.Done 返回 nil
    // valueCtx 的 Done 调用的是 parent.Done() 如果 调用链上一直是 valueCtx 就会一直递归向上调用
    // cancleCtx 和 timerCtx Done() 返回的是 非 nil
    // 综上所述，propagateCancel 方法会顺着 parent 路径往上找，直到找到一个 cancelCtx 并返回它的 done，或者为 nil
	done := parent.Done()
	if done == nil {
		return // parent is never canceled
	}

	select {
    // parent 已经被 cancle 了，直接 cancle
	case <-done:
		// parent is already canceled
		child.cancel(false, parent.Err())
		return
	default:
	}
    // 2. 沿着 Context 链，向上查找第一个 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 {
            // 4.
            // 如果不为空，就把自己加入到这个 cancelCtx 的 child
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			p.children[child] = struct{}{}
		}
		p.mu.Unlock()
	} else {
        // 5.
		// 如果为空，会新起一个 goroutine，由它来监听 parent 的 Done 是否已关闭
		atomic.AddInt32(&goroutines, +1)
		go func() {
			select {
			case <-parent.Done():
				child.cancel(false, parent.Err())
			case <-child.Done():
			}
		}()
	}
}

// parentCancelCtx returns the underlying *cancelCtx for parent.
// It does this by looking up parent.Value(&cancelCtxKey) to find
// the innermost enclosing *cancelCtx and then checking whether
// parent.Done() matches that *cancelCtx. (If not, the *cancelCtx
// has been wrapped in a custom implementation providing a
// different done channel, in which case we should not bypass it.)
func parentCancelCtx(parent Context) (*cancelCtx, bool) {
	done := parent.Done()
	if done == closedchan || done == nil {
		return nil, false
	}
    // 3. 
    // 查找的方法很简单，cancelCtx.Value() 方法里面会判断 key== &cancelCtxKey
    // 因为只有 cancelCtx 会这么判断，所以判断成功肯定是 cancelCtx
    // 除了 emptyCtx 所有类型的 Value() 方法都会调用 c.Context.Value 向上查找，所以会递归向上查找
	p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
	if !ok {
		return nil, false
	}
    // 判断到此处 p 已经是一个 cancelCtx 了
	p.mu.Lock()
    // 7. context 的 value 和 done 方法都会沿着 Context 链向上查找，所以 p.done 应该等于 done
    // 如果不相等，说明 cancleCtx 的 done 对应的 channel 值被更改过
	ok = p.done == done
	p.mu.Unlock()
	if !ok {
		return nil, false
	}
	return p, true
}

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// 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)
	}

    // 递归 cancel 子 cancelCtx
	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 {
        // 把当前 Context 从父 cancelCtx 的 childrens 中删除
		removeChild(c.Context, c)
	}
}

// removeChild removes a context from its parent.
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()
}