Go与Context

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

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

// A canceler is a context type that can be canceled directly. The
// implementations are *cancelCtx and *timerCtx.
type canceler interface {
	cancel(removeFromParent bool, err error)
	Done() <-chan struct{}
}

// 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
}

// &cancelCtxKey is the key that a cancelCtx returns itself for.
var cancelCtxKey int

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 (c *cancelCtx) Err() error {
	c.mu.Lock()
	err := c.err
	c.mu.Unlock()
	return err
}

func (c *cancelCtx) String() string {
	return contextName(c.Context) + ".WithCancel"
}

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

func init() {
	close(closedchan)
}

// cancel closes c.done, cancels each of c's children, and, if
// removeFromParent is true, removes c from its parent's children.
// 取消
func (c *cancelCtx) cancel(removeFromParent bool, err error) {
  // 如果传入的错误为空，则返回 panic
	if err == nil {
		panic("context: internal error: missing cancel error")
	}
  // 如果上下文的错误不为空，说明已经取消过，直接返回即可
	c.mu.Lock()
	if c.err != nil {
		c.mu.Unlock()
		return // already canceled
	}
  // 将错误赋值到上下文
	c.err = err
  // 由于 done 是在调用 Done 的时候才创建，所以此处 done 有可能为 nil
  // 如果是 nil，则将 done 赋值为一个已经关闭的 channel
  // 如果不是 nil，则关闭这个 channel
	if c.done == nil {
		c.done = closedchan
	} else {
		close(c.done)
	}
  // 依次取消子级
	for child := range c.children {
		// NOTE: acquiring the child's lock while holding parent's lock.
		child.cancel(false, err)
	}
  // 清除子级 map
	c.children = nil
	c.mu.Unlock()
  
	// 如果需要从父级移除，则调用 removeChild 从父级中清理掉自己
	if removeFromParent {
		removeChild(c.Context, c)
	}
}

// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
// implement Done and Err. It implements cancel by stopping its timer then
// delegating to cancelCtx.cancel.
type timerCtx struct {
	cancelCtx
	timer *time.Timer // Under cancelCtx.mu.

	deadline time.Time
}

func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
	return c.deadline, true
}

func (c *timerCtx) String() string {
	return contextName(c.cancelCtx.Context) + ".WithDeadline(" +
		c.deadline.String() + " [" +
		time.Until(c.deadline).String() + "])"
}

func (c *timerCtx) cancel(removeFromParent bool, err error) {
  // 调用 cancelCtx 的 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()
  // 如果 timer 不为 nil，则停止计时器
	if c.timer != nil {
		c.timer.Stop()
		c.timer = nil
	}
	c.mu.Unlock()
}

// A valueCtx carries a key-value pair. It implements Value for that key and
// delegates all other calls to the embedded Context.
type valueCtx struct {
	Context
	key, val interface{}
}

func (c *valueCtx) String() string {
	return contextName(c.Context) + ".WithValue(type " +
		reflectlite.TypeOf(c.key).String() +
		", val " + stringify(c.val) + ")"
}

func (c *valueCtx) Value(key interface{}) interface{} {
  // 如果传入的 key 与上下文的 key 相同，则返回值
	if c.key == key {
		return c.val
	}
  // 如果不相同，则从父级查询
	return c.Context.Value(key)
}

var (
	background = new(emptyCtx)
)

// 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.
func Background() Context {
	return background
}

var (
	todo       = new(emptyCtx)
)

// 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).
func TODO() Context {
	return todo
}

// WithValue returns a copy of parent in which the value associated with key is
// val.
//
// Use context Values only for request-scoped data that transits processes and
// APIs, not for passing optional parameters to functions.
//
// The provided key must be comparable and should not be of type
// string or any other built-in type to avoid collisions between
// packages using context. Users of WithValue should define their own
// types for keys. To avoid allocating when assigning to an
// interface{}, context keys often have concrete type
// struct{}. Alternatively, exported context key variables' static
// type should be a pointer or interface.
func WithValue(parent Context, key, val interface{}) Context {
  // 检测父级
	if parent == nil {
		panic("cannot create context from nil parent")
	}
  // 检测key
	if key == nil {
		panic("nil key")
	}
  // 检测key是否是可比较类型
	if !reflectlite.TypeOf(key).Comparable() {
		panic("key is not comparable")
	}
  // 基于父级创建 valueCtx
	return &valueCtx{parent, key, val}
}

// 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) {
	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}
}

// propagateCancel arranges for child to be canceled when parent is.
// 传播取消，负责保证在父级取消的时候，可以正确传播到子级
func propagateCancel(parent Context, child canceler) {
  // 此处的 parent 可以是任意类型的上下文，如果是 emptyCtx 类型，则 parent.Done() 返回的永远是 nil
  // 如果是 cancelCtx 或者 timerCtx，返回的不会是 nil
  // 如果不为 nil，说明永远不会取消，也就不需要一层一层通知子级取消
	done := parent.Done()
	if done == nil {
		return // parent is never canceled
	}

  // 判断 done channel 是否可以读取，可以读取则表明父级已取消，则需要取消子级
	select {
	case <-done:
		// parent is already canceled
		child.cancel(false, parent.Err())
		return
	default:
	}

  // 获取父级的底层cancelCtx
	if p, ok := parentCancelCtx(parent); ok {
    // 如果获取到，则进行取消
		p.mu.Lock()
		if p.err != nil {
			// parent has already been canceled
      // 如果父级已经取消，则直接取消子级即可
			child.cancel(false, p.err)
		} else {
      // 父级没有取消
      // 如果子级为 nil，则需要在父级的 children 里关联子级
      // 这样当父级取消的时候，会依次取消父级 children 里面关联的所有子级
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			p.children[child] = struct{}{}
		}
		p.mu.Unlock()
	} else {
    // 这里有多种情况：
    // 	1. 父级已取消
    //  2. 父级永远不会取消
    //  3. 无法转换为标准 cancelCtx
    //  4. 是一个自定义 Done 的 cancelCtx
    // 前两种情况在函数开头已经判断处理，此处无需处理
    // 后两种情况无法通过父级的 children map 来取消，因此启动一个 Goroutine 来处理
		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.)
// 获取父级的底层cancelCtx
func parentCancelCtx(parent Context) (*cancelCtx, bool) {
  // 如果父级的 done channel 为空，说明不支持 cancel
  // 如果父级的 done channel 为 closedchan，说明父级已经取消
	done := parent.Done()
	if done == closedchan || done == nil {
		return nil, false
	}
  // 判断父级是否可以转换为 cancelCtx，如果无法转为，则返回 nil 和 false
	p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
	if !ok {
		return nil, false
	}
  // 如果可以转换，说明父级是 cancelCtx 类型或其衍生出来的上下文，比如 timerCtx 或者用户自定义的 cancelCtx
  // 需要进一步判断 父级与提取出来的上下文的 done channel 是否一致
  // 如果一致，说明就是标准的 cancelCtx 或 timerCtx，或自定义 cancelCtx 但没有重写 Done()
  // 如果不一致，说明是自定义的 cancelCtx 并且 重写了 Done()，这种情况需要单独处理
	p.mu.Lock()
	ok = p.done == done
	p.mu.Unlock()
	if !ok {
		return nil, false
	}
	return p, true
}

// WithDeadline returns a copy of the parent context with the deadline adjusted
// to be no later than d. If the parent's deadline is already earlier than d,
// WithDeadline(parent, d) is semantically equivalent to parent. The returned
// context's Done channel is closed when the deadline expires, 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 WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
	if parent == nil {
		panic("cannot create context from nil parent")
	}
  // 判断父级是否支持 Deadline，如果支持并且截止时间早于传入的时间，意味着父级一定会在当前要创建的上下文前取消
  // 这样，当父级被取消的时候，其关联的子级也会自动被取消
 	// 所以此处返回一个 cancelCtx 就可以了
	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
		// The current deadline is already sooner than the new one.
		return WithCancel(parent)
	}
  // 创建一个 timerCtx
	c := &timerCtx{
		cancelCtx: newCancelCtx(parent),
		deadline:  d,
	}
  // 传播取消
	propagateCancel(parent, c)
  // 判断当前时间是否已经过了传入的时间，如果已经过了，则直接取消
	dur := time.Until(d)
	if dur <= 0 {
		c.cancel(true, DeadlineExceeded) // deadline has already passed
		return c, func() { c.cancel(false, Canceled) }
	}
  // 当前时间未到截止时间，则创建一个定时器，在定时器到时间收自动 cancel
	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) }
}

// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
//
// Canceling this context releases resources associated with it, so code should
// call cancel as soon as the operations running in this Context complete:
//
// 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
// 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
// 		defer cancel()  // releases resources if slowOperation completes before timeout elapses
// 		return slowOperation(ctx)
// 	}
func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
	return WithDeadline(parent, time.Now().Add(timeout))
}

package main

import (
	"context"
	"fmt"
	"time"
)

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	go Test(ctx)
	time.Sleep(10 * time.Second)
	cancel()
	time.Sleep(1 * time.Second)
}

func Test(ctx context.Context) {
	for range time.Tick(time.Second) {
		select {
		case <-ctx.Done():
			log("context is canceled")
			return
		default:
			log("hahaha")
		}
	}
}

func log(msgs ...interface{}) {
	fmt.Println(time.Now().Format("2006-01-02 15:04:05"), msgs)
}

2022-05-07 10:02:33 [hahaha]
2022-05-07 10:02:34 [hahaha]
2022-05-07 10:02:35 [hahaha]
2022-05-07 10:02:36 [hahaha]
2022-05-07 10:02:37 [hahaha]
2022-05-07 10:02:38 [hahaha]
2022-05-07 10:02:39 [hahaha]
2022-05-07 10:02:40 [hahaha]
2022-05-07 10:02:41 [hahaha]
2022-05-07 10:02:42 [context is canceled]

package main

import (
	"context"
	"fmt"
	"time"
)

func main() {
	ctx, cancel := context.WithDeadline(context.Background(), time.Now().Add(10*time.Second))
	defer cancel()
	Test(ctx)
}

func Test(ctx context.Context) {
	for range time.Tick(time.Second) {
		select {
		case <-ctx.Done():
			log("context is canceled, err: ", ctx.Err())
			return
		default:
			log("hahaha")
		}
	}
}

func log(msgs ...interface{}) {
	fmt.Println(time.Now().Format("2006-01-02 15:04:05"), msgs)
}

2022-05-07 12:23:32 [hahaha]
2022-05-07 12:23:33 [hahaha]
2022-05-07 12:23:34 [hahaha]
2022-05-07 12:23:35 [hahaha]
2022-05-07 12:23:36 [hahaha]
2022-05-07 12:23:37 [hahaha]
2022-05-07 12:23:38 [hahaha]
2022-05-07 12:23:39 [hahaha]
2022-05-07 12:23:40 [hahaha]
2022-05-07 12:23:41 [hahaha]
2022-05-07 12:23:42 [context is canceled, err:  context deadline exceeded]

# WithDeadline
ctx, cancel := context.WithDeadline(context.Background(), time.Now().Add(10*time.Second))
# WithTimeout
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)

package main

import (
	"context"
	"errors"
	"fmt"
)

type testKey string

func main() {
	ctx := context.WithValue(context.Background(), testKey("a"), 1)
	fmt.Println(GetValue(ctx, testKey("a")))
	fmt.Println(GetValue(ctx, testKey("b")))
}

// GetValue 获取Value
func GetValue(ctx context.Context, key testKey) (interface{}, error) {
	v := ctx.Value(key)
	if v == nil {
		return nil, errors.New(fmt.Sprintf("value of %s not found", key))
	}
	return v, nil
}

1 <nil>
<nil> value of b not found