带小伙伴手写 golang context

前言 - context 源码

　　可以先了解官方  轮廓. 这里捎带保存一份当前 context 版本备份. 

// copyright 2014 the go authors. all rights reserved.
// use of this source code is governed by a bsd-style
// license that can be found in the license file.

// package context defines the context type, which carries deadlines,
// cancellation signals, and other request-scoped values across api boundaries
// and between processes.
//
// incoming requests to a server should create a context, and outgoing
// calls to servers should accept a context. the chain of function
// calls between them must propagate the context, optionally replacing
// it with a derived context created using withcancel, withdeadline,
// withtimeout, or withvalue. when a context is canceled, all
// contexts derived from it are also canceled.
//
// the withcancel, withdeadline, and withtimeout functions take a
// context (the parent) and return a derived context (the child) and a
// cancelfunc. calling the cancelfunc cancels the child and its
// children, removes the parent's reference to the child, and stops
// any associated timers. failing to call the cancelfunc leaks the
// child and its children until the parent is canceled or the timer
// fires. the go vet tool checks that cancelfuncs are used on all
// control-flow paths.
//
// programs that use contexts should follow these rules to keep interfaces
// consistent across packages and enable static analysis tools to check context
// propagation:
//
// do not store contexts inside a struct type; instead, pass a context
// explicitly to each function that needs it. the context should be the first
// parameter, typically named ctx:
//
// 	func dosomething(ctx context.context, arg arg) error {
// 		// ... use ctx ...
// 	}
//
// do not pass a nil context, even if a function permits it. pass context.todo
// if you are unsure about which context to use.
//
// use context values only for request-scoped data that transits processes and
// apis, not for passing optional parameters to functions.
//
// the same context may be passed to functions running in different goroutines;
// contexts are safe for simultaneous use by multiple goroutines.
//
// see https://blog.golang.org/context for example code for a server that uses
// contexts.
package context

import (
	"errors"
	"internal/reflectlite"
	"sync"
	"time"
)

// a context carries a deadline, a cancellation signal, and other values across
// api boundaries.
//
// context's methods may be called by multiple goroutines simultaneously.
type context interface {
	// deadline returns the time when work done on behalf of this context
	// should be canceled. deadline returns ok==false when no deadline is
	// set. successive calls to deadline return the same results.
	deadline() (deadline time.time, ok bool)

	// done returns a channel that's closed when work done on behalf of this
	// context should be canceled. done may return nil if this context can
	// never be canceled. successive calls to done return the same value.
	//
	// withcancel arranges for done to be closed when cancel is called;
	// withdeadline arranges for done to be closed when the deadline
	// expires; withtimeout arranges for done to be closed when the timeout
	// elapses.
	//
	// done is provided for use in select statements:
	//
	//  // stream generates values with dosomething and sends them to out
	//  // until dosomething returns an error or ctx.done is closed.
	//  func stream(ctx context.context, out chan<- value) error {
	//  	for {
	//  		v, err := dosomething(ctx)
	//  		if err != nil {
	//  			return err
	//  		}
	//  		select {
	//  		case <-ctx.done():
	//  			return ctx.err()
	//  		case out <- v:
	//  		}
	//  	}
	//  }
	//
	// see https://blog.golang.org/pipelines for more examples of how to use
	// a done channel for cancellation.
	done() <-chan struct{}

	// if done is not yet closed, err returns nil.
	// if done is closed, err returns a non-nil error explaining why:
	// canceled if the context was canceled
	// or deadlineexceeded if the context's deadline passed.
	// after err returns a non-nil error, successive calls to err return the same error.
	err() error

	// value returns the value associated with this context for key, or nil
	// if no value is associated with key. successive calls to value with
	// the same key returns the same result.
	//
	// use context values only for request-scoped data that transits
	// processes and api boundaries, not for passing optional parameters to
	// functions.
	//
	// a key identifies a specific value in a context. functions that wish
	// to store values in context typically allocate a key in a global
	// variable then use that key as the argument to context.withvalue and
	// context.value. a key can be any type that supports equality;
	// packages should define keys as an unexported type to avoid
	// collisions.
	//
	// packages that define a context key should provide type-safe accessors
	// for the values stored using that key:
	//
	// 	// package user defines a user type that's stored in contexts.
	// 	package user
	//
	// 	import "context"
	//
	// 	// user is the type of value stored in the contexts.
	// 	type user struct {...}
	//
	// 	// key is an unexported type for keys defined in this package.
	// 	// this prevents collisions with keys defined in other packages.
	// 	type key int
	//
	// 	// userkey is the key for user.user values in contexts. it is
	// 	// unexported; clients use user.newcontext and user.fromcontext
	// 	// instead of using this key directly.
	// 	var userkey key
	//
	// 	// newcontext returns a new context that carries value u.
	// 	func newcontext(ctx context.context, u *user) context.context {
	// 		return context.withvalue(ctx, userkey, u)
	// 	}
	//
	// 	// fromcontext returns the user value stored in ctx, if any.
	// 	func fromcontext(ctx context.context) (*user, bool) {
	// 		u, ok := ctx.value(userkey).(*user)
	// 		return u, ok
	// 	}
	value(key interface{}) interface{}
}

// canceled is the error returned by context.err when the context is canceled.
var canceled = errors.new("context canceled")

// deadlineexceeded is the error returned by context.err when the context's
// deadline passes.
var deadlineexceeded error = deadlineexceedederror{}

type deadlineexceedederror struct{}

func (deadlineexceedederror) error() string   { return "context deadline exceeded" }
func (deadlineexceedederror) timeout() bool   { return true }
func (deadlineexceedederror) temporary() bool { return true }

// 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)
)

// 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
}

// 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
}

// a cancelfunc tells an operation to abandon its work.
// a cancelfunc does not wait for the work to stop.
// after the first call, subsequent calls to a cancelfunc do nothing.
type cancelfunc func()

// 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) }
}

// newcancelctx returns an initialized cancelctx.
func newcancelctx(parent context) cancelctx {
	return cancelctx{context: parent}
}

// propagatecancel arranges for child to be canceled when parent is.
func propagatecancel(parent context, child canceler) {
	if parent.done() == nil {
		return // parent is never canceled
	}
	if p, ok := parentcancelctx(parent); ok {
		p.mu.lock()
		if p.err != nil {
			// parent has already been canceled
			child.cancel(false, p.err)
		} else {
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			p.children[child] = struct{}{}
		}
		p.mu.unlock()
	} else {
		go func() {
			select {
			case <-parent.done():
				child.cancel(false, parent.err())
			case <-child.done():
			}
		}()
	}
}

// parentcancelctx follows a chain of parent references until it finds a
// *cancelctx. this function understands how each of the concrete types in this
// package represents its parent.
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
		}
	}
}

// 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()
}

// 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{}
}

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

func init() {
	close(closedchan)
}

// 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{} {
	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
}

type stringer interface {
	string() string
}

func contextname(c context) string {
	if s, ok := c.(stringer); ok {
		return s.string()
	}
	return reflectlite.typeof(c).string()
}

func (c *cancelctx) string() string {
	return contextname(c.context) + ".withcancel"
}

// 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)
	}
	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)
	}
}

// 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 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 passed
		return 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) }
}

// 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) {
	c.cancelctx.cancel(false, err)
	if removefromparent {
		// remove this timerctx from its parent cancelctx's children.
		removechild(c.cancelctx.context, c)
	}
	c.mu.lock()
	if c.timer != nil {
		c.timer.stop()
		c.timer = nil
	}
	c.mu.unlock()
}

// 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))
}

// 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 {
	if key == nil {
		panic("nil key")
	}
	if !reflectlite.typeof(key).comparable() {
		panic("key is not comparable")
	}
	return &valuectx{parent, key, val}
}

// 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{}
}

// stringify tries a bit to stringify v, without using fmt, since we don't
// want context depending on the unicode tables. this is only used by
// *valuectx.string().
func stringify(v interface{}) string {
	switch s := v.(type) {
	case stringer:
		return s.string()
	case string:
		return s
	}
	return "<not stringer>"
}

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{} {
	if c.key == key {
		return c.val
	}
	return c.context.value(key)
}　　

golang 标准库 1.7 版本引入 context 包, 用于 golang 函数链安全的管理和控制.  　

说真 golang context 实现非常漂亮, 代码中说明也事无巨细, 整体很赏心悦目.   

那我们带大家宏观过一遍 context 设计布局. 

// context 上下文调用链条
type context interface {
	// deadline 返回是否完成和结束时刻
	deadline() (deadline time.time, ok bool)

	// done 返回是否完成的阻塞 chan
	done() (done <-chan struct{})

	// err done 时候储存 error 信息, canceled or deadlineexceeded
	err() (err error)

	// value context 中获取 key 的值
	value(key interface{}) (val interface{})
}

// emptyctx 永远不会被取消的 context
type emptyctx int

// cancelctx 可被取消的 context
type cancelctx struct { context ... }

// timerctx 带计时器和截止日期的 cancelctx
type timerctx struct { cancelctx ... }

// valuectx 储存键值对 context
type valuectx struct {
	context

	key, val interface{}
}

可以看出 context 是个 interface, context.go 中有四种结构实现了 context interface, 分别

是 emptyctx, cancelctx, timerctx, valuectx. 细看这类数据结构设计思路, 只记录父亲是谁,

单线联系. 可以反着类比普通树结构只记录儿子是谁哈哈, 大众货居然被玩出❀, 真强.

 

正文 - context 手写

　　废话不多说, 开始写代码. 上面 context.go 文件一把梭哈, 对于初学者还是无从学起的. 我们

这里按照结构分模式展开书写, 

├── context
│   ├── cancel.go
│   ├── context.go
│   ├── empty.go
│   ├── timer.go
│   ├── value.go
│   └── with.go

让其看起来好理解些. 

context.go 

package context

import (
	"errors"
	"time"
)

// canceled 取消上下文时 context.err 返回的错误
var canceled = errors.new("context canceled")

// deadlineexceeded 上下文超时时候 context.err 返回的错误
var deadlineexceeded error = deadlineexceedederror{}

type deadlineexceedederror struct{}

func (deadlineexceedederror) error() string   { return "context deadline exceeded" }
func (deadlineexceedederror) timeout() bool   { return true }
func (deadlineexceedederror) temporary() bool { return true }

// context 上下文调用链条
type context interface {
	// deadline 返回是否完成和结束时刻
	deadline() (deadline time.time, ok bool)

	// done 返回是否完成的阻塞 chan
	done() (done <-chan struct{})

	// err done 时候储存 error 信息, canceled or deadlineexceeded
	err() (err error)

	// value context 中获取 key 的值
	value(key interface{}) (val interface{})
}

empty.go

package context

import "time"

// emptyctx 永远不会被取消的 context
type emptyctx int

func (*emptyctx) deadline() (deadline time.time, ok bool) { return }
func (*emptyctx) done() (done <-chan struct{})            { return }
func (*emptyctx) err() (err error)                        { return }
func (*emptyctx) value(key interface{}) (val interface{}) { return }

var (
	background = new(emptyctx)
	todo       = new(emptyctx)
)

// string emptyctx 打印方法
func (e *emptyctx) string() string {
	switch e {
	case background:
		return "context.background"
	case todo:
		return "context.todo"
	}
	return "unknown empty context"
}

// background 不会被取消的 context, 一般用做* context
func background() context {
	return background
}

// todo 当你不知道用什么 context 时候, 记住那就用这个
func todo() context {
	return todo
}

cancel.go 

package context

import (
	"reflect"
	"sync"
)

type stringer interface {
	string() string
}

func contextname(c context) string {
	if s, ok := c.(stringer); ok {
		return s.string()
	}
	return reflect.typeof(c).string()
}

// canceler 可以直接取消的上下文, 详情见 *cancelctx 和 *timerctx
type canceler interface {
	cancel(removefromparent bool, err error)
	done() (done <-chan struct{})
}

// cancelctx 可被取消的 context
type cancelctx struct {
	context

	mu       sync.mutex            // 互斥锁保证 goroutine 安全
	done     chan struct{}         // 慢创建, 首次取消才会被调用的开关
	children map[canceler]struct{} // 首次 context 取消后待取消的 child context
	err      error                 // 首次取消 context 保留的 error
}

func (c *cancelctx) done() (done <-chan struct{}) {
	c.mu.lock()
	if c.done == nil {
		c.done = make(chan struct{})
	}
	done = c.done
	c.mu.unlock()
	return
}

func (c *cancelctx) err() (err error) {
	c.mu.lock()
	err = c.err
	c.mu.unlock()
	return
}

func (c *cancelctx) string() string {
	return contextname(c) + ".withcancel"
}

// newcancelctx returns an initialized cancelctx
func newcancelctx(parent context) cancelctx {
	return cancelctx{context: parent}
}

(这里用 "reflect" 标准包来表述 "internal/reflectlite" 内部才能使用的 "reflect" 包)

timer.go

package context

import (
	"time"
)

// timerctx 带计时器和截止日期的 cancelctx
type timerctx struct {
	cancelctx

	timer    *time.timer // 依赖 cancelctx.mu
	deadline time.time   // context 截止时间
}

func (c *timerctx) deadline() (time.time, bool) {
	return c.deadline, true
}

func (c *timerctx) string() string {
	return contextname(c.cancelctx.context) + ".withdeadline(" + c.deadline.string() + " [" + time.until(c.deadline).string() + "])"
}

value.go

package context

import "reflect"

// valuectx 储存键值对 context
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)
}

// stringify tries a bit to stringify v, without using fmt, since we don't
// want context depending on the unicode tables. this is only used by *valuectx.string()
func stringify(v interface{}) string {
	switch s := v.(type) {
	case stringer:
		return s.string()
	case string:
		return s
	}
	return "<not stringer>"
}

func (c *valuectx) string() string {
	return contextname(c.context) + ".withvalue(type " + reflect.typeof(c.key).string() + ", val " + stringify(c.val) + ")"
}

with.go

package context

import (
	"reflect"
	"time"
)

// withvalue returns a copy of parent in which the value associated with key is val
func withvalue(parent context, key, val interface{}) context {
	if key == nil {
		panic("nil key")
	}

	// key 不具备可比性时候, 会引发运行时恐慌 panic
	if !reflect.typeof(key).comparable() {
		panic("key is not comparable")
	}

	return &valuectx{parent, key, val}
}

// parentcancelctx 获取 parent cancelctx
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
		}
	}
}

// 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()
}

// closedchan 可重复使用且已经关闭通道
var closedchan = make(chan struct{})

func init() {
	close(closedchan)
}

// cancel cancelctx 取消操作, 关闭 c.done, 取消每个 child context, removefromparent is true 从 parent 删除 child
func (c *cancelctx) cancel(removefromparent bool, err 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)
	}
	for child := range c.children {
		// note: 保留 parent 锁, 继续获取 child 锁
		child.cancel(false, err)
	}
	c.children = nil
	c.mu.unlock()

	if removefromparent {
		removechild(c.context, c)
	}
}

// cancel timerctx 取消操作
func (c *timerctx) cancel(removefromparent bool, err error) {
	c.cancelctx.cancel(false, err)
	if removefromparent {
		// remove this timerctx from its parent cancelctx's children
		removechild(c.cancelctx.context, c)
	}

	c.mu.lock()
	if c.timer != nil {
		c.timer.stop()
		c.timer = nil
	}
	c.mu.unlock()
}

// propagatecancel parent 取消 map 中添加 child 子项
func propagatecancel(parent context, child canceler) {
	if parent.done() == nil {
		return // parent is never canceled
	}

	if p, ok := parentcancelctx(parent); ok {
		p.mu.lock()
		if p.err != nil {
			// parent has already been canceled
			child.cancel(false, p.err)
		} else {
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			p.children[child] = struct{}{}
		}
		p.mu.unlock()
	} else {
		go func() {
			select {
			case <-parent.done():
				child.cancel(false, parent.err())
			case <-child.done():
			}
		}()
	}
}

// cancelfunc cancel func 行为
type cancelfunc func()

// withcancel 基于 parent context 构造可取消的 child context
func withcancel(parent context) (ctx context, cancel cancelfunc) {
	c := newcancelctx(parent)
	propagatecancel(parent, &c)
	return &c, func() { c.cancel(true, canceled) }
}

// withdeadline 返回 child context 并调整 parent deadline
func withdeadline(parent context, d time.time) (context, cancelfunc) {
	if cur, ok := parent.deadline(); ok && cur.before(d) {
		// 当前截止日期早于新设置的截止日期, 直接使用当前截止日期
		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 passed
		return 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) }
}

// withtimeout returns withdeadline(parent, time.now().add(timeout))
func withtimeout(parent context, timeout time.duration) (context, cancelfunc) {
	return withdeadline(parent, time.now().add(timeout))
}　

看到 with.go 是不是有种 mmp 感觉, 第一问还好, 第二问也爽爽, 这第三问怎么就有了压轴最后一问思索感 ~ 

其实还好, 多关怀下 propagatecancel 和 cancelctx.cancel 操作怎么打配合.  其实上面源码中最难写的是

reflect 和 time (errors 和 sync 接触多好理解) 有兴趣的同行可以深入研究 . 水文该说再见了, 希望大家有

所得 ~ 

后记 - 代码和注释并存

　　错误是难免的, 欢迎勘误 ~ 共同成长提高 ❉

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