带小伙伴手写 golang context

前言 - context 源码

　　可以先了解官方 context.go 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, 一般用做顶级 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 ❉

 <窗外雨> - https://music.163.com/#/song?id=514543382