context主要用于跨多个Goroutine设置截止时间、同步信号、传递上下文请求值，没了解过Context的先看看这个Golang Context 源码分析
源码
// 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"
"sync/atomic"
"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.
// Context接口定义了四个方法
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.
// 获取截止时间
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.
// The close of the Done channel may happen asynchronously,
// after the cancel function returns.
//
// 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.
// 获取信号通道，用于判断父Context是否已取消
// <-chan struct{} 这里struct{}是常用的占位手法，不占用内存空间
// 因为是只读通道，当该通道关闭时，所有的子Context就可以结合select来及时获得通知，从而达到层层广播的效果，类似多米诺骨牌
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.
// 通道取消的错误信息，用于区分是哪种原因取消了
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
// }
// 获取key对应的value值
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.
// 空的context
// 不会过期，没有截止时间，没有<k, v>
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.
// 空Context的别名，用于构造所有context的根
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).
// 空Context的别名，用于占位
// 比如你现在不知道要传什么context，就可以用TODO来占位
func TODO() Context {
return todo
}
// A CancelFunc tells an operation to abandon its work.
// A CancelFunc does not wait for the work to stop.
// A CancelFunc may be called by multiple goroutines simultaneously.
// After the first call, subsequent calls to a CancelFunc do nothing.
// 定义取消操作
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.
// 构建可取消的Context
func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
// 父Context不能为nil，这也为什么要对外提供Background的原因
if parent == nil {
panic("cannot create context from nil parent")
}
// 新建一个cancelCtx
c := newCancelCtx(parent)
// 将新建的cancelCtx向上挂靠到最近的可取消父Context
// 对于WithCancel来说，这里有一个隐藏约定，就是所有的可取消的Context的Done()方法返回的channel都不会是nil，而所有不可取消的都是nil
propagateCancel(parent, &c)
// 返回Context和对应的cancel方法
return &c, func() { c.cancel(true, Canceled) }
}
// newCancelCtx returns an initialized cancelCtx.
// 新建并返回cancelCtx
func newCancelCtx(parent Context) cancelCtx {
return cancelCtx{Context: parent}
}
// goroutines counts the number of goroutines ever created; for testing.
// 测试用例使用，不用关心
var goroutines int32
// propagateCancel arranges for child to be canceled when parent is.
// 挂靠可取消父Context
// 该方法接受两个接口类型的参数，一个是Context，一个是canceler
// 因为取消Context的方式有多种，目前context包默认实现了两种
// canceler接口包含了不可导出的cancel方法，所以用户是无法自己实现canceler接口的
// 所以该方法接收canceler接口类型而不是具体的结构体比如cancelCtx
func propagateCancel(parent Context, child canceler) {
// 获取父Context的取消通道
// 因为Context以组合的方式来层层嵌套
// 所以调用Done()方法也会逆序层层检查并调用
// 当done == nil的时候，说明整个Context树都没有可取消的Context (上面说的约定)
done := parent.Done()
if done == nil {
return // parent is never canceled
}
// 这里判断parent是否已经取消，如果取消，则立刻调用child的cancel方法执行对应取消操作
select {
case <-done:
// parent is already canceled
// 注意这里传的第一个参数是false，代表不需要从parent的child集合中删除该child，理由很简单，因为还没挂靠上去
// 具体可见cancel方法的实现
child.cancel(false, parent.Err())
return
default:
}
// 是否找到可挂靠的cancelCtx
// 这里需要做区分，如果是cancelCtx或者cancelCtx的包装类型且没有重写更改Done()和Value()方法和对应的动作，就可以通过child集合的方式进行挂靠
// 否则就只能起个协程，以监听通道信号的方式进行挂靠
if p, ok := parentCancelCtx(parent); ok {
// 加锁
// 这里加锁粒度要注意，因为考虑并发情况，判断p.err的时候就需要加锁，而且p.err设置只可能是在取消的时候且必须加锁，这里一旦加了锁，那么p.err就无法被设置，也就无法变更了
p.mu.Lock()
if p.err != nil {
// parent has already been canceled
child.cancel(false, p.err)
} else {
if p.children == nil {
// 延迟初始化，同时对p.children的读写操作都要加锁
// 这里又是struct{}的妙用，通过map实现了集合
p.children = make(map[canceler]struct{})
}
p.children[child] = struct{}{}
}
p.mu.Unlock()
} else {
// 什么情况下认为有可取消Context但是没找到cancelCtx呢
// 当用户自定义Done()或者Value()并且改变了原来的行为时，可能就会导致，具体看后面例子
atomic.AddInt32(&goroutines, +1) //测试用的，不用关心
go func() { // 开个协程，监听父Context和自己的取消信号
select {
case <-parent.Done():
child.cancel(false, parent.Err())
case <-child.Done(): //这里也要监听自己的，因为有可能child自动取消了，比如定时器Context
}
}()
}
}
// &cancelCtxKey is the key that a cancelCtx returns itself for.
// 这是一个标识，用于复用Value()方法来找到cancelCtx
// 注意这是一个不可导出的变量
var cancelCtxKey int
// 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.)
// 找到父cancelCtx
func parentCancelCtx(parent Context) (*cancelCtx, bool) {
// 获得信号通道
done := parent.Done()
// 这里没有像propagateCancel方法使用select来判断是否关闭，也没有执行cancel操作，这个会向上抛给propagateCancel方法，通过case <-parent.Done():来操作cancel
// 因为这里明确就是找到cancelCtx，而cancelCtx一旦取消了，done肯定就是closedchan
// 而且这里又判断了一次done == nil是因为还有removeChild方法调用了该方法
if done == closedchan || done == nil {
return nil, false
}
// 只有cancelCtx实现的Value方法，才能通过Value(&cancelCtxKey)拿到自身
p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
if !ok {
return nil, false
}
// 这里需要做进一步判断，因为通过value找到的Context的Done()方法可能被用户自定义覆盖了，这就不能按照cancelCtx来处理，因为其对应的cancel操作是会操作cancelCtx.done的，而这个done跟Done()方法返回的可能不是同一个，就会造成取消行为的歧义
// 同时加了一把锁
p.mu.Lock()
ok = p.done == done
p.mu.Unlock()
if !ok {
return nil, false
}
return p, true
}
// removeChild removes a context from its parent.
// 从parent的child集合中删除自己
func removeChild(parent Context, child canceler) {
// 找到之前挂载的父Context
p, ok := parentCancelCtx(parent)
if !ok {
return
}
// 修改行为，加锁
p.mu.Lock()
// 有可能在加锁之前，父Context执行了取消，p.children == nil
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.
// 取消接口
// 注意这里定义的cancel方法是不可导出的，不支持用户自定义
type canceler interface {
cancel(removeFromParent bool, err error)
Done() <-chan struct{}
}
// closedchan is a reusable closed channel.
// 可重用的已关闭的channel
var closedchan = make(chan struct{})
// init()方法，关闭closedchan
// 该方法在调用其他方法之前已执行
func init() {
close(closedchan)
}
// A cancelCtx can be canceled. When canceled, it also cancels any children
// that implement canceler.
// 可取消Context
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) Value(key interface{}) interface{} {
// 还记得吧，这个特殊的cancelCtxKey可以用来标记返回自身
if key == &cancelCtxKey {
return c
}
return c.Context.Value(key)
}
func (c *cancelCtx) Done() <-chan struct{} {
// 常规加锁操作
c.mu.Lock()
// 这里会延迟初始化，主要是为了将done的初始化包装到cancelCtx的方法中，这样就算用户包装了cancelCtx，也能触发done的初始化
if c.done == nil {
c.done = make(chan struct{})
}
d := c.done
c.mu.Unlock()
return d
}
// 错误处理，不细说
func (c *cancelCtx) Err() error {
c.mu.Lock()
err := c.err
c.mu.Unlock()
return err
}
// 下面三个方法都是为了支持打印操作，不细说
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.
// cancelCtx的取消操作
func (c *cancelCtx) cancel(removeFromParent bool, err error) {
// 必须要有取消信息
if err == nil {
panic("context: internal error: missing cancel error")
}
c.mu.Lock()
// 已经取消过了
if c.err != nil {
c.mu.Unlock()
return // already canceled
}
c.err = err
// 还未初始化的done，就给一个已关闭的channel
if c.done == nil {
c.done = closedchan
} else {
close(c.done)
}
// 取消所有的子Context
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()
// 是否从父Context的child集合中删除自己
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.
// 对cancelCtx的包装，支持截止时间
func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
if parent == nil {
panic("cannot create context from nil parent")
}
// 如果父Context的截止时间比当前截止时间更早，那直接作为cancelCtx挂着就行了
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,
}
// 将c挂靠到可取消的父Context
propagateCancel(parent, c)
dur := time.Until(d)
// 如果到了截止时间
if dur <= 0 {
c.cancel(true, DeadlineExceeded) // deadline has already passed
return c, func() { c.cancel(false, Canceled) }
}
// 这里要加锁，因为一旦挂靠了，就可能被触发cancel操作
c.mu.Lock()
defer c.mu.Unlock()
// 如果没有被触发取消
if c.err == nil {
// 启一个定时器，一旦到点就会执行func()，也就是cancel操作
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
}
// 获取截止时间
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() + "])"
}
// timerCtx的cancel操作
func (c *timerCtx) cancel(removeFromParent bool, err error) {
// 注意这里是false，因为挂靠的是包装的timerCtx
c.cancelCtx.cancel(false, err)
if removeFromParent {
// Remove this timerCtx from its parent cancelCtx's children.
removeChild(c.cancelCtx.Context, c)
}
c.mu.Lock()
// 停掉定时器
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)
// }
// WithDeadline的包装，支持超时时间段
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.
// 传递<k,v>的Context
func WithValue(parent Context, key, val interface{}) Context {
if parent == nil {
panic("cannot create context from nil parent")
}
if key == nil {
panic("nil key")
}
// 注意必须是可比较的类型，slice、map和函数是不可以的
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) + ")"
}
// 获取<k,v>
func (c *valueCtx) Value(key interface{}) interface{} {
if c.key == key {
return c.val
}
// 这里会层层解套调用父Context的Value()方法来实现全遍历
return c.Context.Value(key)
}
使用建议
以下是来自官方的使用Context的建议，比较好读，不翻译了
- 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.
- 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.
举个栗子
WithValue
func main() {
ctx := context.WithValue(context.Background(), 1, 1)
ctx = withName(ctx)
WithValueTest(ctx)
}
func withName(ctx context.Context) context.Context {
return context.WithValue(ctx, "name", "joker")
}
func WithValueTest(ctx context.Context) {
fmt.Println(ctx.Value(1).(int)) // 1
fmt.Println(ctx.Value("name").(string)) // joker
v, ok := ctx.Value("no-set").(int)
fmt.Println(v, ok) // 0 false
}
注意，这只是个用法示例，实际使用中Context传递的应该是作用于整个请求的数据，比如request_id，token之类的，自定义的数据尽量通过参数传递，否则当Context嵌套层数一多，你自己可能都搞不清楚传了哪些，哪些节点能获取到哪些数据
WithCancel
func main() {
ctx, cancel := context.WithCancel(context.Background())
go handler(ctx)
time.Sleep(time.Second * 3)
cancel()
time.Sleep(time.Second * 2)
}
func handler(ctx context.Context) {
for {
select {
case <-ctx.Done():
fmt.Println("handler canceled")
return
default:
fmt.Println("handler running")
time.Sleep(time.Second)
}
}
}
// output
handler running
handler running
handler running
handler running
handler canceled
WithTimeout
func main() {
ctx, _ := context.WithTimeout(context.Background(), time.Second * 2)
go handler(ctx)
time.Sleep(time.Second * 4)
}
func handler(ctx context.Context) {
for {
select {
case <-ctx.Done():
fmt.Println("handler canceled")
return
default:
fmt.Println("handler running")
time.Sleep(time.Second)
}
}
}
// output
handler running
handler running
handler canceled
总结
context解决了并发控制的问题，但是设计上面并不够优雅，需要所有涉及到的方法/函数层层传递Context类型参数，而且对于Value()方法的实现，是递归的链式处理，性能不是很好，且使用的范围有限，对初学者很容易误用滥用导致后期维护困难
