导语
当需要在多个 Goroutine 中传递上下文信息时，可以使用 Context 实现
Context
Context 除了用来传递上下文信息，还可以用来传递终结执行子任务的相关信号，终止多个执行子任务的 Goroutine。
context.Context 接口数据结构：
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// 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.
// 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.
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 any) any
}
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Deadline：返回 Context 被取消的时间，也就是完成工作的截至日期；Done：返回一个 channel，这个 channel 会在当前工作完成或者上下文被取消之后关闭，多次调用 Done 方法会返回同一个 channel；Err：放回 Context 结束的原因，只会在 Done 返回的 channel 被关闭时才会返回非空的值，如果 Context 被取消，会返回 Canceled 错误；如果 Context 超时，会返回 DeadlineExceeded 错误；Value：可用于从 Context 中获取传递的键值信息；
Example
在 Web 请求的处理过程中，一个请求可能启动多个 goroutine 协同工作，这些 goroutine 之间可能需要共享请求的信息，且当请求被取消或者执行超时时，该请求对应的所有 goroutine 都需要快速结束，释放资源，Context 就是为了解决上述场景而开发的。
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package main
import (
"context"
"fmt"
"time"
)
const DB_ADDRESS = "db_address"
const CALCULATE_VALUE = "calculate_value"
func readDB(ctx context.Context, cost time.Duration) {
fmt.Println("DB address is ", ctx.Value(DB_ADDRESS))
select {
case <-time.After(cost):
fmt.Println("read data from db")
case <-ctx.Done():
fmt.Println(ctx.Err())
}
}
func calculate(ctx context.Context, cost time.Duration) {
fmt.Println("calculate value is", ctx.Value(CALCULATE_VALUE))
select {
case <-time.After(cost): // 模拟数据计算
fmt.Println("calculate finish")
case <-ctx.Done():
fmt.Println(ctx.Err()) // 任务取消的原因
// 一些清理工作
}
}
func main() {
ctx := context.Background()
// Add Context info
ctx = context.WithValue(ctx, DB_ADDRESS, "localhost:3306")
ctx = context.WithValue(ctx, CALCULATE_VALUE, "123")
ctx, cancel := context.WithTimeout(ctx, time.Second*2)
defer cancel()
go readDB(ctx, time.Second*4)
go calculate(ctx, time.Second*4)
time.Sleep(time.Second * 5)
}
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使用 Context，能够有效地在一组 goroutine 中传递共享值、取消信号、deadline 等信息，及时关闭不需要的 goroutine。
Reference
