errgroup

什么是errrgroup

在开发并发程序时，错误的收集和传播往往比较繁琐，有时候当一个错误发声时，我们需要停止所有相关任务，有时候却不是。sync.ErrGroup刚好可以解决我们上述的痛点，它提供错误传播，以及利用context的方式来决定是否要停止相关任务。

errrgroup.Group结构体

type Group struct {
    cancel func()
    wg sync.WaitGroup
    errOnce sync.Once
    err     error
}
``

三个对外api

```go
unc WithContext(ctx context.Context) (*Group, context.Context)
func (g *Group) Go(f func() error)
func (g *Group) Wait() error

使用errrgroup

只返回错误

package main

import (
    "fmt"
    "golang.org/x/sync/errgroup"
    "net/http"
)

func main() {
    var g errgroup.Group
    var urls = []string{
        "http://www.golang.org/",
        "http://www.111111111111111111111111.com/", //这个地址不存在
        "http://www.google.com/",
        "http://www.somestupidname.com/",
    }
    for _, url := range urls {
        url := url 
        g.Go(func() error {
            resp, err := http.Get(url)
            if err == nil {
                resp.Body.Close()
            }
            return err
        })
    }
    if err := g.Wait(); err == nil {
        fmt.Println("Successfully fetched all URLs.")
    }else {
        fmt.Println(err)
    }
}

$ go run main.go
www.111111111111111111111111.com:80: unknown error host unreachable

使用 errgroup.WithContext

package main

import (
    "context"
    "fmt"
    "golang.org/x/sync/errgroup"
    "net/http"
    "time"
)

func main() {
    ctx ,_:=context.WithTimeout(context.Background(),3*time.Second)
    var g ,_=  errgroup.WithContext(ctx)
    var urls = []string{
        "http://www.golang.org/",
        "http://www.111111111111111111111111.com/",
        "http://www.google.com/",
        "http://www.somestupidname.com/",
    }
    for _, url := range urls {
        url := url
        g.Go(func() error {
            ch := make(chan error)
            go func() {
                time.Sleep(4e9)
                resp, err := http.Get(url)
                if err == nil {
                    resp.Body.Close()
                }
                ch <- err
            }()
            select {
            case err:= <-ch :
                return err
            case <-ctx.Done():
                return ctx.Err()
            }
        })
    }
    if err := g.Wait(); err == nil {
        fmt.Println("Successfully fetched all URLs.")
    }else {
        fmt.Println(err)
    }
}

$ go run main.go
context deadline exceeded

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