golang errgroup

errgroup为goroutine组提供了通知、错误传递、上下文取消功能。

一般在一组goroutine出现错误需要取消全部goroutine的情况使用到

成员与方法

errGroup主要有以下结构

• cancel func() : context的cancel方法
• wg sync.WiatGroup: 封装sync.WaitGroup
• errOnce: 保证只接受一次错误
• err error: 保存第一个返回的错误
• sem chan token: 控制goroutine活动数量（token实际上是struct{}）

有以下方法

• WithContext(ctx context.Context) (*Group, context.Context): 使传入context转换为一个可取消的context，并将cancel方法传入errGroup

  func WithContext(ctx context.Context) (*Group, context.Context) {
      ctx, cancel := context.WithCancel(ctx)
      return &Group{cancel: cancel}, ctx
  }
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• Go(f func() error): 保存errGroup传入方法f的第一次错误，并取消errGroup中的context

  func (g *Group) Go(f func() error) {
  	if g.sem != nil {
  		g.sem <- token{}
  	}
  
  	g.wg.Add(1)
  	go func() {
  		defer g.done()
  
  		if err := f(); err != nil {
  			g.errOnce.Do(func() {
  				g.err = err
  				if g.cancel != nil {
  					g.cancel()
  				}
  			})
  		}
  	}()
  }
  
  func (g *Group) done() {
  	if g.sem != nil {
  		<-g.sem
  	}
  	g.wg.Done()
  }
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• TryGo(f func() error) bool: 能返回开启协程额度是否足够的Go(f func() error)方法

• Wait() error: 等待errGroup Go中方法运行结束

  func (g *Group) Wait() error {
      g.wg.Wait()
      if g.cancel != nil {
          g.cancel()
      }
      return g.err
  }
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• SetLimit(n int): 设置限制活动goroutine的最大数量（负数代表没有限制）。通过固定长度n的channel是实现的，setLimit设置固定额度，每个活动的goroutine消费该额度，然后处理完之后释放额度

  func (g *Group) SetLimit(n int) {
  	if n < 0 {
  		g.sem = nil
  		return
  	}
  	if len(g.sem) != 0 {
  		panic(fmt.Errorf("errgroup: modify limit while %v goroutines in the group are still active", len(g.sem)))
  	}
  	g.sem = make(chan token, n)
  }
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示例

多协程计算多文件md5 sum

以下是官方利用errGroup开启1个goroutine检查文件和20个goroutine遍历目标路径文件计算md5 sum。若只要有一个文件检查出现就直接返回错误，终止后面20个goroutine的sum计算

func ExampleGroup_pipeline() {
	m, err := MD5All(context.Background(), ".")
	if err != nil {
		log.Fatal(err)
	}

	for k, sum := range m {
		fmt.Printf("%s:\t%x\n", k, sum)
	}
}

type result struct {
	path string
	sum  [md5.Size]byte
}

// MD5All reads all the files in the file tree rooted at root and returns a map
// from file path to the MD5 sum of the file's contents. If the directory walk
// fails or any read operation fails, MD5All returns an error.
func MD5All(ctx context.Context, root string) (map[string][md5.Size]byte, error) {
	// ctx is canceled when g.Wait() returns. When this version of MD5All returns
	// - even in case of error! - we know that all of the goroutines have finished
	// and the memory they were using can be garbage-collected.
	g, ctx := errgroup.WithContext(ctx)
	paths := make(chan string)

	g.Go(func() error {
		defer close(paths)
		return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
			if err != nil {
				return err
			}
			if !info.Mode().IsRegular() {
				return nil
			}
			select {
			case paths <- path:
			case <-ctx.Done():
				return ctx.Err()
			}
			return nil
		})
	})

	// Start a fixed number of goroutines to read and digest files.
	c := make(chan result)
	const numDigesters = 20
	for i := 0; i < numDigesters; i++ {
		g.Go(func() error {
			for path := range paths {
				data, err := ioutil.ReadFile(path)
				if err != nil {
					return err
				}
				select {
				case c <- result{path, md5.Sum(data)}:
				case <-ctx.Done():
					return ctx.Err()
				}
			}
			return nil
		})
	}
	go func() {
		g.Wait()
		close(c)
	}()

	m := make(map[string][md5.Size]byte)
	for r := range c {
		m[r.path] = r.sum
	}
	// Check whether any of the goroutines failed. Since g is accumulating the
	// errors, we don't need to send them (or check for them) in the individual
	// results sent on the channel.
	if err := g.Wait(); err != nil {
		return nil, err
	}
	return m, nil
}

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