type token struct{}
// A Group is a collection of goroutines working on subtasks that are part of
// the same overall task.
//
// A zero Group is valid, has no limit on the number of active goroutines,
// and does not cancel on error.
type Group struct {
	cancel func()
	wg sync.WaitGroup
	sem chan token
	errOnce sync.Once
	err     error
}

// WithContext returns a new Group and an associated Context derived from ctx.
//
// The derived Context is canceled the first time a function passed to Go
// returns a non-nil error or the first time Wait returns, whichever occurs
// first.
func WithContext(ctx context.Context) (*Group, context.Context) {
	ctx, cancel := context.WithCancel(ctx)
	return &Group{cancel: cancel}, ctx
}

// Wait blocks until all function calls from the Go method have returned, then
// returns the first non-nil error (if any) from them.
func (g *Group) Wait() error {
	g.wg.Wait()
	if g.cancel != nil {
		g.cancel()
	}
	return g.err
}

// Go calls the given function in a new goroutine.
// It blocks until the new goroutine can be added without the number of
// active goroutines in the group exceeding the configured limit.
//
// The first call to return a non-nil error cancels the group; its error will be
// returned by Wait.
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()
}

// SetLimit limits the number of active goroutines in this group to at most n.
// A negative value indicates no limit.
//
// Any subsequent call to the Go method will block until it can add an active
// goroutine without exceeding the configured limit.
//
// The limit must not be modified while any goroutines in the group are active.
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)
}

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

func (g *Group) Go(f func() error) {
	if g.sem != nil {
		g.sem <- token{}
	}
	g.wg.Add(1)
	go func() {
		defer g.done()
                ...
	}()
}

func (g *Group) done() {
	if g.sem != nil {
		<-g.sem
	}
	g.wg.Done()
}

// TryGo calls the given function in a new goroutine only if the number of
// active goroutines in the group is currently below the configured limit.
//
// The return value reports whether the goroutine was started.
func (g *Group) TryGo(f func() error) bool {
	if g.sem != nil {
		select {
		case g.sem <- token{}:
			// Note: this allows barging iff channels in general allow barging.
		default:
			return false
		}
	}
	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()
				}
			})
		}
	}()
	return true
}

package main
import (
    "errors"
    "fmt"
    "time"
    "golang.org/x/sync/errgroup"
)
func main() {
    var g errgroup.Group
    // 啟動第一個子任務,它執行成功
    g.Go(func() error {
        time.Sleep(5 * time.Second)
        fmt.Println("exec #1")
        return nil
    })
    // 啟動第二個子任務，它執行失敗
    g.Go(func() error {
        time.Sleep(10 * time.Second)
        fmt.Println("exec #2")
        return errors.New("failed to exec #2")
    })
    // 啟動第三個子任務，它執行成功
    g.Go(func() error {
        time.Sleep(15 * time.Second)
        fmt.Println("exec #3")
        return nil
    })
    // 等待三個任務都完成
    if err := g.Wait(); err == nil {
        fmt.Println("Successfully exec all")
    } else {
        fmt.Println("failed:", err)
    }
}

package main
import (
    "errors"
    "fmt"
    "time"
    "golang.org/x/sync/errgroup"
)
func main() {
    var g errgroup.Group
    var result = make([]error, 3)
    // 啟動第一個子任務,它執行成功
    g.Go(func() error {
        time.Sleep(5 * time.Second)
        fmt.Println("exec #1")
        result[0] = nil // 保存成功或者失敗的結果
        return nil
    })
    // 啟動第二個子任務，它執行失敗
    g.Go(func() error {
        time.Sleep(10 * time.Second)
        fmt.Println("exec #2")
        result[1] = errors.New("failed to exec #2") // 保存成功或者失敗的結果
        return result[1]
    })
    // 啟動第三個子任務，它執行成功
    g.Go(func() error {
        time.Sleep(15 * time.Second)
        fmt.Println("exec #3")
        result[2] = nil // 保存成功或者失敗的結果
        return nil
    })
    if err := g.Wait(); err == nil {
        fmt.Printf("Successfully exec all. result: %v\n", result)
    } else {
        fmt.Printf("failed: %v\n", result)
    }
}

package main
import (
	"context"
	"crypto/md5"
	"fmt"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"
	"golang.org/x/sync/errgroup"
)
// Pipeline demonstrates the use of a Group to implement a multi-stage
// pipeline: a version of the MD5All function with bounded parallelism from
// https://blog.golang.org/pipelines.
func main() {
	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
}