Golang GroupWaiter (a wrapper aroung WaitGroup)

group_waiter.go

package main

import (
	"errors"
	"fmt"
	"sync"
	"time"
)

type GroupWaiter struct {
	waitGroup *sync.WaitGroup
	errors    []error
}

func NewGroupWaiter() *GroupWaiter {
	return &GroupWaiter{
		waitGroup: new(sync.WaitGroup),
		errors:    make([]error, 0),
	}
}

func (gw *GroupWaiter) Do(f func() error) {
	gw.waitGroup.Add(1)
	gw.errors = append(gw.errors, nil)
	index := len(gw.errors)

	go func() {
		defer gw.waitGroup.Done()
		err := f()
		if err != nil {
			gw.errors[index-1] = err
		}
	}()
}

// Returns erros from single go routines and a cumulative errors with messages from all (if there were errors)
func (gw *GroupWaiter) Wait() ([]error, error) {
	gw.waitGroup.Wait()
	var result []error
	var errMessage = ""
	for n, _ := range gw.errors {
		if gw.errors[n] != nil {
			result = append(result, gw.errors[n])
			if len(errMessage) > 0 {
				errMessage += "\n"
			}
			errMessage += "ERR:" + gw.errors[n].Error()
		}
	}

	if len(result) > 0 {
		return result, errors.New(errMessage)
	}

	return result, nil
}

func main() {
	gw := NewGroupWaiter()
	gw.Do(func() error {
		for i := 0; i < 10; i++ {
			fmt.Println(i)
			time.Sleep(time.Second / 3)
		}
		fmt.Println("Done")
		return errors.New("Random error 1")
	})
	gw.Do(func() error {
		for i := 0; i < 10; i++ {
			fmt.Println(i)
			time.Sleep(time.Second / 4)
		}
		fmt.Println("Done")
		return errors.New("Random error 2")
	})
	gw.Do(func() error {
		for i := 0; i < 10; i++ {
			fmt.Println(i)
			time.Sleep(time.Second / 5)
		}
		fmt.Println("Done")
		return nil
	})
	_, err := gw.Wait()
	if err != nil {
		fmt.Println("ERROR:", err.Error())
	}
}