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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package errgroup provides synchronization, error propagation, and Context
// cancelation for groups of goroutines working on subtasks of a common task.
package errgroup
import (
// A Group is a collection of goroutines working on subtasks that are part of
// the same overall task.
// A zero Group is valid and does not cancel on error.
type Group struct {
cancel func()
wg sync.WaitGroup
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 {
if g.cancel != nil {
return g.err
// Go calls the given function in a new goroutine.
// 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) {
go func() {
defer g.wg.Done()
if err := f(); err != nil {
g.errOnce.Do(func() {
g.err = err
if g.cancel != nil {