1// Copyright 2016 The Go Authors. All rights reserved.
 2// Use of this source code is governed by a BSD-style
 3// license that can be found in the LICENSE file.
 4
 5// Package errgroup provides synchronization, error propagation, and Context
 6// cancelation for groups of goroutines working on subtasks of a common task.
 7package errgroup
 8
 9import (
10	"context"
11	"sync"
12)
13
14// A Group is a collection of goroutines working on subtasks that are part of
15// the same overall task.
16//
17// A zero Group is valid and does not cancel on error.
18type Group struct {
19	cancel func()
20
21	wg sync.WaitGroup
22
23	errOnce sync.Once
24	err     error
25}
26
27// WithContext returns a new Group and an associated Context derived from ctx.
28//
29// The derived Context is canceled the first time a function passed to Go
30// returns a non-nil error or the first time Wait returns, whichever occurs
31// first.
32func WithContext(ctx context.Context) (*Group, context.Context) {
33	ctx, cancel := context.WithCancel(ctx)
34	return &Group{cancel: cancel}, ctx
35}
36
37// Wait blocks until all function calls from the Go method have returned, then
38// returns the first non-nil error (if any) from them.
39func (g *Group) Wait() error {
40	g.wg.Wait()
41	if g.cancel != nil {
42		g.cancel()
43	}
44	return g.err
45}
46
47// Go calls the given function in a new goroutine.
48//
49// The first call to return a non-nil error cancels the group; its error will be
50// returned by Wait.
51func (g *Group) Go(f func() error) {
52	g.wg.Add(1)
53
54	go func() {
55		defer g.wg.Done()
56
57		if err := f(); err != nil {
58			g.errOnce.Do(func() {
59				g.err = err
60				if g.cancel != nil {
61					g.cancel()
62				}
63			})
64		}
65	}()
66}