✨ Short-circuit new goroutines if an error already occured

errgroup.go

@@ -13,6 +13,7 @@ import (
 	"context"
 	"fmt"
 	"sync"
+	"sync/atomic"
 )
 
 type token struct{}
@@ -30,7 +31,7 @@ type Group struct {
 	sem chan token
 
 	errOnce sync.Once
-	err     error
+	err     atomic.Value
 }
 
 func (g *Group) done() {
@@ -50,14 +51,22 @@ func WithContext(ctx context.Context) (*Group, context.Context) {
 	return &Group{cancel: cancel}, ctx
 }
 
+func (g *Group) error() error {
+	v := g.err.Load()
+	if v == nil {
+		return nil
+	}
+	return v.(error)
+}
+
 // 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(g.err)
+		g.cancel(g.error())
 	}
-	return g.err
+	return g.error()
 }
 
 // Go calls the given function in a new goroutine.
@@ -70,6 +79,9 @@ func (g *Group) Wait() error {
 // cancel the associated Context, if any. The error will be returned
 // by Wait.
 func (g *Group) Go(f func() error) {
+	if g.error() != nil {
+		return
+	}
 	if g.sem != nil {
 		g.sem <- token{}
 	}
@@ -92,9 +104,9 @@ func (g *Group) Go(f func() error) {
 
 		if err := f(); err != nil {
 			g.errOnce.Do(func() {
-				g.err = err
+				g.err.Store(err)
 				if g.cancel != nil {
-					g.cancel(g.err)
+					g.cancel(err)
 				}
 			})
 		}
@@ -106,6 +118,9 @@ func (g *Group) Go(f func() error) {
 //
 // The return value reports whether the goroutine was started.
 func (g *Group) TryGo(f func() error) bool {
+	if g.error() != nil {
+		return false
+	}
 	if g.sem != nil {
 		select {
 		case g.sem <- token{}:
@@ -121,9 +136,9 @@ func (g *Group) TryGo(f func() error) bool {
 
 		if err := f(); err != nil {
 			g.errOnce.Do(func() {
-				g.err = err
+				g.err.Store(err)
 				if g.cancel != nil {
-					g.cancel(g.err)
+					g.cancel(err)
 				}
 			})
 		}

errgroup_test.go

@@ -98,6 +98,38 @@ func ExampleGroup_parallel() {
 	// video result for "golang"
 }
 
+// FirstError demonstrates that g.Go becomes a no-op if a previous g.Go
+// has returned an error.
+func ExampleGroup_firstError() {
+	err1 := errors.New("errgroup_test: 1")
+	err2 := errors.New("errgroup_test: 2")
+
+	g := new(errgroup.Group)
+
+	ch := make(chan struct{})
+
+	g.Go(func() error {
+		fmt.Printf("Returning %s\n", err1)
+		ch <- struct{}{}
+		return err1
+	})
+
+	<-ch
+
+	g.Go(func() error {
+		// This should never run
+		fmt.Printf("Returning %s\n", err2)
+		return err2
+	})
+
+	err := g.Wait()
+	fmt.Printf("Got %s\n", err)
+
+	// Output:
+	// Returning errgroup_test: 1
+	// Got errgroup_test: 1
+}
+
 func TestZeroGroup(t *testing.T) {
 	err1 := errors.New("errgroup_test: 1")
 	err2 := errors.New("errgroup_test: 2")
@@ -256,6 +288,7 @@ func TestCancelCause(t *testing.T) {
 		{errs: []error{nil}, want: nil},
 		{errs: []error{errDoom}, want: errDoom},
 		{errs: []error{errDoom, nil}, want: errDoom},
+		{errs: []error{nil, errDoom}, want: errDoom},
 	}
 
 	for _, tc := range cases {