✨ 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)
 				}
 			})
 		}