From b8f71b04a11b4978e39593b55feaa61ea40fe08e Mon Sep 17 00:00:00 2001
From: Frederik Ring <frederik.ring@gmail.com>
Date: Thu, 10 Mar 2022 11:09:39 +0100
Subject: [PATCH] Use errgroup for running commands in parallel (#83)

---
 cmd/backup/exec.go | 24 +++++++++++-------------
 go.mod             |  1 +
 go.sum             |  2 ++
 3 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/cmd/backup/exec.go b/cmd/backup/exec.go
index 00f1c05..9499396 100644
--- a/cmd/backup/exec.go
+++ b/cmd/backup/exec.go
@@ -13,12 +13,12 @@ import (
 	"io/ioutil"
 	"os"
 	"strings"
-	"sync"
 
 	"github.com/cosiner/argv"
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/filters"
 	"github.com/docker/docker/pkg/stdcopy"
+	"golang.org/x/sync/errgroup"
 )
 
 func (s *script) exec(containerRef string, command string) ([]byte, []byte, error) {
@@ -97,29 +97,27 @@ func (s *script) runLabeledCommands(label string) error {
 		return nil
 	}
 
-	wg := sync.WaitGroup{}
-	wg.Add(len(containersWithCommand))
+	g := new(errgroup.Group)
 
-	var cmdErrors []error
 	for _, container := range containersWithCommand {
-		go func(c types.Container) {
+		c := container
+		g.Go(func() error {
 			cmd, _ := c.Labels[label]
 			s.logger.Infof("Running %s command %s for container %s", label, cmd, strings.TrimPrefix(c.Names[0], "/"))
 			stdout, stderr, err := s.exec(c.ID, cmd)
-			if err != nil {
-				cmdErrors = append(cmdErrors, err)
-			}
 			if s.c.ExecForwardOutput {
 				os.Stderr.Write(stderr)
 				os.Stdout.Write(stdout)
 			}
-			wg.Done()
-		}(container)
+			if err != nil {
+				return fmt.Errorf("runLabeledCommands: error executing command: %w", err)
+			}
+			return nil
+		})
 	}
 
-	wg.Wait()
-	if len(cmdErrors) != 0 {
-		return join(cmdErrors...)
+	if err := g.Wait(); err != nil {
+		return fmt.Errorf("runLabeledCommands: error from errgroup: %w", err)
 	}
 	return nil
 }
diff --git a/go.mod b/go.mod
index 99683b5..bcc4891 100644
--- a/go.mod
+++ b/go.mod
@@ -47,6 +47,7 @@ require (
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/rs/xid v1.3.0 // indirect
 	golang.org/x/net v0.0.0-20210226172049-e18ecbb05110 // indirect
+	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c // indirect
 	golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1 // indirect
 	golang.org/x/text v0.3.6 // indirect
 	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
diff --git a/go.sum b/go.sum
index 1b7ec47..04d34ec 100644
--- a/go.sum
+++ b/go.sum
@@ -806,6 +806,8 @@ golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e/go.mod h1:RxMgew5VJxzue5/jJ
 golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20201020160332-67f06af15bc9/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20201207232520-09787c993a3a/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20210220032951-036812b2e83c h1:5KslGYwFpkhGh+Q16bwMP3cOontH8FOep7tGV86Y7SQ=
+golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20180905080454-ebe1bf3edb33/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20180909124046-d0be0721c37e/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=