Try to prevent data race

pkg/dockerimage/push.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"strings"
+	"sync"
 )
 
 const (
@@ -14,16 +15,37 @@ const (
 	remoteDigestPrefix = "latest: digest: sha256:"
 )
 
+var (
+	DigestOutputError = digestOutputError{
+		errors.New("no digest in output found"),
+	}
+)
+
+type digestOutputError struct {
+	error
+}
+
+func (err digestOutputError) Is(other error) bool {
+	_, ok := other.(digestOutputError)
+	return ok
+}
+
 // Push calls docker and pushes a tag.
 // Returns the remote digest for the pushed image.
 func PushWithWriters(tag string, out io.Writer, errOut io.Writer) (string, error) {
 	r, w := io.Pipe()
 	defer r.Close()
 	defer w.Close()
 
-	digest := make(chan string)
-	defer close(digest)
+	digest := make(chan string, 1)
+	errs := make(chan error, 1)
+	errG := sync.WaitGroup{}
+
+	errG.Add(1)
 	go func() {
+		defer close(digest)
+		defer errG.Done()
+
 		scanner := bufio.NewScanner(r)
 		for scanner.Scan() {
 			if !strings.HasPrefix(scanner.Text(), remoteDigestPrefix) {
@@ -33,18 +55,32 @@ func PushWithWriters(tag string, out io.Writer, errOut io.Writer) (string, error
 			digest <- scanner.Text()[len(remoteDigestPrefix) : len(remoteDigestPrefix)+64]
 			return
 		}
+		errs <- DigestOutputError
 	}()
-	err := executeDockerCommandWithWriters(io.MultiWriter(out, w), errOut, "push", tag)
-	if err != nil {
-		return "", err
-	}
 
-	select {
-	case d := <-digest:
+	errG.Add(1)
+	go func() {
+		defer errG.Done()
+
+		errs <- executeDockerCommandWithWriters(io.MultiWriter(out, w), errOut, "push", tag)
+	}()
+
+	go func() {
+		defer close(errs)
+
+		errG.Wait()
+	}()
+
+	d, ok := <-digest
+	if ok {
 		return d, nil
-	default:
-		return "", errors.New("no digest in output found")
 	}
+	for err := range errs {
+		if err != nil {
+			return "", err
+		}
+	}
+	panic("should have resulted in error before")
 }
 
 // Push calls docker and pushes a tag.