diff --git a/go/tools/bzltestutil/wrap.go b/go/tools/bzltestutil/wrap.go
index 19b9f0825c..6aca56932b 100644
--- a/go/tools/bzltestutil/wrap.go
+++ b/go/tools/bzltestutil/wrap.go
@@ -15,6 +15,7 @@
 package bzltestutil
 
 import (
+	"bufio"
 	"bytes"
 	"fmt"
 	"io"
@@ -59,10 +60,57 @@ func shouldAddTestV() bool {
 	return false
 }
 
+// streamMerger intelligently merges an input stdout and stderr stream and dumps
+// the output to the writer `inner`. Additional synchronization is applied to
+// ensure that one line at a time is written to the inner writer.
+type streamMerger struct {
+	OutW, ErrW *io.PipeWriter
+	mutex sync.Mutex
+	inner io.Writer
+	wg sync.WaitGroup
+	outR, errR *bufio.Reader
+}
+
+func NewStreamMerger(w io.Writer) *streamMerger {
+	outR, outW := io.Pipe()
+	errR, errW := io.Pipe()
+	return &streamMerger{
+		inner: w,
+		OutW: outW,
+		ErrW: errW,
+		outR: bufio.NewReader(outR),
+		errR: bufio.NewReader(errR),
+	}
+}
+
+func (m *streamMerger) Start() {
+	m.wg.Add(2)
+	process := func(r *bufio.Reader) {
+		for {
+			s, err := r.ReadString('\n')
+			if len(s) > 0 {
+				m.mutex.Lock()
+				io.WriteString(m.inner, s)
+				m.mutex.Unlock()
+			}
+			if err == io.EOF {
+				break
+			}
+		}
+		m.wg.Done()
+	}
+	go process(m.outR)
+	go process(m.errR)
+}
+
+func (m *streamMerger) Wait() {
+	m.wg.Wait()
+}
+
 func Wrap(pkg string) error {
 	var jsonBuffer bytes.Buffer
 	jsonConverter := NewConverter(&jsonBuffer, pkg, Timestamp)
-	pipeRead, pipeWrite := io.Pipe()
+	streamMerger := NewStreamMerger(jsonConverter)
 
 	args := os.Args[1:]
 	if shouldAddTestV() {
@@ -74,20 +122,13 @@ func Wrap(pkg string) error {
 	}
 	cmd := exec.Command(exePath, args...)
 	cmd.Env = append(os.Environ(), "GO_TEST_WRAP=0")
-	cmd.Stderr = io.MultiWriter(os.Stderr, pipeWrite)
-	cmd.Stdout = io.MultiWriter(os.Stdout, pipeWrite)
-	var wg sync.WaitGroup
-	wg.Add(1)
-	go func() {
-		_, err := io.Copy(jsonConverter, pipeRead)
-		if err != nil {
-			panic(err)
-		}
-		wg.Done()
-	}()
+	cmd.Stderr = io.MultiWriter(os.Stderr, streamMerger.ErrW)
+	cmd.Stdout = io.MultiWriter(os.Stdout, streamMerger.OutW)
+	streamMerger.Start()
 	err := cmd.Run()
-	pipeWrite.Close()
-	wg.Wait()
+	streamMerger.ErrW.Close()
+	streamMerger.OutW.Close()
+	streamMerger.Wait()
 	jsonConverter.Close()
 	if out, ok := os.LookupEnv("XML_OUTPUT_FILE"); ok {
 		werr := writeReport(jsonBuffer, pkg, out)