cmd/storj-sim: ensure prefix writer can be used concurrently (#2243)

cmd/storj-sim/prefix.go

@@ -44,6 +44,8 @@ func NewPrefixWriter(defaultPrefix string, dst io.Writer) *PrefixWriter {
 type prefixWriter struct {
 	*PrefixWriter
 	prefix string
+
+	local  sync.Mutex
 	id     string
 	buffer []byte
 }
@@ -54,7 +56,12 @@ func (writer *PrefixWriter) Prefixed(prefix string) io.Writer {
 	writer.prefixlen = max(writer.prefixlen, len(prefix))
 	writer.mu.Unlock()
 
-	return &prefixWriter{writer, prefix, "", make([]byte, 0, writer.maxline)}
+	return &prefixWriter{
+		PrefixWriter: writer,
+		prefix:       prefix,
+		id:           "",
+		buffer:       make([]byte, 0, writer.maxline),
+	}
 }
 
 // Write implements io.Writer that prefixes lines.
@@ -68,6 +75,9 @@ func (writer *prefixWriter) Write(data []byte) (int, error) {
 		return 0, nil
 	}
 
+	writer.local.Lock()
+	defer writer.local.Unlock()
+
 	var newID string
 	if writer.id == "" {
 		if start := bytes.Index(data, []byte("Node ")); start > 0 {

cmd/storj-sim/prefix_test.go

@@ -0,0 +1,36 @@
+// Copyright (C) 2019 Storj Labs, Inc.
+// See LICENSE for copying information.
+
+package main
+
+import (
+	"io/ioutil"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sync/errgroup"
+)
+
+func TestPrefixWriter(t *testing.T) {
+	root := NewPrefixWriter("", ioutil.Discard)
+	alpha := root.Prefixed("alpha")
+	beta := root.Prefixed("beta")
+
+	var group errgroup.Group
+	defer func() {
+		require.NoError(t, group.Wait())
+	}()
+
+	group.Go(func() error {
+		_, err := alpha.Write([]byte{1, 2, 3})
+		return err
+	})
+	group.Go(func() error {
+		_, err := alpha.Write([]byte{3, 2, 1})
+		return err
+	})
+	group.Go(func() error {
+		_, err := beta.Write([]byte{1, 2, 3})
+		return err
+	})
+}