io/ioutil: fix Discard data race

Fixes #4589

R=golang-dev, iant, dvyukov
CC=golang-dev
https://golang.org/cl/7011047

src/pkg/io/ioutil/blackhole.go

@@ -2,12 +2,22 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !race
-
 package ioutil
 
-var blackHoleBuf = make([]byte, 8192)
+var blackHoleBuf = make(chan []byte, 1)
 
 func blackHole() []byte {
-	return blackHoleBuf
+	select {
+	case b := <-blackHoleBuf:
+		return b
+	default:
+	}
+	return make([]byte, 8192)
+}
+
+func blackHolePut(p []byte) {
+	select {
+	case blackHoleBuf <- p:
+	default:
+	}
 }

src/pkg/io/ioutil/blackhole_race.go

@@ -1,13 +0,0 @@
-// Copyright 2012 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build race
-
-package ioutil
-
-// Replaces the normal fast implementation with slower but formally correct one.
-
-func blackHole() []byte {
-	return make([]byte, 8192)
-}

src/pkg/io/ioutil/ioutil.go

@@ -132,6 +132,7 @@ func (devNull) Write(p []byte) (int, error) {
 
 func (devNull) ReadFrom(r io.Reader) (n int64, err error) {
 	buf := blackHole()
+	defer blackHolePut(buf)
 	readSize := 0
 	for {
 		readSize, err = r.Read(buf)