io/ioutil: use sync.Pool in Discard

And merge the blackhole.go file back into ioutil,
where it once was. It was only in a separate file
because it used to have race-vs-!race versions.

R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/44060044

src/pkg/io/ioutil/blackhole.go

-// 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.
-
-package ioutil
-
-var blackHoleBuf = make(chan []byte, 1)
-
-func blackHole() []byte {
-	select {
-	case b := <-blackHoleBuf:
-		return b
-	default:
-	}
-	return make([]byte, 8192)
-}
-
-func blackHolePut(p []byte) {
-	select {
-	case blackHoleBuf <- p:
-	default:
-	}
-}

src/pkg/io/ioutil/ioutil.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"os"
 	"sort"
+	"sync"
 )
 
 // readAll reads from r until an error or EOF and returns the data it read
@@ -136,14 +137,21 @@ func (devNull) WriteString(s string) (int, error) {
 	return len(s), nil
 }
 
+var blackHolePool = sync.Pool{
+	New: func() interface{} {
+		b := make([]byte, 8192)
+		return &b
+	},
+}
+
 func (devNull) ReadFrom(r io.Reader) (n int64, err error) {
-	buf := blackHole()
-	defer blackHolePut(buf)
+	bufp := blackHolePool.Get().(*[]byte)
 	readSize := 0
 	for {
-		readSize, err = r.Read(buf)
+		readSize, err = r.Read(*bufp)
 		n += int64(readSize)
 		if err != nil {
+			blackHolePool.Put(bufp)
 			if err == io.EOF {
 				return n, nil
 			}