bytes, strings: add Reader.ReadAt race tests

Tests for the race detector to catch anybody
trying to mutate Reader in ReadAt.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/86700043

src/pkg/bytes/reader.go

@@ -43,6 +43,7 @@ func (r *Reader) Read(b []byte) (n int, err error) {
 }
 
 func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
+	// cannot modify state - see io.ReaderAt
 	if off < 0 {
 		return 0, errors.New("bytes: invalid offset")
 	}

src/pkg/bytes/reader_test.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"sync"
 	"testing"
 )
 
@@ -98,6 +99,22 @@ func TestReaderAt(t *testing.T) {
 	}
 }
 
+func TestReaderAtConcurrent(t *testing.T) {
+	// Test for the race detector, to verify ReadAt doesn't mutate
+	// any state.
+	r := NewReader([]byte("0123456789"))
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			var buf [1]byte
+			r.ReadAt(buf[:], int64(i))
+		}(i)
+	}
+	wg.Wait()
+}
+
 func TestReaderWriteTo(t *testing.T) {
 	for i := 0; i < 30; i += 3 {
 		var l int

src/pkg/strings/reader.go

@@ -42,6 +42,7 @@ func (r *Reader) Read(b []byte) (n int, err error) {
 }
 
 func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
+	// cannot modify state - see io.ReaderAt
 	if off < 0 {
 		return 0, errors.New("strings: invalid offset")
 	}

src/pkg/strings/reader_test.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"os"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -98,6 +99,22 @@ func TestReaderAt(t *testing.T) {
 	}
 }
 
+func TestReaderAtConcurrent(t *testing.T) {
+	// Test for the race detector, to verify ReadAt doesn't mutate
+	// any state.
+	r := strings.NewReader("0123456789")
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			var buf [1]byte
+			r.ReadAt(buf[:], int64(i))
+		}(i)
+	}
+	wg.Wait()
+}
+
 func TestWriteTo(t *testing.T) {
 	const str = "0123456789"
 	for i := 0; i <= len(str); i++ {