bytes, strings: fix Reader WriteTo return value on 0 bytes copied

Fixes #4421

R=golang-dev, dave, minux.ma, mchaten, rsc
CC=golang-dev
https://golang.org/cl/6855083

src/pkg/bytes/reader.go

@@ -125,7 +125,7 @@ func (r *Reader) Seek(offset int64, whence int) (int64, error) {
 func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
 	r.prevRune = -1
 	if r.i >= len(r.s) {
-		return 0, io.EOF
+		return 0, nil
 	}
 	b := r.s[r.i:]
 	m, err := w.Write(b)

src/pkg/bytes/reader_test.go

@@ -8,6 +8,7 @@ import (
 	. "bytes"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
 	"testing"
 )
@@ -88,16 +89,20 @@ func TestReaderAt(t *testing.T) {
 }
 
 func TestReaderWriteTo(t *testing.T) {
-	for i := 3; i < 30; i += 3 {
-		s := data[:len(data)/i]
-		r := NewReader(testBytes[:len(testBytes)/i])
+	for i := 0; i < 30; i += 3 {
+		var l int
+		if i > 0 {
+			l = len(data) / i
+		}
+		s := data[:l]
+		r := NewReader(testBytes[:l])
 		var b Buffer
 		n, err := r.WriteTo(&b)
 		if expect := int64(len(s)); n != expect {
 			t.Errorf("got %v; want %v", n, expect)
 		}
 		if err != nil {
-			t.Errorf("got error = %v; want nil", err)
+			t.Errorf("for length %d: got error = %v; want nil", l, err)
 		}
 		if b.String() != s {
 			t.Errorf("got string %q; want %q", b.String(), s)
@@ -107,3 +112,26 @@ func TestReaderWriteTo(t *testing.T) {
 		}
 	}
 }
+
+// verify that copying from an empty reader always has the same results,
+// regardless of the presence of a WriteTo method.
+func TestReaderCopyNothing(t *testing.T) {
+	type nErr struct {
+		n   int64
+		err error
+	}
+	type justReader struct {
+		io.Reader
+	}
+	type justWriter struct {
+		io.Writer
+	}
+	discard := justWriter{ioutil.Discard} // hide ReadFrom
+
+	var with, withOut nErr
+	with.n, with.err = io.Copy(discard, NewReader(nil))
+	withOut.n, withOut.err = io.Copy(discard, justReader{NewReader(nil)})
+	if with != withOut {
+		t.Errorf("behavior differs: with = %#v; without: %#v", with, withOut)
+	}
+}

src/pkg/strings/reader.go

@@ -124,7 +124,7 @@ func (r *Reader) Seek(offset int64, whence int) (int64, error) {
 func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
 	r.prevRune = -1
 	if r.i >= len(r.s) {
-		return 0, io.EOF
+		return 0, nil
 	}
 	s := r.s[r.i:]
 	m, err := io.WriteString(w, s)

src/pkg/strings/reader_test.go

@@ -90,7 +90,7 @@ func TestReaderAt(t *testing.T) {
 
 func TestWriteTo(t *testing.T) {
 	const str = "0123456789"
-	for i := 0; i < len(str); i++ {
+	for i := 0; i <= len(str); i++ {
 		s := str[i:]
 		r := strings.NewReader(s)
 		var b bytes.Buffer
@@ -99,7 +99,7 @@ func TestWriteTo(t *testing.T) {
 			t.Errorf("got %v; want %v", n, expect)
 		}
 		if err != nil {
-			t.Errorf("got error = %v; want nil", err)
+			t.Errorf("for length %d: got error = %v; want nil", len(s), err)
 		}
 		if b.String() != s {
 			t.Errorf("got string %q; want %q", b.String(), s)