bytes, strings: add (*Reader).WriteTo

Fixes #4031.

R=golang-dev, bradfitz, remyoudompheng, r, dave
CC=golang-dev
https://golang.org/cl/6632046

2 files changed, 43 insertions, 1 deletions

diff --git a/src/pkg/strings/reader.go b/src/pkg/strings/reader.go
index 8569805552..98325ce75b 100644
--- a/src/pkg/strings/reader.go
+++ b/src/pkg/strings/reader.go
@@ -10,7 +10,7 @@ import (
 	"unicode/utf8"
 )
 
-// A Reader implements the io.Reader, io.ReaderAt, io.Seeker,
+// A Reader implements the io.Reader, io.ReaderAt, io.Seeker, io.WriterTo,
 // io.ByteScanner, and io.RuneScanner interfaces by reading
 // from a string.
 type Reader struct {
@@ -120,6 +120,25 @@ func (r *Reader) Seek(offset int64, whence int) (int64, error) {
 	return abs, nil
 }
 
+// WriteTo implements the io.WriterTo interface.
+func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
+	r.prevRune = -1
+	if r.i >= len(r.s) {
+		return 0, io.EOF
+	}
+	s := r.s[r.i:]
+	m, err := io.WriteString(w, s)
+	if m > len(s) {
+		panic("strings.Reader.WriteTo: invalid WriteString count")
+	}
+	r.i += m
+	n = int64(m)
+	if m != len(s) && err == nil {
+		err = io.ErrShortWrite
+	}
+	return
+}
+
 // NewReader returns a new Reader reading from s.
 // It is similar to bytes.NewBufferString but more efficient and read-only.
 func NewReader(s string) *Reader { return &Reader{s, 0, -1} }
diff --git a/src/pkg/strings/reader_test.go b/src/pkg/strings/reader_test.go
index a99ae2a0ea..bab91fc719 100644
--- a/src/pkg/strings/reader_test.go
+++ b/src/pkg/strings/reader_test.go
@@ -5,6 +5,7 @@
 package strings_test
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"os"
@@ -86,3 +87,25 @@ func TestReaderAt(t *testing.T) {
 		}
 	}
 }
+
+func TestWriteTo(t *testing.T) {
+	const str = "0123456789"
+	for i := 0; i < len(str); i++ {
+		s := str[i:]
+		r := strings.NewReader(s)
+		var b bytes.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)
+		}
+		if b.String() != s {
+			t.Errorf("got string %q; want %q", b.String(), s)
+		}
+		if r.Len() != 0 {
+			t.Errorf("reader contains %v bytes; want 0", r.Len())
+		}
+	}
+}