bytes: avoid duplicate malloc/copy in Buffer.ReadString

Twice faster and twice less garbage.

R=golang-dev, dave, daniel.morsing, bradfitz
CC=golang-dev
https://golang.org/cl/6849128

src/pkg/bytes/buffer.go

@@ -360,16 +360,24 @@ func (b *Buffer) UnreadByte() error {
 // ReadBytes returns err != nil if and only if the returned data does not end in
 // delim.
 func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
+	slice, err := b.readSlice(delim)
+	// return a copy of slice. The buffer's backing array may
+	// be overwritten by later calls.
+	line = append(line, slice...)
+	return
+}
+
+// readSlice is like readBytes but returns a reference to internal buffer data.
+func (b *Buffer) readSlice(delim byte) (line []byte, err error) {
 	i := IndexByte(b.buf[b.off:], delim)
-	size := i + 1
+	end := b.off + i + 1
 	if i < 0 {
-		size = len(b.buf) - b.off
+		end = len(b.buf)
 		err = io.EOF
 	}
-	line = make([]byte, size)
+	line = b.buf[b.off:end]
-	copy(line, b.buf[b.off:])
+	b.off = end
-	b.off += size
+	return line, err
-	return
 }
 
 // ReadString reads until the first occurrence of delim in the input,
@@ -379,8 +387,8 @@ func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
 // ReadString returns err != nil if and only if the returned data does not end
 // in delim.
 func (b *Buffer) ReadString(delim byte) (line string, err error) {
-	bytes, err := b.ReadBytes(delim)
+	slice, err := b.readSlice(delim)
-	return string(bytes), err
+	return string(slice), err
 }
 
 // NewBuffer creates and initializes a new Buffer using buf as its initial

src/pkg/bytes/buffer_test.go

@@ -375,6 +375,41 @@ func TestReadBytes(t *testing.T) {
 	}
 }
 
+func TestReadString(t *testing.T) {
+	for _, test := range readBytesTests {
+		buf := NewBufferString(test.buffer)
+		var err error
+		for _, expected := range test.expected {
+			var s string
+			s, err = buf.ReadString(test.delim)
+			if s != expected {
+				t.Errorf("expected %q, got %q", expected, s)
+			}
+			if err != nil {
+				break
+			}
+		}
+		if err != test.err {
+			t.Errorf("expected error %v, got %v", test.err, err)
+		}
+	}
+}
+
+func BenchmarkReadString(b *testing.B) {
+	const n = 32 << 10
+
+	data := make([]byte, n)
+	data[n-1] = 'x'
+	b.SetBytes(int64(n))
+	for i := 0; i < b.N; i++ {
+		buf := NewBuffer(data)
+		_, err := buf.ReadString('x')
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
 func TestGrow(t *testing.T) {
 	x := []byte{'x'}
 	y := []byte{'y'}