encoding/line: make it an io.Reader too

R=agl1, bradfitzwork, rsc
CC=golang-dev
https://golang.org/cl/4066043

src/pkg/encoding/line/line.go

@@ -28,6 +28,26 @@ func NewReader(input io.Reader, maxLineLength int) *Reader {
 	}
 }
 
+// Read reads from any buffered data past the last line read, or from the underlying
+// io.Reader if the buffer is empty.
+func (l *Reader) Read(p []byte) (n int, err os.Error) {
+	l.removeConsumedFromBuffer()
+	if len(l.buf) > 0 {
+		n = copy(p, l.buf)
+		l.consumed += n
+		return
+	}
+	return l.in.Read(p)
+}
+
+func (l *Reader) removeConsumedFromBuffer() {
+	if l.consumed > 0 {
+		n := copy(l.buf, l.buf[l.consumed:])
+		l.buf = l.buf[:n]
+		l.consumed = 0
+	}
+}
+
 // ReadLine tries to return a single line, not including the end-of-line bytes.
 // If the line was found to be longer than the maximum length then isPrefix is
 // set and the beginning of the line is returned. The rest of the line will be
@@ -36,11 +56,7 @@ func NewReader(input io.Reader, maxLineLength int) *Reader {
 // the Reader and is only valid until the next call to ReadLine. ReadLine
 // either returns a non-nil line or it returns an error, never both.
 func (l *Reader) ReadLine() (line []byte, isPrefix bool, err os.Error) {
-	if l.consumed > 0 {
+	l.removeConsumedFromBuffer()
-		n := copy(l.buf, l.buf[l.consumed:])
-		l.buf = l.buf[:n]
-		l.consumed = 0
-	}
 
 	if len(l.buf) == 0 && l.err != nil {
 		err = l.err

src/pkg/encoding/line/line_test.go

@@ -6,6 +6,7 @@ package line
 
 import (
 	"bytes"
+	"io"
 	"os"
 	"testing"
 )
@@ -87,3 +88,23 @@ func TestLineTooLong(t *testing.T) {
 		t.Errorf("bad result for third line: %x", line)
 	}
 }
+
+func TestReadAfterLines(t *testing.T) {
+	line1 := "line1"
+	restData := "line2\nline 3\n"
+	inbuf := bytes.NewBuffer([]byte(line1 + "\n" + restData))
+	outbuf := new(bytes.Buffer)
+	maxLineLength := len(line1) + len(restData)/2
+	l := NewReader(inbuf, maxLineLength)
+	line, isPrefix, err := l.ReadLine()
+	if isPrefix || err != nil || string(line) != line1 {
+		t.Errorf("bad result for first line: isPrefix=%v err=%v line=%q", isPrefix, err, string(line))
+	}
+	n, err := io.Copy(outbuf, l)
+	if int(n) != len(restData) || err != nil {
+		t.Errorf("bad result for Read: n=%d err=%v", n, err)
+	}
+	if outbuf.String() != restData {
+		t.Errorf("bad result for Read: got %q; expected %q", outbuf.String(), restData)
+	}
+}