bufio: change ReadSlice to match description

On error, ReadSlice is defined to return the available data
and advance past that data, but it was not behaving that
way for err == ErrBufferFull, making it harder for callers to
handle well.

R=r
CC=golang-dev
https://golang.org/cl/1480041

src/pkg/bufio/bufio.go

@@ -229,7 +229,8 @@ func (b *Reader) ReadSlice(delim byte) (line []byte, err os.Error) {
 
 		// Buffer is full?
 		if b.Buffered() >= len(b.buf) {
-			return nil, ErrBufferFull
+			b.r = b.w
+			return b.buf, ErrBufferFull
 		}
 	}
 	panic("not reached")
@@ -259,20 +260,9 @@ func (b *Reader) ReadBytes(delim byte) (line []byte, err os.Error) {
 			break
 		}
 
-		// Read bytes out of buffer.
-		buf := make([]byte, b.Buffered())
-		var n int
-		n, e = b.Read(buf)
-		if e != nil {
-			frag = buf[0:n]
-			err = e
-			break
-		}
-		if n != len(buf) {
-			frag = buf[0:n]
-			err = errInternal
-			break
-		}
+		// Make a copy of the buffer.
+		buf := make([]byte, len(frag))
+		copy(buf, frag)
 
 		// Grow list if needed.
 		if full == nil {

src/pkg/bufio/bufio_test.go

@@ -407,3 +407,15 @@ func TestWriteString(t *testing.T) {
 		t.Errorf("WriteString wants %q gets %q", s, string(buf.Bytes()))
 	}
 }
+
+func TestBufferFull(t *testing.T) {
+	buf, _ := NewReaderSize(strings.NewReader("hello, world"), 5)
+	line, err := buf.ReadSlice(',')
+	if string(line) != "hello" || err != ErrBufferFull {
+		t.Errorf("first ReadSlice(,) = %q, %v", line, err)
+	}
+	line, err = buf.ReadSlice(',')
+	if string(line) != "," || err != nil {
+		t.Errorf("second ReadSlice(,) = %q, %v", line, err)
+	}
+}