bytes: fix UnreadByte failure after ReadBytes

Fixes #4583.

R=golang-dev, minux.ma, bradfitz, rsc, dave
CC=golang-dev
https://golang.org/cl/6976050

src/pkg/bytes/buffer.go

@@ -367,7 +367,7 @@ func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
 	return
 }
 
-// readSlice is like readBytes but returns a reference to internal buffer data.
+// 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)
 	end := b.off + i + 1
@@ -377,6 +377,7 @@ func (b *Buffer) readSlice(delim byte) (line []byte, err error) {
 	}
 	line = b.buf[b.off:end]
 	b.off = end
+	b.lastRead = opRead
 	return line, err
 }
 

src/pkg/bytes/buffer_test.go

@@ -453,3 +453,25 @@ func TestReadEmptyAtEOF(t *testing.T) {
 		t.Errorf("wrong count; got %d want 0", n)
 	}
 }
+
+func TestUnreadByte(t *testing.T) {
+	b := new(Buffer)
+	b.WriteString("abcdefghijklmnopqrstuvwxyz")
+
+	_, err := b.ReadBytes('m')
+	if err != nil {
+		t.Fatalf("ReadBytes: %v", err)
+	}
+
+	err = b.UnreadByte()
+	if err != nil {
+		t.Fatalf("UnreadByte: %v", err)
+	}
+	c, err := b.ReadByte()
+	if err != nil {
+		t.Fatalf("ReadByte: %v", err)
+	}
+	if c != 'm' {
+		t.Errorf("ReadByte = %q; want %q", c, 'm')
+	}
+}