NewCursorFile fails if the Log is already attached to VSM.

pkg/log/cursor.go

@@ -94,10 +94,15 @@ func (log *Log) NewCursor() (*Cursor, error) {
 
 // NewCursorFile creates a Cursor to read from a binary log file,
 // created by varnishlog -w.
+//
+// Fails if the Log object is already attached to a live Varnish log.
 func (log *Log) NewCursorFile(path string) (*Cursor, error) {
 	if err := log.checkNil(); err != nil {
 		return nil, err
 	}
+	if log.vsm != nil {
+		return nil, errors.New("Already attached to a Varnish log")
+	}
 	C.VSL_ResetError(log.vsl)
 	cursor := C.VSL_CursorFile(log.vsl, C.CString(path), 0)
 	if cursor == nil {

pkg/log/cursor_e2e_test.go

@@ -69,6 +69,21 @@ func TestNewCursor(t *testing.T) {
 	defer cn.Delete()
 }
 
+func TestIllegalCursorFile(t *testing.T) {
+	l := New()
+	defer l.Release()
+	if err := l.Attach(""); err != nil {
+		t.Fatal("Attach(default):", err)
+		return
+	}
+	c, err := l.NewCursorFile(testFile)
+	if err == nil {
+		t.Fatal("Expected NewCursorFile() to fail for attached Log")
+		return
+	}
+	c.Delete()
+}
+
 func TestE2ECursor(t *testing.T) {
 	l := New()
 	defer l.Release()