container/list: mark must be an element of the list

The methods MoveAfter and MoveBefore of the container/list package did silently corrupt the interal structure of the list if a mark element is used which is not an element of the list.

LGTM=gri
R=golang-codereviews, gobot, gri
CC=golang-codereviews
https://golang.org/cl/60980043

src/pkg/container/list/list.go

@@ -180,18 +180,18 @@ func (l *List) MoveToBack(e *Element) {
 }
 
 // MoveBefore moves element e to its new position before mark.
-// If e is not an element of l, or e == mark, the list is not modified.
+// If e or mark is not an element of l, or e == mark, the list is not modified.
 func (l *List) MoveBefore(e, mark *Element) {
-	if e.list != l || e == mark {
+	if e.list != l || e == mark || mark.list != l {
 		return
 	}
 	l.insert(l.remove(e), mark.prev)
 }
 
 // MoveAfter moves element e to its new position after mark.
-// If e is not an element of l, or e == mark, the list is not modified.
+// If e or mark is not an element of l, or e == mark, the list is not modified.
 func (l *List) MoveAfter(e, mark *Element) {
-	if e.list != l || e == mark {
+	if e.list != l || e == mark || mark.list != l {
 		return
 	}
 	l.insert(l.remove(e), mark)

src/pkg/container/list/list_test.go

@@ -324,3 +324,20 @@ func TestInsertAfterUnknownMark(t *testing.T) {
 	l.InsertAfter(1, new(Element))
 	checkList(t, &l, []interface{}{1, 2, 3})
 }
+
+// Test that a list l is not modified when calling MoveAfter or MoveBefore with a mark that is not an element of l.
+func TestMoveUnkownMark(t *testing.T) {
+	var l1 List
+	e1 := l1.PushBack(1)
+
+	var l2 List
+	e2 := l2.PushBack(2)
+
+	l1.MoveAfter(e1, e2)
+	checkList(t, &l1, []interface{}{1})
+	checkList(t, &l2, []interface{}{2})
+
+	l1.MoveBefore(e1, e2)
+	checkList(t, &l1, []interface{}{1})
+	checkList(t, &l2, []interface{}{2})
+}