Clear old parent reference on resetting image parent

On migration 2 different images can end up with same
content addressable ID, meaning `SetParent` will be called
multiple times. Previous version did not clear the old
in-memory reference.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

image/store.go

@@ -231,6 +231,9 @@ func (is *store) SetParent(id, parent ID) error {
 	if parentMeta == nil {
 		return fmt.Errorf("unknown parent image ID %s", parent.String())
 	}
+	if parent, err := is.GetParent(id); err == nil && is.images[parent] != nil {
+		delete(is.images[parent].children, id)
+	}
 	parentMeta.children[id] = struct{}{}
 	return is.fs.SetMetadata(id, "parent", []byte(parent))
 }

image/store_test.go

@@ -233,6 +233,62 @@ func TestSearchAfterDelete(t *testing.T) {
 	}
 }
 
+func TestParentReset(t *testing.T) {
+	tmpdir, err := ioutil.TempDir("", "images-fs-store")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+	fs, err := NewFSStoreBackend(tmpdir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	is, err := NewImageStore(fs, &mockLayerGetReleaser{})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	id, err := is.Create([]byte(`{"comment": "abc1", "rootfs": {"type": "layers"}}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	id2, err := is.Create([]byte(`{"comment": "abc2", "rootfs": {"type": "layers"}}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	id3, err := is.Create([]byte(`{"comment": "abc3", "rootfs": {"type": "layers"}}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := is.SetParent(id, id2); err != nil {
+		t.Fatal(err)
+	}
+
+	ids := is.Children(id2)
+	if actual, expected := len(ids), 1; expected != actual {
+		t.Fatalf("wrong number of children: %d, got %d", expected, actual)
+	}
+
+	if err := is.SetParent(id, id3); err != nil {
+		t.Fatal(err)
+	}
+
+	ids = is.Children(id2)
+	if actual, expected := len(ids), 0; expected != actual {
+		t.Fatalf("wrong number of children after parent reset: %d, got %d", expected, actual)
+	}
+
+	ids = is.Children(id3)
+	if actual, expected := len(ids), 1; expected != actual {
+		t.Fatalf("wrong number of children after parent reset: %d, got %d", expected, actual)
+	}
+
+}
+
 type mockLayerGetReleaser struct{}
 
 func (ls *mockLayerGetReleaser) Get(layer.ChainID) (layer.Layer, error) {