Let image manager return a copy of image list.

pkg/kubelet/images/image_gc_manager.go

@@ -124,7 +124,7 @@ func (i *imageCache) set(images []container.Image) {
 func (i *imageCache) get() []container.Image {
 	i.RLock()
 	defer i.RUnlock()
-	return i.images
+	return append([]container.Image{}, i.images...)
 }
 
 // Information about the images we track.

pkg/kubelet/images/image_gc_manager_test.go

@@ -548,6 +548,16 @@ func TestValidateImageGCPolicy(t *testing.T) {
 	}
 }
 
+func TestImageCacheReturnCopiedList(t *testing.T) {
+	cache := &imageCache{}
+	testList := []container.Image{{ID: "1"}, {ID: "2"}}
+	cache.set(testList)
+	list := cache.get()
+	assert.Len(t, list, 2)
+	list[0].ID = "3"
+	assert.Equal(t, cache.get(), testList)
+}
+
 func uint64Ptr(i uint64) *uint64 {
 	return &i
 }