Remove check causing informers to miss notifications

Fix DeltaFIFO bug that caused the sync delta created by a relist
for object ID X to be dropped if the DeltaFIFO already stored a
Delete delta for X. This caused SharedIndexInformer to miss create
notifications. Also, add unit test to expose the bug.

Kubernetes-commit: 9e4be54895e8c0655408b4766cd78de17b6d5b7c

tools/cache/delta_fifo_test.go

@@ -85,6 +85,33 @@ func TestDeltaFIFO_basic(t *testing.T) {
 	}
 }
 
+// TestDeltaFIFO_replaceWithDeleteDeltaIn tests that a `Sync` delta for an
+// object `O` with ID `X` is added when .Replace is called and `O` is among the
+// replacement objects even if the DeltaFIFO already stores in terminal position
+// a delta of type `Delete` for ID `X`. Not adding the `Sync` delta causes
+// SharedIndexInformers to miss `O`'s create notification, see https://github.com/kubernetes/kubernetes/issues/83810
+// for more details.
+func TestDeltaFIFO_replaceWithDeleteDeltaIn(t *testing.T) {
+	oldObj := mkFifoObj("foo", 1)
+	newObj := mkFifoObj("foo", 2)
+
+	f := NewDeltaFIFO(testFifoObjectKeyFunc, keyLookupFunc(func() []testFifoObject {
+		return []testFifoObject{oldObj}
+	}))
+
+	f.Delete(oldObj)
+	f.Replace([]interface{}{newObj}, "")
+
+	actualDeltas := Pop(f)
+	expectedDeltas := Deltas{
+		Delta{Type: Deleted, Object: oldObj},
+		Delta{Type: Sync, Object: newObj},
+	}
+	if !reflect.DeepEqual(expectedDeltas, actualDeltas) {
+		t.Errorf("expected %#v, got %#v", expectedDeltas, actualDeltas)
+	}
+}
+
 func TestDeltaFIFO_requeueOnPop(t *testing.T) {
 	f := NewDeltaFIFO(testFifoObjectKeyFunc, nil)