Fix updates not backfilling correctly

The backfill was applied to a copy of the item, so the changes were lost. This broke batching, which would apply to the first fetch but not to the second.

core/source.go

@@ -276,8 +276,8 @@ func updateWithFetchedItemsTx(
 	}
 
 	// Bulk update the existing items
-	for _, item := range updatedItems {
-		BackfillItem(&item, existingItemsById[item.Id])
+	for i := range updatedItems {
+		BackfillItem(&updatedItems[i], existingItemsById[updatedItems[i].Id])
 	}
 	if err = UpdateItems(db, updatedItems); err != nil {
 		return 0, 0, err

core/source_test.go

@@ -379,16 +379,27 @@ func TestSourceBatching(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if items[0].Id == "i" {
-		item1 = items[0]
-		item2 = items[1]
-	} else {
-		item1 = items[1]
-		item2 = items[0]
-	}
+	item1 = items[0]
+	item2 = items[1]
 	if item2.Tts != item1.Tts-1 {
 		t.Fatalf("expected different tts based on batch time, for %d and %d", item1.Tts, item2.Tts)
 	}
+
+	add, del, err = UpdateWithFetchedItems(db, "s", nil, []Item{item2}, now.Add(10*time.Second))
+	if add != 0 || del != 0 || err != nil {
+		t.Fatalf("expected only updates: %v", err)
+	}
+
+	itemsAgain, err := GetAllItemsForSource(db, "s", 0, 100)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if itemsAgain[1].Id != item2.Id {
+		t.Error("unexpected return order")
+	}
+	if itemsAgain[1].Tts != item2.Tts {
+		t.Fatalf("batch tts not backfilled: %d vs %d", item2.Tts, itemsAgain[0].Tts)
+	}
 }
 
 func TestSourceLastUpdated(t *testing.T) {