fix(storage): make Query show Prefix consistently (#2661)

A Query with SetAttrSelection currently hides Prefix results,
which are returned otherwise. This is a fix to make sure
Prefixes are not inadvertently filtered.

Fixes #1763.

storage/integration_test.go

@@ -686,6 +686,43 @@ func TestIntegration_Objects(t *testing.T) {
 	testObjectsIterateSelectedAttrs(t, bkt, objects)
 	testObjectsIterateAllSelectedAttrs(t, bkt, objects)
 	testObjectIteratorWithOffset(t, bkt, objects)
+	t.Run("testObjectsIterateSelectedAttrsDelimiter", func(t *testing.T) {
+		query := &Query{Prefix: "", Delimiter: "/"}
+		if err := query.SetAttrSelection([]string{"Name"}); err != nil {
+			t.Fatalf("selecting query attrs: %v", err)
+		}
+
+		var gotNames []string
+		var gotPrefixes []string
+		it := bkt.Objects(context.Background(), query)
+		for {
+			attrs, err := it.Next()
+			if err == iterator.Done {
+				break
+			}
+			if err != nil {
+				t.Fatalf("iterator.Next: %v", err)
+			}
+			if attrs.Name != "" {
+				gotNames = append(gotNames, attrs.Name)
+			} else if attrs.Prefix != "" {
+				gotPrefixes = append(gotPrefixes, attrs.Prefix)
+			}
+
+			if attrs.Bucket != "" {
+				t.Errorf("Bucket field not selected, want empty, got = %v", attrs.Bucket)
+			}
+		}
+
+		sortedNames := []string{"obj1", "obj2"}
+		if !cmp.Equal(sortedNames, gotNames) {
+			t.Errorf("names = %v, want %v", gotNames, sortedNames)
+		}
+		sortedPrefixes := []string{"obj/"}
+		if !cmp.Equal(sortedPrefixes, gotPrefixes) {
+			t.Errorf("prefixes = %v, want %v", gotPrefixes, sortedPrefixes)
+		}
+	})
 
 	// Test Reader.
 	for _, obj := range objects {
@@ -1136,7 +1173,7 @@ func testObjectsIterateSelectedAttrs(t *testing.T, bkt *BucketHandle, objects []
 			break
 		}
 		if err != nil {
-			log.Fatal(err)
+			t.Fatalf("iterator.Next: %v", err)
 		}
 		gotNames = append(gotNames, attrs.Name)
 
@@ -1177,7 +1214,7 @@ func testObjectsIterateAllSelectedAttrs(t *testing.T, bkt *BucketHandle, objects
 			break
 		}
 		if err != nil {
-			log.Fatal(err)
+			t.Fatalf("iterator.Next: %v", err)
 		}
 		count++
 	}

storage/storage.go

@@ -1386,7 +1386,7 @@ func (q *Query) SetAttrSelection(attrs []string) error {
 
 	if len(fieldSet) > 0 {
 		var b bytes.Buffer
-		b.WriteString("items(")
+		b.WriteString("prefixes,items(")
 		first := true
 		for field := range fieldSet {
 			if !first {