fix Rows bug where Pilosa would crash without 'field' argument.

executor.go

@@ -2761,7 +2761,10 @@ func newGroupByIterator(rowIDs []RowIDs, children []*pql.Call, filter *Row, inde
 
 	ignorePrev := false
 	for i, call := range children {
-		fieldName := call.Args["field"].(string) // this has already been validated by this point
+		fieldName, ok := call.Args["field"].(string)
+		if !ok {
+			return nil, errors.Errorf("%s call must have 'field' argument", call.Name)
+		}
 		gbi.fields[i].Field = fieldName
 		// Fetch fragment.
 		frag := holder.fragment(index, fieldName, viewStandard, shard)

executor_test.go

@@ -2672,6 +2672,39 @@ func TestExecutor_Execute_Rows(t *testing.T) {
 	if !reflect.DeepEqual(rows, pilosa.RowIdentifiers{Rows: []uint64{11, 12}}) {
 		t.Fatalf("unexpected rows: %+v", rows)
 	}
+
+}
+
+func TestExecutor_Execute_Rows_Error(t *testing.T) {
+	c := test.MustRunCluster(t, 3)
+	defer c.Close()
+	c.CreateField(t, "i", pilosa.IndexOptions{}, "general")
+
+	tests := []struct {
+		query string
+		error string
+	}{
+		{
+			query: "GroupBy(Rows())",
+			error: "Rows call must have 'field' argument",
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
+			r, err := c[0].API.Query(context.Background(), &pilosa.QueryRequest{
+				Index: "i",
+				Query: test.query,
+			})
+			if err == nil {
+				t.Fatalf("should have gotten an error on invalid rows query, but got %#v", r)
+			}
+			if !strings.Contains(err.Error(), test.error) {
+				t.Fatalf("unexpected error message: %s", err.Error())
+			}
+		})
+	}
+
 }
 
 func TestExecutor_Execute_Rows_Keys(t *testing.T) {