iterx: paging iteration working

We used to rely upon NumRows to determine if there are
new pages available. This is not correct since the server
is allowed to return empty pages with has_more_data flag
set and the needed data to do this is not exposed by
the gocql driver.

We simply remove these checks and let the driver decide
when to stop reading.

Co-authored-by: Henrik Johansson <henrik@scylladb.com>
Co-authored-by: Piotr Sarna <sarna@scylladb.com>

iterx.go

@@ -80,11 +80,6 @@ func (iter *Iterx) scanAny(dest interface{}, structOnly bool) bool {
 		return false
 	}
 
-	// no results or query error
-	if iter.Iter.NumRows() == 0 {
-		return false
-	}
-
 	base := reflectx.Deref(value.Type())
 	scannable := isScannable(base)
 
@@ -132,11 +127,6 @@ func (iter *Iterx) scanAll(dest interface{}, structOnly bool) bool {
 		return false
 	}
 
-	// no results or query error
-	if iter.Iter.NumRows() == 0 {
-		return false
-	}
-
 	slice, err := baseType(value.Type(), reflect.Slice)
 	if err != nil {
 		iter.err = err
@@ -180,7 +170,7 @@ func (iter *Iterx) scanAll(dest interface{}, structOnly bool) bool {
 
 		// allocate memory for the page data
 		if !alloc {
-			v = reflect.MakeSlice(slice, 0, iter.Iter.NumRows())
+			v = reflect.MakeSlice(slice, 0, iter.NumRows())
 			alloc = true
 		}
 
@@ -212,11 +202,6 @@ func (iter *Iterx) StructScan(dest interface{}) bool {
 		return false
 	}
 
-	// no results or query error
-	if iter.Iter.NumRows() == 0 {
-		return false
-	}
-
 	if !iter.started {
 		columns := columnNames(iter.Iter.Columns())
 		m := iter.Mapper

iterx_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/gocql/gocql"
 	"github.com/scylladb/gocqlx"
 	. "github.com/scylladb/gocqlx/gocqlxtest"
+	"github.com/scylladb/gocqlx/qb"
 	"gopkg.in/inf.v0"
 )
 
@@ -327,3 +328,46 @@ func TestNotFound(t *testing.T) {
 		}
 	})
 }
+
+func TestPaging(t *testing.T) {
+	session := CreateSession(t)
+	defer session.Close()
+	if err := ExecStmt(session, `CREATE TABLE gocqlx_test.paging_table (id int PRIMARY KEY, val int)`); err != nil {
+		t.Fatal("create table:", err)
+	}
+	if err := ExecStmt(session, `CREATE INDEX id_val_index ON gocqlx_test.paging_table (val)`); err != nil {
+		t.Fatal("create index:", err)
+	}
+	stmt, names := qb.Insert("gocqlx_test.paging_table").Columns("id", "val").ToCql()
+	q := gocqlx.Query(session.Query(stmt), names)
+	for i := 0; i < 5000; i++ {
+		if err := q.Bind(i, i).Exec(); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	type Paging struct {
+		ID  int
+		Val int
+	}
+
+	t.Run("iter", func(t *testing.T) {
+		stmt, names := qb.Select("gocqlx_test.paging_table").
+			Where(qb.Lt("val")).
+			AllowFiltering().
+			Columns("id", "val").ToCql()
+		it := gocqlx.Query(session.Query(stmt, 100).PageSize(10), names).Iter()
+		defer it.Close()
+		var cnt int
+		for {
+			p := &Paging{}
+			if !it.StructScan(p) {
+				break
+			}
+			cnt++
+		}
+		if cnt != 100 {
+			t.Fatal("expected 100", "got", cnt)
+		}
+	})
+}