Make Query.Iter to skip through empty iters when pagining is manual

In some cases it confuses user to have no rows in the iterator.
This simple solution makes Query.Iter to retry on empty iterator.

cassandra_test.go

@@ -419,6 +419,11 @@ func TestPagingWithAllowFiltering(t *testing.T) {
 
 				for {
 					iter := qry.PageState(currentPageState).Iter()
+
+					// Here we make sure that all iterator, but last one have some data in it
+					if !iter.LastPage() && iter.NumRows() == 0 {
+						t.Errorf("expected at least one row, but got 0")
+					}
 					for iter.Scan(&c1, &f1) {
 						if c1 != f1 {
 							t.Fatalf("expected c1 and f1 values to be the same, but got c1=%d f1=%d", c1, f1)
@@ -428,10 +433,13 @@ func TestPagingWithAllowFiltering(t *testing.T) {
 					if err := iter.Close(); err != nil {
 						t.Fatal("select:", err.Error())
 					}
-					newPageState := iter.PageState()
-					if len(newPageState) == 0 || len(currentPageState) == len(newPageState) && bytes.Compare(newPageState, currentPageState) == 0 {
+					if iter.LastPage() {
 						break
 					}
+					newPageState := iter.PageState()
+					if len(currentPageState) == len(newPageState) && bytes.Compare(newPageState, currentPageState) == 0 {
+						t.Fatalf("page state did not change")
+					}
 					currentPageState = newPageState
 				}
 
@@ -2355,7 +2363,7 @@ func TestManualQueryPaging(t *testing.T) {
 			fetched++
 		}
 
-		if len(iter.PageState()) > 0 {
+		if !iter.LastPage() {
 			// more pages
 			iter = query.PageState(iter.PageState()).Iter()
 		} else {

session.go

@@ -1393,9 +1393,24 @@ func (q *Query) Iter() *Iter {
 	if isUseStatement(q.stmt) {
 		return &Iter{err: ErrUseStmt}
 	}
-	// if the query was specifically run on a connection then re-use that
-	// connection when fetching the next results
+
+	if !q.disableAutoPage {
+		return q.executeQuery()
+	}
+
+	// Retry on empty page if pagination is manual
+	iter := q.executeQuery()
+	for iter.err == nil && iter.numRows == 0 && !iter.LastPage() {
+		q.PageState(iter.PageState())
+		iter = q.executeQuery()
+	}
+	return iter
+}
+
+func (q *Query) executeQuery() *Iter {
 	if q.conn != nil {
+		// if the query was specifically run on a connection then re-use that
+		// connection when fetching the next results
 		return q.conn.executeQuery(q.Context(), q)
 	}
 	return q.session.executeQuery(q)
@@ -1771,6 +1786,11 @@ func (iter *Iter) PageState() []byte {
 	return iter.meta.pagingState
 }
 
+// LastPage returns true if there are no more pages to fetch.
+func (iter *Iter) LastPage() bool {
+	return len(iter.meta.pagingState) == 0
+}
+
 // NumRows returns the number of rows in this pagination, it will update when new
 // pages are fetched, it is not the value of the total number of rows this iter
 // will return unless there is only a single page returned.