presto-iceberg/src/main/java/io/prestosql/plugin/iceberg/util/PageListBuilder.java/PageListBuilder/beginRow
class PageListBuilder:    public void beginRow()
    {
        checkArgument(channel == -1, "already in row");
        if (pageBuilder.isFull()) {
            pages.add(pageBuilder.build());
            pageBuilder.reset();
        }
        pageBuilder.declarePosition();
        channel = 0;
    }