[vector file writer] use field type to save attribute values

(relying on attribute value type isn't reliable under certain
scenarios, including memory layers created through a processing
python algorithm)

python/plugins/processing/tests/testdata/expected/import_photos.gml

@@ -19,7 +19,7 @@
       <ogr:directory>/home/nyall/dev/QGIS/python/plugins/processing/tests/testdata/custom/photos</ogr:directory>
       <ogr:altitude>422.191</ogr:altitude>
       <ogr:direction>59.9153</ogr:direction>
-      <ogr:longitude>149.2751666666667</ogr:longitude>
+      <ogr:longitude>149.27516666666668</ogr:longitude>
       <ogr:latitude>-37.2305</ogr:latitude>
       <ogr:timestamp>2012/03/06 14:28:40</ogr:timestamp>
     </ogr:import_photos>

src/core/qgsvectorfilewriter.cpp

@@ -2088,6 +2088,7 @@ gdal::ogr_feature_unique_ptr QgsVectorFileWriter::createFeature( const QgsFeatur
     int ogrField = it.value();
 
     QVariant attrValue = feature.attribute( fldIdx );
+    QgsField field = mFields.at( fldIdx );
 
     if ( !attrValue.isValid() || attrValue.isNull() )
     {
@@ -2106,17 +2107,16 @@ gdal::ogr_feature_unique_ptr QgsVectorFileWriter::createFeature( const QgsFeatur
 
     if ( mFieldValueConverter )
     {
+      field = mFieldValueConverter->fieldDefinition( field );
       attrValue = mFieldValueConverter->convert( fldIdx, attrValue );
     }
 
-    switch ( attrValue.type() )
+    switch ( field.type() )
     {
       case QVariant::Int:
-      case QVariant::UInt:
         OGR_F_SetFieldInteger( poFeature.get(), ogrField, attrValue.toInt() );
         break;
       case QVariant::LongLong:
-      case QVariant::ULongLong:
         OGR_F_SetFieldInteger64( poFeature.get(), ogrField, attrValue.toLongLong() );
         break;
       case QVariant::Bool:

tests/src/python/test_qgsvectorfilewriter.py

@@ -94,6 +94,27 @@ def testWrite(self):
 
         writeShape(self.mMemoryLayer, 'writetest.shp')
 
+    def testWriteWithLongLongField(self):
+        ml = QgsVectorLayer('NoGeometry?crs=epsg:4326&field=fldlonglong:long',
+                            'test2', 'memory')
+        provider = ml.dataProvider()
+        feat = QgsFeature()
+        feat.setAttributes([2262000000])
+        provider.addFeatures([feat])
+
+        filename = os.path.join(str(QDir.tempPath()), 'with_longlong_field')
+        crs = QgsCoordinateReferenceSystem()
+        crs.createFromId(4326, QgsCoordinateReferenceSystem.EpsgCrsId)
+        rc, errmsg = QgsVectorFileWriter.writeAsVectorFormat(ml, filename, 'utf-8', crs, 'GPKG')
+
+        # open the resulting geopackage
+        vl = QgsVectorLayer(filename + '.gpkg', '', 'ogr')
+        self.assertTrue(vl.isValid())
+
+        # test values
+        idx = vl.fields().indexFromName('fldlonglong')
+        self.assertEqual(vl.getFeature(1).attributes()[idx], 2262000000)
+
     def testWriteWithBoolField(self):
 
         # init connection string