[Backport 3.10][VirtualLayer] Catch exception while updating virtual layer stats

src/providers/virtual/qgsvirtuallayerprovider.cpp

@@ -568,20 +568,30 @@ void QgsVirtualLayerProvider::updateStatistics() const
                 .arg( hasGeometry ? QStringLiteral( ",Min(MbrMinX(%1)),Min(MbrMinY(%1)),Max(MbrMaxX(%1)),Max(MbrMaxY(%1))" ).arg( quotedColumn( mDefinition.geometryField() ) ) : QString(),
                       mTableName,
                       subset );
-  Sqlite::Query q( mSqlite.get(), sql );
-  if ( q.step() == SQLITE_ROW )
+
+  try
   {
-    mFeatureCount = q.columnInt64( 0 );
-    if ( hasGeometry )
+    Sqlite::Query q( mSqlite.get(), sql );
+    if ( q.step() == SQLITE_ROW )
     {
-      double x1, y1, x2, y2;
-      x1 = q.columnDouble( 1 );
-      y1 = q.columnDouble( 2 );
-      x2 = q.columnDouble( 3 );
-      y2 = q.columnDouble( 4 );
-      mExtent = QgsRectangle( x1, y1, x2, y2 );
+      mFeatureCount = q.columnInt64( 0 );
+      if ( hasGeometry )
+      {
+        double x1, y1, x2, y2;
+        x1 = q.columnDouble( 1 );
+        y1 = q.columnDouble( 2 );
+        x2 = q.columnDouble( 3 );
+        y2 = q.columnDouble( 4 );
+        mExtent = QgsRectangle( x1, y1, x2, y2 );
+      }
+      mCachedStatistics = true;
     }
-    mCachedStatistics = true;
+  }
+  catch ( std::runtime_error &e )
+  {
+    pushError( tr( "Error while executing feature count request : %1" ).arg( e.what() ) );
+    mFeatureCount = 0;
+    return;
   }
 }
 
@@ -709,4 +719,3 @@ QGISEXTERN QgsProviderGuiMetadata *providerGuiMetadataFactory()
   return new QgsVirtualLayerProviderGuiMetadata();
 }
 #endif
-

tests/src/python/test_provider_virtual.py

@@ -1187,6 +1187,26 @@ def test_subset_string(self):
         self.assertEqual(gpkg_virtual_layer.subsetString(), '"join_value" = \'twenty\'')
         self.assertEqual(gpkg_virtual_layer.featureCount(), 1)
 
+    def test_feature_count_on_error(self):
+        """Test that triggered exception while getting feature count on a badly defined
+        virtual layer is correctly caught (see https://github.com/qgis/QGIS/issues/34378)"""
+
+        l1 = QgsVectorLayer(os.path.join(self.testDataDir, "france_parts.shp"), "france", "ogr",
+                            QgsVectorLayer.LayerOptions(False))
+        self.assertEqual(l1.isValid(), True)
+        QgsProject.instance().addMapLayer(l1)
+
+        df = QgsVirtualLayerDefinition()
+        df.setQuery('select error')
+
+        vl = QgsVectorLayer(df.toString(), "testq", "virtual")
+        self.assertEqual(vl.isValid(), False)
+        self.assertEqual(vl.featureCount(), 0)
+        ids = [f.id() for f in vl.getFeatures()]
+        self.assertEqual(len(ids), 0)
+
+        QgsProject.instance().removeMapLayer(l1.id())
+
 
 if __name__ == '__main__':
     unittest.main()