Create all layers before filling in the data

QGIS (or GDAL) prefer the gpkg structure to not change after
first opening

libqfieldsync/offliners.py

@@ -200,17 +200,9 @@ def ogr_escape(self, data: str):
         # CSLFetchNameValue compares the name of an option followed by a `=` or `:` and treats everything after as value.
         return data
 
-    def convert_to_offline_layer(
-        self,
-        layer: QgsVectorLayer,
-        data_source: ogr.DataSource,
-        offline_gpkg_path: str,
-        feature_request: QgsFeatureRequest = QgsFeatureRequest(),
-    ) -> str:
-        """
-        Will create a copy of ``layer`` in the GeoPackage specified as ``data_source`` which is stored at ``offline_gpkg_path``.
-        It will replace the dataProvider of the original layer.
-        """
+    def create_layer(
+        self, layer: QgsVectorLayer, data_source: ogr.DataSource, offline_gpkg_path: str
+    ) -> None:
 
         identifier = hashlib.sha256(
             layer.dataProvider().dataSourceUri().encode()
@@ -253,7 +245,27 @@ def convert_to_offline_layer(
                 )
 
         ogr_layer.SyncToDisk()
-        qgis_uri = f"{offline_gpkg_path}|layername={identifier}"  # |option:QGIS_FORCE_WAL=ON' TODO: check why forcewal is enabled
+
+    def convert_to_offline_layer(
+        self,
+        layer: QgsVectorLayer,
+        data_source: ogr.DataSource,
+        offline_gpkg_path: str,
+        feature_request: QgsFeatureRequest = QgsFeatureRequest(),
+    ) -> str:
+        """
+        Will create a copy of ``layer`` in the GeoPackage specified as ``data_source`` which is stored at ``offline_gpkg_path``.
+        It will replace the dataProvider of the original layer.
+        """
+
+        identifier = hashlib.sha256(
+            layer.dataProvider().dataSourceUri().encode()
+        ).hexdigest()
+
+        # See https://github.com/qgis/QGIS/commit/fe671f8bba3825c6b6e83b7c4693d2a3113b90fa for background on QGIS_FORCE_WAL
+        qgis_uri = (
+            f"{offline_gpkg_path}|layername={identifier}|option:QGIS_FORCE_WAL=ON"
+        )
 
         qgis_layer_options = QgsVectorLayer.LayerOptions(
             QgsProject.instance().transformContext()
@@ -382,6 +394,10 @@ class LayerInfo(NamedTuple):
 
             datasource_mapping[datasource_hash].append(LayerInfo(layer, subset_string))
 
+        for datasource_hash, layer_infos in datasource_mapping.items():
+            layer_to_offline = layer_infos[0].layer
+            self.create_layer(layer_to_offline, data_source, offline_gpkg_path)
+
         for datasource_hash, layer_infos in datasource_mapping.items():
             request = QgsFeatureRequest()
             # All layers for given `datasource_hash` are pointing to the very same file/datasource.