[gdal-dev] Strange (expected?) behaviour exporting multipart geometry to ESRI Shapefile format

[Andrea Giudiceandrea]

Hi GDAL devs,
trying find the root cause of a bug [1] reported in the QGIS GitHub 
repository, I've noticed a strange behaviour when a multipart geometry 
is exported to an ESRI Shapefile layer.

I've created a minimal GeoPackage layer file [2] which contains 1 
multipart polygon geometry (MultiPolygon) feature consisting of 3 parts.
The 3 parts are 3 clockwise polygons external rings.

The following command for such GeoPackage layer:
ogrinfo Polygon_3.gpkg Polygon_3 -geom=SUMMARY

reports:
OGRFeature(Polygon_3):0
   MULTIPOLYGON : 3 geometries:
POLYGON : 7 points
POLYGON : 9 points
POLYGON : 21 points

It seems to me something strange happens converting the GeoPackage layer 
to an ESRI Shapefile layer using e.g:
ogr2ogr -f "ESRI Shapefile" Polygon_3.shp Polygon_3.gpkg

ogrinfo for the resulting ESRI Shapefile layer reports:
OGRFeature(Polygon_3):0
   FID (Integer64) = 0
   MULTIPOLYGON : 2 geometries:
POLYGON : 9 points
POLYGON : 21 points, 1 inner rings (7 points)

That is, the 7 points part has been written in the Shapefile layer as a 
counter clockwise polygon and "attached" to the 21 points part (which 
remains a clockwise polygon external ring) as his inner ring. The 9 
points part remains a clockwise polygon external ring.

On the contrary, converting the GeoPackage layer to GML or FlatGeobuf or 
GeoJSON or Spatialite formats, the resulting layer correctly contains 1 
MultiPolygon feature consisting of 3 clockwise Polygon parts like the 
original layer and ogrinfo reports:
OGRFeature(Polygon_3):0
   MULTIPOLYGON : 3 geometries:
POLYGON : 7 points
POLYGON : 9 points
POLYGON : 21 points

I've checked the orientation of the polygons parts using QGIS 3.22.3 for 
all the layers and also ArcMap/ArcGIS 9.3.1 for the Shapefile layer.

QGIS reports 3 parts for the geometry in the GeoPackage, GML, 
FlatGeobuf, GeoJSON or Spatialite layers, while it reports 2 parts for 
the ESRI Shapefile layer.

Is this an expected behaviour of the ESRI Shapefile driver writing 
multipart features geometries?

Best regards.

Andrea Giudiceandrea

[1] https://github.com/qgis/QGIS/issues/47288
[2] https://drive.google.com/file/d/10HiCARTYJqhAXUckK43nU4MzV1wUbLg9