CastToXYZ on empty geometry returns weird data with EnableGpkgMode()

[Pieter Roggemans]

Hello Sandro,

In the beginning of 2024 I reported a crash when running some functions on an empty geometry in a GPKG file: https://groups.google.com/g/spatialite-users/c/nzwISNn0eO8/m/uh2rWEW3AAAJ

I'm not sure it this is the same, but I encountered a similar but possibly slightly different issue.

When running the following query in GDAL on an empty geometry the xmin returned is the max int value (1.7976931348623157e+308).

> SELECT ST_MinX(CastToXYZ(geom)) AS minx FROM "src_lyr"

However, when I run this query in plain SQLite + Spatialite this does return the expected NULL, so I'm not able to reproduce it "standalone".

The ~reason of this difference is that ST_MinX is implemented in GDAL as well..., so when I run this query using GDAL it is the GDAL version that takes precedence, and that version returns maxint.

I reported an issue in the GDAL issue tracker: https://github.com/OSGeo/gdal/issues/13557

Rouault looked into it, and apparently CastToXYZ returns odd data when ran on an empty geometry with EnableGpkgMode enabled. 

Quote from Rouault: "It returns a GeoPackage header that with the Empty bit set to 0 (so a non-empty geometry) and the ExtentXY bit set (whereas the minx,miny,maxx,maxy are set to +/- DBL_MAX)"

He implemented a workaround in GDAL to catch this case, but it would be a lot better if this would be fixed at the source.

In the following geopackage fid=47 is an empty geometry:

https://github.com/geofileops/geofileops/blob/main/tests/data/polygon-parcel.gpkg

Here a script that gives some info, but doesn't reproduce the problem ;-).

> -- Init
> SELECT load_extension('mod_spatialite');
> SELECT EnableGpkgMode();
>
> SELECT GeomFromText('GeometryCollection()');
>
> SELECT * FROM "parcels";
> -- Returns 16 which is a bit weird, but it isn't 0 so OK enough for me.
> SELECT ST_IsEmpty(geom) FROM "parcels" WHERE fid = 47;
> SELECT ST_NPoints(geom) FROM "parcels" WHERE fid = 47;
> -- Returns "GEOMETRYCOLLECTION()"
> SELECT ST_AsText(geom) FROM "parcels" WHERE fid = 47;
> -- Returns NULL in when ran in SQLITE, but apparently the data returned by  CastToXYZ(geom)
> -- is pretty weird
> SELECT ST_MinX(CastToXYZ(geom)) FROM "parcels" WHERE fid = 47;
> -- I reported a crash on empty geometries in the beginning of 2024 which was fixed but hasn't
> -- been released yet... so this stil crashes.
> SELECT ST_ReducePrecision(geom, 0.001) FROM "parcels" WHERE fid = 47;

Regards,

Pieter