Error saving file only after converting multipolygon to single polygons

[secretmango]

What is the bug or the crash?

I have a geopackage layer with a huge multipolygon on it. It is the result of an overpass turbo OSM extraction, then I cut off the overlap with a different layer. Then I changed the coordinate system to a projected pseudo-mercator one.

All fine until now.

I want to convert the huge polygon into separate polygons, so that I can geoprocess them individually.

I use the "vector - geometry - multi- to single polygon" tool and it works instantly.

Then I try to save the temporary layer and always get the same crash.

Steps to reproduce the issue

1. Convert a multipolygon layer to single polygons. I will attach the geopackage for you.

demo-file-multipolygon.gpkg.zip

2. try to save or export the resulting single-polygon layer in geopackage format
3. it always errors out

Temporary Draft layer cannot be made permanent.
Error: Error writing object:
Object creation error (OGR-Fehler: failed to execute insert : UNIQUE constraint failed: this-file-cannot-be-saved.fid)
Objekterzeugungsfehler (OGR-Fehler: failed to execute insert : UNIQUE constraint failed: this-file-cannot-be-saved.fid)
... repeat
... stops after 1001 failed attempts

Versions

QGIS-Version	3.34.11-Prizren	QGIS-Codezweig	Release 3.34
Kompiliert gegen Qt	5.15.14	Laufendes Qt	5.15.15
Python-Version	3.12.6
GDAL-Version	3.8.5
PROJ-Version	9.3.1
EPSG-Registraturdatenbankversion	v10.098 (2023-11-24)
GEOS-Version	3.12.2-CAPI-1.18.2
SQLite-Version	3.45.1
PDAL-Version	2.6.3
PostgreSQL-Client-Version	16.4
SpatiaLite-Version	5.1.0
QWT-Version	6.2.0
QScintilla2-Version	2.13.4
BS-Version	Fedora Linux 40 (Toolbx Container Image)
Aktive Python-Erweiterungen
alkisplugin	2.0.56
address_search	0.1
SAGisALKISSearch	1.0.2
projektcheck	1.3
qgis-maptiler-plugin	3.3.2
OSMDownloader	1.0.3
MetaSearch	0.3.6
db_manager	0.1.20
grassprovider	2.12.99
processing	2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

this only happens with this exact converted file. All others didn't give an error. renaming, checking the name, checking again, changing locations, ... all dont work.

Restarting QGis also didnt work.

Using a BTRFS Filesystem.

[secretmango]

I see that all these files get saved, but stop at around 100KB. They are also saved to my home folder, which is strange. Normally the files are saved to the same folder the qgz project is in, which is not my home folder.

example-saved-file.zip

[secretmango]

So @nyalldawson , the issue is that the converted layer lacks unique IDs per polygon?

I will happily test the latest commit, but would also appreciate a workaround in the meantime, as you seem to know what it going wrong here.

Thanks!

[secretmango]

Hi, could you tell me when a new release is planned, so that this fix ends up on devices?

Or in the meantime if you know of any workaround?

I currently cannot proceed my work with this error 😅

I am trying to build latest QGis myself using QtCreator, after giving up on RPM packaging for now. I am unsure if Qt6 packages can already be used, as to my knowledge QGis is still fully using Qt5?

[troopa81]

Hi, could you tell me when a new release is planned, so that this fix ends up on devices?

QGIS 3.40 released the 25th of october, no backport is planned so far.

I am trying to build latest QGis myself using QtCreator, after giving up on RPM packaging for now. I am unsure if Qt6 packages can already be used, as to my knowledge QGis is still fully using Qt5?

Both Qt6 and Qt5 are working now, but the official is still Qt5. I don't know the policy for package on redhat/fedora but you'd probably have to build it yourself.

[nyalldawson]

@secretmango

Or in the meantime if you know of any workaround?

The easiest workaround would be to remove the fid field from the output layer before saving as gpkg. Alternatively if you directly set the processing tool to store the result in a gpkg it will automatically do this for you (on all current versions)

[secretmango]

thanks! Well that was obvious, they are all the same XD

Instead I did

1. Open field calculator
2. Click "update existing field"
3. Select the "fid" column
4. as function use @row_number

done