Empty Temporary scratch layer made permanent is not editable.

[ValPinnaSardinia]

Issue

After recording a weird behaviour with some layers, I have realised that when a new Temporary scratch layer is made permanent (as .shp) before adding to it any new feature or field, it becomes not editable.
The new layer does not accept new features or new fields. Also, trying to use the Field calculator to create a new field, it appears the message This layer does not support adding new provider fields. You can only add virtual fields.

Also removing and readding the layer to the project doesn't solve the problem.

How to Reproduce

1. Create a new Temporary scratch layer (with any geometry type) without adding any field.
2. Make it permanent as shapefile .shp clicking in the icon in the Layers Panel or using the voice in the menu.
3. Toggle off the editing mode.
4. Toggle on the editing mode.
5. The Add Feature icon it will be not selectable.
6. Open the Attribute Table: the add new Field icon is not selectable.
7. Open the Field Calculator: it will appear the message This layer does not support adding new provider fields. You can only add virtual fields.

QGIS and OS versions

Tried on QGIS 3.10.10, 3.14.16 and dev 3.15 on Windows 8 and 10

[gioman]

@ValPinnaSardinia Cannot confirm here, at least on Linux.

[pigreco]

I confirm

OSGeo4W64 WIn 10

[gioman]

@pigreco @ValPinnaSardinia works fine on Windows too here. Please try with a new QGIS profile.

[ValPinnaSardinia]

I have tried with a new clean profile and I confirm that I have the same problem.

[gioman]

Ok, probably I'm doing it wrong, I will test again later.

[bellanz80]

I confirm

[agiudiceandrea]

@gioman I can confirm also with 3.4 (GDAL/OGR 2.4.0) and 2.18 (GDAL/OGR 2.2.4) on Windows.

Minimal procedure:

1. Create a new Temporary scratch layer (with any geometry type) without adding any field.
2. Make it permanent as shapefile .shp clicking in the icon in the Layers Panel or using the voice in the menu.
3. The Add Feature icon it will be not selectable.
4. Open the Attribute Table: the add new Field icon is not selectable.
5. Open the Field Calculator: it will appear the message This layer does not support adding new provider fields. You can only add virtual fields.

It seems that in QGIS it is not possible to add a geometry feature or a field to a shapefile without at least 1 field. So it's not editable de facto.

If the shapefile without at least 1 field contains geometries, then is possible to edit the feature geometries but not to add new feature geometries or fields.

Another way to generate a shapefile without at least 1 field is: create a shapefile with 1 field (it is not possible to create a shapefile without at least 1 field using the QGIS GUI) and then remove the field and remove and readd the shapefile.

In https://gdal.org/drivers/vector/shapefile.html it is stated that:
".dbf files are required to have at least one field. If none are created by the application an “FID” field will be automatically created and populated with the record number."

Anyway I think this is a bug. It should be possible to add fields to a shapefile without fields (or, at least, a warning should be displayed when a vector layer without fields is "made permanent" or saved to a shapefile" or when all the fields are deleted from a shapefile).

With ArcGIS/ArcMap 9 it is possible without problem to add features and fields to a shapefile without fields.

In the ESRI Shapefile Technical Description there are no constraints on the number of field in the dbf attribute table.

The problem should be in the ogr data provider and in the way it calculates the layer capabilities.

For a shapefile layer without at least 1 field the following capabilities are reported by the ogr data provider:
'Rename Attributes, Create Spatial Index, Create Attribute Indexes, Fast Access to Features at ID, Change Geometries'
so it lacks of the 'Add Attributes' capability.

[gioman]

The problem should be in the ogr data provider and in the way it calculates the layer capabilities.

@agiudiceandrea or ogr itself? from what you describe seems an upstream issue(?).

[agiudiceandrea]

or ogr itself? from what you describe seems an upstream issue(?).

Using ogrinfo cli tool it is possible to add a field to a shapefile without fields.

[agiudiceandrea]

Confirmed also with QGIS 3.14 on Ubuntu Linux 18.04 (GDAL/OGR 2.2.3) and 20.04 (GDAL/OGR 3.0.4).

[agiudiceandrea]

It seems the relevant code is at

QGIS/src/core/providers/ogr/qgsogrprovider.cpp

Lines 2832 to 2849 in 73626bd

	// OGR doesn't handle shapefiles without attributes, ie. missing DBFs well, fixes #803
	if ( mGDALDriverName == QLatin1String( "ESRI Shapefile" ) )
	{
	ability |= CreateSpatialIndex;
	ability |= CreateAttributeIndex;
	if ( mAttributeFields.size() == 0 )
	{
	QgsMessageLog::logMessage( tr( "Shapefiles without attribute are considered read-only." ), tr( "OGR" ) );
	ability &= ~( AddFeatures | DeleteFeatures | ChangeAttributeValues | AddAttributes | DeleteAttributes );
	}
	if ( ( ability & ChangeAttributeValues ) == 0 )
	{
	// on readonly shapes OGR reports that it can delete features although it can't RandomWrite
	ability &= ~( AddAttributes | DeleteFeatures );
	}
	}

The capability limitation for shapefiles w/o attributes was introduced a long long time ago: bf62b39
Probably the OGR driver is improved in the mean time...

[elpaso]

@rouault is this a limitation of the GDAL/OGR library or can we fix it QGIS-side?

[agiudiceandrea]

Hi @elpaso and @rouault, any news about this issue?