[QGIS-Developer] GDAL processing algorithms issues due the default output layer extensions weirdly changed

[Andrea Giudiceandrea]

Hi developers,
various QGIS users have reported during the latest months (mostly since 
September 2023, and a couple also back in July 2023) on various channels 
a strange issue which makes the GDAL processing algorithms to fail when 
the temporary output is set.

Looking at the provided processing logs, it turns out that the issue is 
due to the fact that the default output vector layer extension was set 
to .xtf "Interlis 2" (instead of the default .gpkg "GeoPackage") or that 
the default output raster layer extension was set to .nc "NetCDF" 
(instead of the default .tif "GeoTIFF").

The weird think is that all the users affected by the issue have 
reported that they had not deliberately changed these settings.

Currently there is no issue report open on GitHub since they have been 
closed one by the reporter and one as duplicate.
Anyway the issues reports were about the processing issues and not about 
the "who changed the default output layer extensions?" issue.

I tried without success to find out what's going on and I hope some 
developers can have a look and find the root cause of the issue, since 
the issue continues to be reported by the users.
Actually the issue can also affect both the QGIS and the GRASS 
processing algorithms that have a raster output, although not reported yet.

Since both the layer extensions .xtf "Interlis 2" and .nc "NetCDF" are 
the last one (i.e. index -1) in the default output vector layer 
extension list and the the default output raster layer extension list, 
It may be possible that the issue may be due to the recent changes in 
the settings storing / retrieving logic.

You'll find the links to a dozen reports (either on GitHub, 
stackexchange, qgis-user ml, Facebook groups) listed in the comments of 
the closed issue report at https://github.com/qgis/QGIS/issues/54952

Best regards.

Andrea Giudiceandrea


P.S. while writing this message I found an issue report 
https://github.com/qgis/QGIS/issues/53204 and a PR 
https://github.com/qgis/QGIS/pull/53458 allegedly fixing it both dating 
May 2023 precisely relating to the DefaultOutputRasterLayerExt and the 
DefaultOutputVectorLayerExt settings.