[QGIS-Developer] What to do about incorrect SAGA algorithms?

[Nyall Dawson]

Hi list,

Just raising the discussion about what we should do with SAGA
Processing algorithms when the results generated by SAGA are known to
be incorrect (i.e. there's a bug in SAGA itself).

Specifically, the SAGA difference and symmetric difference algorithms
generate incorrect results. See https://issues.qgis.org/issues/21354,
and the results from
https://github.com/qgis/QGIS/blob/master/python/plugins/processing/tests/testdata/saga_algorithm_tests.yaml#L318
. This seems to be a general problem with the algorithms, not isolated
to particular input data.

My gut feeling is: we should "deprecate" these algorithms (which means
they continue to work for existing models, but are hidden from the UI
and cannot be run from the toolbox or added to new models). I don't
see the value in exposing broken algorithms when we have robust
alternatives (the native QGIS difference/symmetric difference
algorithms are very fast, stable, and well tested), and potentially a
lot of harm.

Thoughts?

Nyall