qgis_process: impossible to execute some algorithms for lack of project context

[agiudiceandrea]

Describe the bug

Some algorithms need information stored in the project properties, like ellipsoid to use, or crs of the project, area and length measurement units, in order to properly execute the job.

It seems impossible to run those algs using qgis_process because it is not possible to specify a project containing the needed information nor to provide the necessary information directly passing them as additional parameters.

A probably not comprehensive list of algorithms that use context.project() follows:

qgis:exportaddgeometrycolumns (only methods 1 e 2)
qgis:fieldcalculator
qgis:distancetonearesthublinetohub
qgis:distancetonearesthubpoints
qgis:distancematrix
qgis:pointstopath
qgis:randompointsalongline
qgis:randompointsinsidepolygons
qgis:executesql
qgis:rastercalculator
qgis:postgisexecuteandloadsql
qgis:tilesxyzdirectory

native:antimeridiansplit
native:sumlinelengths
native:calculatevectoroverlaps
native:nearestneighbouranalysis
native:refactorfields
native:linedensity
native:aggregate
native:hublines
native:categorizeusingstyle
native:networkanalysis
native:rasterize
native:loadlayer
native:setprojectvariable
native:stylefromproject
native:bookmarkstolayer

How to Reproduce

qgis_process run qgis:randompointsinsidepolygons --STRATEGY=0 --VALUE=1000 --INPUT="./ProvCM01012020_g/ProvCM01012020_g_WGS84.shp" --OUTPUT="./output/random_point_prov.shp"

/usr/lib/python3/dist-packages/qgis/utils.py:743: DeprecationWarning: the
imp module is deprecated in favour of importlib; see the module's
documentation for alternative uses
  mod = _builtin_import(name, globals, locals, fromlist, level)
Problem with SAGA installation: SAGA was not found or is not correctly
installed
----------------
Inputs
----------------

INPUT:  ./ProvCM01012020_g/ProvCM01012020_g_WGS84.shp
OUTPUT: ./output/random_point_prov.shp
STRATEGY:       0
VALUE:  1000

Logged critical: Traceback (most recent call last):
  File
"/usr/share/qgis/python/plugins/processing/algs/qgis/RandomPointsPolygons.py",
line 160, in processAlgorithm
    da.setEllipsoid(context.project().ellipsoid())
AttributeError: 'NoneType' object has no attribute 'ellipsoid'

Traceback (most recent call last):
  File
"/usr/share/qgis/python/plugins/processing/algs/qgis/RandomPointsPolygons.py",
line 160, in processAlgorithm
    da.setEllipsoid(context.project().ellipsoid())
AttributeError: 'NoneType' object has no attribute 'ellipsoid'
ERROR:  Traceback (most recent call last):
  File
"/usr/share/qgis/python/plugins/processing/algs/qgis/RandomPointsPolygons.py",
line 160, in processAlgorithm
    da.setEllipsoid(context.project().ellipsoid())
AttributeError: 'NoneType' object has no attribute 'ellipsoid'

QGIS and OS versions

QGIS 3.14.1

Additional context

Ref: #34617