[symbol] fix crasher when rendering a symbol layer subsequent to rendering a geometry generator layer (fixes #19121)

[nirvn]

Description

@m-kuhn , this PR fixes reported issue 19121 (https://issues.qgis.org/issues/19121).

Long story short, the geometry generator's render function was passing on a referenced QgsRenderContext, which modified its geometry(), replacing it with another QgsGeometry that'd scope out, and led to crash when a parent symbol rendered a subsequent symbol layer.

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[m-kuhn]

Interesting, I can't follow how the geometry would still be referenced after it scoped out. I think QgsFeature::setGeometry takes a copy?

[nirvn]

In renderFeature(), line 703 sets the QgsRenderContext::geometry() to a constGet() pointer (https://github.com/qgis/QGIS/blob/master/src/core/symbology/qgssymbol.cpp#L703) of a const QgsGeometry declared on line 695 (https://github.com/qgis/QGIS/blob/master/src/core/symbology/qgssymbol.cpp#L695) which scopes out.

Since the QgsRenderContext is passed on as a reference in QgsGeometryGeneratorSymbolLayer::render on line 204 (https://github.com/qgis/QGIS/blob/master/src/core/symbology/qgsgeometrygeneratorsymbollayer.cpp#L204), which itself is passed on as a context for the parent QgsFillSymbol::renderPolygon() loop on line 1787 (https://github.com/qgis/QGIS/blob/master/src/core/symbology/qgssymbol.cpp#L1787), it ends up killing the referred geometry for symbol layers being rendered after the geometry generator layer.

[nirvn]

In addition, as I understand it, even if the geometry wouldn't scope out, we'd still be modifying the context geometry with that of the geometry generated one, which would then mess with the subsequent symbol layers by having those use the wrong geometry.

[m-kuhn]

Wow, that's superdangerous code there. I wonder if it's not rather the code you are pointing to now, that should be changed, i.e. it should set the old geometry back once its own geometry scopes out.

[nirvn]

@m-kuhn , sounds like an alternative approach that would be more future proof indeed.

Do you want to come up with a commit of your own that'd supersede this here?

[nirvn]

Whatever approach we settle on, we should make sure it doesn't slow down our rendering loop. That's partly why I didn't want to touch renderFeature() to begin with.

[m-kuhn]

What do you think about #7359 ?

[nirvn]

Superseded by PR #7359 , closing.