[processing] Don't run validity checks for buffer alg

Buffering doesn't care about validity and can sometimes be used
to repair geometries

src/analysis/processing/qgsalgorithmbuffer.cpp

@@ -81,7 +81,7 @@ QgsBufferAlgorithm *QgsBufferAlgorithm::createInstance() const
 
 QVariantMap QgsBufferAlgorithm::processAlgorithm( const QVariantMap &parameters, QgsProcessingContext &context, QgsProcessingFeedback *feedback )
 {
-  std::unique_ptr< QgsFeatureSource > source( parameterAsSource( parameters, QStringLiteral( "INPUT" ), context ) );
+  std::unique_ptr< QgsProcessingFeatureSource > source( parameterAsSource( parameters, QStringLiteral( "INPUT" ), context ) );
   if ( !source )
     throw QgsProcessingException( invalidSourceError( parameters, QStringLiteral( "INPUT" ) ) );
 
@@ -98,7 +98,7 @@ QVariantMap QgsBufferAlgorithm::processAlgorithm( const QVariantMap &parameters,
   double miterLimit = parameterAsDouble( parameters, QStringLiteral( "MITER_LIMIT" ), context );
   double bufferDistance = parameterAsDouble( parameters, QStringLiteral( "DISTANCE" ), context );
   bool dynamicBuffer = QgsProcessingParameters::isDynamic( parameters, QStringLiteral( "DISTANCE" ) );
-  QgsExpressionContext expressionContext = createExpressionContext( parameters, context, dynamic_cast< QgsProcessingFeatureSource * >( source.get() ) );
+  QgsExpressionContext expressionContext = createExpressionContext( parameters, context, source.get() );
   QgsProperty bufferProperty;
   if ( dynamicBuffer )
   {
@@ -108,7 +108,8 @@ QVariantMap QgsBufferAlgorithm::processAlgorithm( const QVariantMap &parameters,
   long count = source->featureCount();
 
   QgsFeature f;
-  QgsFeatureIterator it = source->getFeatures();
+  // buffer doesn't care about invalid features, and buffering can be used to repair geometries
+  QgsFeatureIterator it = source->getFeatures( QgsFeatureRequest(), QgsProcessingFeatureSource::FlagSkipGeometryValidityChecks );
 
   double step = count > 0 ? 100.0 / count : 1;
   int current = 0;

src/analysis/processing/qgsalgorithmmultiringconstantbuffer.cpp

@@ -113,6 +113,11 @@ QgsFields QgsMultiRingConstantBufferAlgorithm::outputFields( const QgsFields &in
   return fields;
 }
 
+QgsProcessingFeatureSource::Flag QgsMultiRingConstantBufferAlgorithm::sourceFlags() const
+{
+  return QgsProcessingFeatureSource::FlagSkipGeometryValidityChecks;
+}
+
 QgsFeatureList QgsMultiRingConstantBufferAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &context, QgsProcessingFeedback *feedback )
 {
   double currentDistance = 0;

src/analysis/processing/qgsalgorithmmultiringconstantbuffer.h

@@ -49,6 +49,7 @@ class QgsMultiRingConstantBufferAlgorithm : public QgsProcessingFeatureBasedAlgo
     QString outputName() const override;
     QgsFields outputFields( const QgsFields &inputFields ) const override;
     QgsProcessing::SourceType outputLayerType() const override { return QgsProcessing::TypeVectorPolygon; }
+    QgsProcessingFeatureSource::Flag sourceFlags() const override;
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type inputWkbType ) const override { Q_UNUSED( inputWkbType ); return QgsWkbTypes::Polygon; }
     bool prepareAlgorithm( const QVariantMap &parameters, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
     QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;