Skip invalid returned features

Author: nyalldawson

python/core/processing/qgsprocessingalgorithm.sip

@@ -827,7 +827,10 @@ class QgsProcessingFeatureBasedAlgorithm : QgsProcessingAlgorithm
  geometry with the centroid of the original feature geometry for a 'centroid' type
  algorithm).
 
- Implementations should return the modified feature.
+ Implementations should return the modified feature. Returning an invalid feature (e.g.
+ a default constructed QgsFeature) will indicate that this feature should be 'skipped',
+ and will not be added to the algorithm's output. Subclasses can use this approach to
+ filter the incoming features as desired.
 
  The provided ``feedback`` object can be used to push messages to the log and for giving feedback
  to users. Note that handling of progress reports and algorithm cancelation is handled by

src/core/processing/qgsprocessingalgorithm.cpp

@@ -656,7 +656,8 @@ QVariantMap QgsProcessingFeatureBasedAlgorithm::processAlgorithm( const QVariant
     }
 
     QgsFeature transformed = processFeature( f, feedback );
-    sink->addFeature( transformed, QgsFeatureSink::FastInsert );
+    if ( transformed.isValid() )
+      sink->addFeature( transformed, QgsFeatureSink::FastInsert );
 
     feedback->setProgress( current * step );
     current++;

src/core/processing/qgsprocessingalgorithm.h

@@ -798,7 +798,10 @@ class CORE_EXPORT QgsProcessingFeatureBasedAlgorithm : public QgsProcessingAlgor
      * geometry with the centroid of the original feature geometry for a 'centroid' type
      * algorithm).
      *
-     * Implementations should return the modified feature.
+     * Implementations should return the modified feature. Returning an invalid feature (e.g.
+     * a default constructed QgsFeature) will indicate that this feature should be 'skipped',
+     * and will not be added to the algorithm's output. Subclasses can use this approach to
+     * filter the incoming features as desired.
      *
      * The provided \a feedback object can be used to push messages to the log and for giving feedback
      * to users. Note that handling of progress reports and algorithm cancelation is handled by