Add method to take results from another processing context and

add to the current context

With appropriate note and tests to ensure that both the current
context and that which the results being taken from share the
same thread affinity

Author: nyalldawson

python/core/processing/qgsprocessingcontext.sip

@@ -237,6 +237,15 @@ Destination project
 .. seealso:: thread()
 %End
 
+    void takeResultsFrom( QgsProcessingContext &context );
+%Docstring
+ Takes the results from another ``context`` and merges them with the results currently
+ stored in this context. This includes settings like any layers loaded in the temporaryLayerStore()
+ and layersToLoadOnCompletion().
+ This is only safe to call when both this context and the other ``context`` share the same
+ thread() affinity, and that thread is the current thread.
+%End
+
   private:
     QgsProcessingContext( const QgsProcessingContext &other );
 };

src/core/processing/qgsprocessingcontext.h

@@ -340,10 +340,29 @@ class CORE_EXPORT QgsProcessingContext
      */
     void pushToThread( QThread *thread )
     {
-      Q_ASSERTX( QThread::currentThread() == thread(), "QgsProcessingContext::pushToThread", "Cannot push context to another thread unless the current thread matches the existing context thread affinity" );
+      Q_ASSERT_X( QThread::currentThread() == QgsProcessingContext::thread(), "QgsProcessingContext::pushToThread", "Cannot push context to another thread unless the current thread matches the existing context thread affinity" );
       tempLayerStore.moveToThread( thread );
     }
 
+    /**
+     * Takes the results from another \a context and merges them with the results currently
+     * stored in this context. This includes settings like any layers loaded in the temporaryLayerStore()
+     * and layersToLoadOnCompletion().
+     * This is only safe to call when both this context and the other \a context share the same
+     * thread() affinity, and that thread is the current thread.
+     */
+    void takeResultsFrom( QgsProcessingContext &context )
+    {
+      QMap< QString, LayerDetails > loadOnCompletion = context.layersToLoadOnCompletion();
+      QMap< QString, LayerDetails >::const_iterator llIt = loadOnCompletion.constBegin();
+      for ( ; llIt != loadOnCompletion.constEnd(); ++llIt )
+      {
+        mLayersToLoadOnCompletion.insert( llIt.key(), llIt.value() );
+      }
+      context.setLayersToLoadOnCompletion( QMap< QString, LayerDetails >() );
+      tempLayerStore.transferLayersFromStore( context.temporaryLayerStore() );
+    }
+
   private:
 
     QgsProcessingContext::Flags mFlags = 0;

tests/src/core/testqgsprocessing.cpp

@@ -583,12 +583,18 @@ void TestQgsProcessing::context()
   context.setInvalidGeometryCheck( QgsFeatureRequest::GeometrySkipInvalid );
   QCOMPARE( context.invalidGeometryCheck(), QgsFeatureRequest::GeometrySkipInvalid );
 
+  QgsVectorLayer *vector = new QgsVectorLayer( "Polygon", "vector", "memory" );
+  context.temporaryLayerStore()->addMapLayer( vector );
+  QCOMPARE( context.temporaryLayerStore()->mapLayer( vector->id() ), vector );
+
   QgsProcessingContext context2;
   context2.copyThreadSafeSettings( context );
   QCOMPARE( context2.defaultEncoding(), context.defaultEncoding() );
   QCOMPARE( context2.invalidGeometryCheck(), context.invalidGeometryCheck() );
   QCOMPARE( context2.flags(), context.flags() );
   QCOMPARE( context2.project(), context.project() );
+  // layers from temporaryLayerStore must not be copied by copyThreadSafeSettings
+  QVERIFY( context2.temporaryLayerStore()->mapLayers().isEmpty() );
 
   // layers to load on completion
   QgsVectorLayer *v1 = new QgsVectorLayer( "Polygon", "V1", "memory" );
@@ -606,6 +612,11 @@ void TestQgsProcessing::context()
   QCOMPARE( context.layersToLoadOnCompletion().values().at( 0 ).name, QStringLiteral( "v1" ) );
   QCOMPARE( context.layersToLoadOnCompletion().keys().at( 1 ), v2->id() );
   QCOMPARE( context.layersToLoadOnCompletion().values().at( 1 ).name, QStringLiteral( "v2" ) );
+
+  // ensure that copyThreadSafeSettings doesn't copy layersToLoadOnCompletion()
+  context2.copyThreadSafeSettings( context );
+  QVERIFY( context2.layersToLoadOnCompletion().isEmpty() );
+
   layers.clear();
   layers.insert( v2->id(), QgsProcessingContext::LayerDetails( QStringLiteral( "v2" ), &p ) );
   context.setLayersToLoadOnCompletion( layers );
@@ -616,8 +627,20 @@ void TestQgsProcessing::context()
   QCOMPARE( context.layersToLoadOnCompletion().count(), 2 );
   QCOMPARE( context.layersToLoadOnCompletion().keys().at( 0 ), v1->id() );
   QCOMPARE( context.layersToLoadOnCompletion().keys().at( 1 ), v2->id() );
-  delete v1;
-  delete v2;
+
+  context.temporaryLayerStore()->addMapLayer( v1 );
+  context.temporaryLayerStore()->addMapLayer( v2 );
+
+  // test takeResultsFrom
+  context2.takeResultsFrom( context );
+  QVERIFY( context.temporaryLayerStore()->mapLayers().isEmpty() );
+  QVERIFY( context.layersToLoadOnCompletion().isEmpty() );
+  // should now be in context2
+  QCOMPARE( context2.temporaryLayerStore()->mapLayer( v1->id() ), v1 );
+  QCOMPARE( context2.temporaryLayerStore()->mapLayer( v2->id() ), v2 );
+  QCOMPARE( context2.layersToLoadOnCompletion().count(), 2 );
+  QCOMPARE( context2.layersToLoadOnCompletion().keys().at( 0 ), v1->id() );
+  QCOMPARE( context2.layersToLoadOnCompletion().keys().at( 1 ), v2->id() );
 }
 
 void TestQgsProcessing::mapLayers()