fixup! USDLayerWriter : Refactor using parallelReduceLocations

- Add unit test to expose bug.
- Fix child merging operation, which was using path prefix from the children rather than the parent.
- Rename merging functions to make the problem/solution clearer.
- Remove unnecessary `path` argument.

python/GafferUSDTest/USDLayerWriterTest.py

@@ -418,5 +418,35 @@ def testDispatch( self ) :
 
 		self.assertTrue( ( self.temporaryDirectory() / "test.usda" ).is_file() )
 
+	def testIdenticalNamesInDifferentGroups( self ) :
+
+		# /plane  <- modified in layer
+		# /group
+		#   /plane  <- identical
+
+		plane = GafferScene.Plane()
+
+		group = GafferScene.Group()
+		group["in"][0].setInput( plane["out"] )
+
+		parent = GafferScene.Parent()
+		parent["in"].setInput( plane["out"] )
+		parent["children"][0].setInput( group["out"] )
+		parent["parent"].setValue( "/" )
+
+		pathFilter = GafferScene.PathFilter()
+		pathFilter["paths"].setValue( IECore.StringVectorData( [ "/plane" ] ) )
+
+		primitiveVariables = GafferScene.PrimitiveVariables()
+		primitiveVariables["in"].setInput( parent["out"] )
+		primitiveVariables["filter"].setInput( pathFilter["out"] )
+		primitiveVariables["primitiveVariables"].addChild( Gaffer.NameValuePlug( "a", 10 ) )
+
+		layerFileName, compositionFileName = self.__writeLayerAndComposition( parent["out"], primitiveVariables["out"] )
+
+		reader = GafferScene.SceneReader()
+		reader["fileName"].setValue( compositionFileName )
+		self.assertScenesEqual( primitiveVariables["out"], reader["out"], checks = self.allSceneChecks - { "sets" } )
+
 if __name__ == "__main__":
 	unittest.main()

src/GafferUSD/USDLayerWriter.cpp

@@ -85,22 +85,20 @@ struct Filters
 	PathMatcher deleteAttributes;
 	ScenePlug::ScenePath localPath;
 
-	// Merges in filters from sibling locations.
-	void add( const Filters &other )
+	void mergeSibling( const Filters &sibling )
 	{
-		fullyPruned = fullyPruned && other.fullyPruned;
-		prune.addPaths( other.prune );
-		deleteAttributes.addPaths( other.deleteAttributes );
-		deleteObject.addPaths( other.deleteObject );
+		fullyPruned = fullyPruned && sibling.fullyPruned;
+		prune.addPaths( sibling.prune );
+		deleteAttributes.addPaths( sibling.deleteAttributes );
+		deleteObject.addPaths( sibling.deleteObject );
 	}
 
-	// Merges in filters from child locations.
-	void addWithPrefix( const Filters &other )
+	void mergeChildren( const Filters &children )
 	{
-		fullyPruned = fullyPruned && other.fullyPruned;
-		prune.addPaths( other.prune, other.localPath );
-		deleteAttributes.addPaths( other.deleteAttributes, other.localPath );
-		deleteObject.addPaths( other.deleteObject, other.localPath );
+		fullyPruned = fullyPruned && children.fullyPruned;
+		prune.addPaths( children.prune, localPath );
+		deleteAttributes.addPaths( children.deleteAttributes, localPath );
+		deleteObject.addPaths( children.deleteObject, localPath );
 	}
 
 };
@@ -110,7 +108,7 @@ struct Filters
 // - Objects with identical hashes will be included in `Filter::deleteObject`.
 // - Attributes with identical hashes will be included in `Filter::deleteAttributes`.
 // - Subtrees which are identical in all properties will be included in `Filter::prune`.
-Filters buildFilters( const GafferScene::ScenePlug *baseScene, const GafferScene::ScenePlug *layerScene, const std::vector<float> &frames, const ScenePlug::ScenePath &path )
+Filters buildFilters( const GafferScene::ScenePlug *baseScene, const GafferScene::ScenePlug *layerScene, const std::vector<float> &frames )
 {
 	Context::EditableScope childNamesScope( Context::current() );
 
@@ -181,17 +179,16 @@ Filters buildFilters( const GafferScene::ScenePlug *baseScene, const GafferScene
 		},
 		[]( Filters &result, const Filters &childrenResult )
 		{
-			result.addWithPrefix( childrenResult );
+			result.mergeChildren( childrenResult );
 			if( result.fullyPruned )
 			{
 				result.prune.addPath( result.localPath );
 			}
 		},
-		[]( Filters &result, const Filters &sibling )
+		[]( Filters &result, const Filters &siblingResult )
 		{
-			result.add( sibling );
-		},
-		path
+			result.mergeSibling( siblingResult );
+		}
 	);
 }
 
@@ -521,7 +518,7 @@ void USDLayerWriter::executeSequence( const std::vector<float> &frames ) const
 
 	// Figure out the filters for our Prune, DeleteObject and DeleteAttribute
 	// nodes.
-	const Filters filters = buildFilters( basePlug(), layerPlug(), frames, ScenePlug::ScenePath() );
+	const Filters filters = buildFilters( basePlug(), layerPlug(), frames );
 
 	// Pass the filter settings via context variables since we can't call
 	// `Plug::setValue()` from `executeSequence()` because it would violate