IECoreUSD : Support IECore::Object references when determining primvar type.

`IECore::Object` avoids some data duplication by writing an internal object's
data a single time while saving, and then writing a reference path to the data
container if other internal objects share the same pointer.

One case in which this situation has presented itself is creating a "Pref"
primvar to hold the rest positions of a primitive. In the following example,
the created "Pref" primitive variable receives a shallow copy of the "P"
primitive variable. So, the internal "data" and "indices" members point
to the same shared data. Therefore, a reference is created when the primitive
object saves itself.
```
box = IECoreScene.MeshPrimitive.createBox(
	imath.Box3f( imath.V3f(-1), imath.V3f(1) )
)
box['Pref'] = box['P']
```

Author: bradleyhenke

contrib/IECoreUSD/src/IECoreUSD/SceneCacheData.cpp

@@ -661,7 +661,7 @@ void SceneCacheData::loadPrimVars( const SceneInterface::Path& currentPath, TfTo
 	// variables
 	SceneInterface::Path variablesPath;
 	variablesPath.push_back( g_ioRoot );
-	for ( auto& p : currentPath )
+	for ( const auto& p : currentPath )
 	{
 		// avoid injecting the internal root because
 		// the path would be invalid in the IndexedIO hierarchy
@@ -675,20 +675,21 @@ void SceneCacheData::loadPrimVars( const SceneInterface::Path& currentPath, TfTo
 
 	variablesPath.insert( variablesPath.end(), g_staticIoVariablesPath.begin(), g_staticIoVariablesPath.end() );
 
-	if ( auto variables = m_sceneio->directory( variablesPath, IndexedIO::MissingBehaviour::NullIfMissing ) )
+	if ( const auto variables = m_sceneio->directory( variablesPath, IndexedIO::MissingBehaviour::NullIfMissing ) )
 	{
 		IndexedIO::EntryIDList variableLists;
 		variables->entryIds( variableLists );
-		for( auto& var: variableLists )
+
+		for( const auto& var: variableLists )
 		{
-			auto it = find( g_defaultPrimVars.cbegin(), g_defaultPrimVars.cend(), var.value() );
+			const auto it = find( g_defaultPrimVars.cbegin(), g_defaultPrimVars.cend(), var.value() );
 			if( it != g_defaultPrimVars.cend() )
 			{
 				continue;
 			}
 
 			// interpolation
-			auto variableIO = variables->subdirectory( var, IndexedIO::MissingBehaviour::NullIfMissing );
+			const auto variableIO = variables->subdirectory( var, IndexedIO::MissingBehaviour::NullIfMissing );
 			int interpolationValue = 0;
 			TfToken usdInterpolation;
 			if ( !variableIO || !variableIO->hasEntry( g_ioInterpolation ) )
@@ -701,17 +702,45 @@ void SceneCacheData::loadPrimVars( const SceneInterface::Path& currentPath, TfTo
 			usdInterpolation = PrimitiveAlgo::toUSD( static_cast<PrimitiveVariable::Interpolation>( interpolationValue ) );
 
 			// data type
-			auto dataType = variableIO->subdirectory( g_ioData, IndexedIO::MissingBehaviour::NullIfMissing );
-			std::string dataTypeValue;
-			if( !dataType || !dataType->hasEntry( g_ioType ) )
+			if( !variableIO->hasEntry( g_ioData ) )
+			{
+				IECore::msg( IECore::Msg::Warning, "SceneCacheData::loadPrimVars", boost::format( "Unable to find data for Primitive Variable \"%s\" at location \"%s\"." ) % var % primPath );
+				continue;
+			}
+
+			ConstIndexedIOPtr dataIO;
+			const IndexedIO::Entry dataEntry = variableIO->entry( g_ioData );
+			if( dataEntry.entryType() == IndexedIO::File)
+			{
+				if( dataEntry.dataType() != IndexedIO::InternedStringArray )
+				{
+					IECore::msg( IECore::Msg::Warning, "SceneCacheData::loadPrimVars", boost::format( "Unable to find reference to data for Primitive Variable \"%s\" at location \"%s\"." ) % var % primPath );
+					continue;
+				}
+				// IECore::Object has saved a reference to the data, so we need to follow the link to the referenced
+				// directory in order to find the dataType
+				IndexedIO::EntryIDList referencedPath( dataEntry.arrayLength() );
+				InternedString *p = referencedPath.data();
+				variableIO->read( g_ioData, p, dataEntry.arrayLength() );
+				dataIO = variableIO->directory( referencedPath, IndexedIO::MissingBehaviour::NullIfMissing );
+			}
+			else
+			{
+				// Get the data subdirectory
+				dataIO = variableIO->subdirectory( g_ioData, IndexedIO::MissingBehaviour::NullIfMissing );
+			}
+
+			if( !dataIO || !dataIO->hasEntry( g_ioType ) )
 			{
 				IECore::msg( IECore::Msg::Warning, "SceneCacheData::loadPrimVars", boost::format( "Unable to find data type for Primitive Variable \"%s\" at location \"%s\"." ) % var % primPath );
 				continue;
 			}
-			dataType->read( g_ioType, dataTypeValue );
+
+			std::string dataTypeValue;
+			dataIO->read( g_ioType, dataTypeValue );
 
 			// interpretation
-			auto interpretationData = dataType->subdirectory( g_ioData, IndexedIO::MissingBehaviour::NullIfMissing );
+			const auto interpretationData = dataIO->subdirectory( g_ioData, IndexedIO::MissingBehaviour::NullIfMissing );
 			IntDataPtr interpretationValue = nullptr;
 			if ( interpretationData && interpretationData->hasEntry( g_interpretation ) )
 			{

contrib/IECoreUSD/test/IECoreUSD/SceneCacheFileFormatTest.py

@@ -527,6 +527,9 @@ def testCustomPrimvars( self ):
 		mesh["customPoint"] = IECoreScene.PrimitiveVariable( IECoreScene.PrimitiveVariable.Interpolation.Vertex, v )
 		mesh["customIndexedInt"] = IECoreScene.PrimitiveVariable( IECoreScene.PrimitiveVariable.Interpolation.Uniform, IECore.IntVectorData( [12] ), IECore.IntVectorData( [ 0 ] * vertexSize ) )
 
+		# Test a shallow copy of a primvar which causes IECore.Object to write a reference into the IndexedIO file
+		mesh["Pref"] = mesh["P"]
+
 		box.writeObject( mesh, 1.0 )
 		del m, box
 
@@ -557,6 +560,14 @@ def testCustomPrimvars( self ):
 		customIndexedInt = prim.GetAttribute( "primvars:customIndexedInt" )
 		self.assertEqual( customIndexedInt.GetMetadata( "interpolation" ), pxr.UsdGeom.Tokens.uniform )
 
+		# Pref
+		Pref = prim.GetAttribute( "primvars:Pref" )
+		self.assertIsNotNone( Pref )
+		self.assertFalse( Pref.GetMetadata( "custom" ) )
+		self.assertEqual( Pref.GetMetadata( "typeName" ), pxr.Sdf.ValueTypeNames.Point3fArray )
+		self.assertEqual( Pref.Get( 24.0 ), pxr.UsdGeom.Mesh( prim ).GetPointsAttr().Get( 24.0 ) )
+		self.assertEqual( Pref.GetMetadata( "interpolation" ), pxr.UsdGeom.Tokens.vertex )
+
 		# round trip
 		exportPath = os.path.join( self.temporaryDirectory(), "testUSDExportCustomPrimVar.scc" )
 		stage.Export( exportPath )
@@ -581,6 +592,9 @@ def testCustomPrimvars( self ):
 		# indices
 		self.assertEqual( mesh["customIndexedInt"].indices, IECore.IntVectorData( [0] * vertexSize ) )
 
+		# copy
+		self.assertEqual( mesh["Pref"], mesh["P"] )
+
 	def testCornersAndCreases( self ):
 		fileName = os.path.join( self.temporaryDirectory(), "testUSDCornerAndCreases.scc" )
 		m = IECoreScene.SceneCache( fileName, IECore.IndexedIO.OpenMode.Write )