Restore session layer properly or create one when root-layer is not editable.

lib/mayaUsd/base/CMakeLists.txt

@@ -4,11 +4,13 @@
 target_sources(${PROJECT_NAME} 
     PRIVATE
         debugCodes.cpp
+        tokens.cpp
 )
 
 set(HEADERS
     api.h
     debugCodes.h
+    tokens.h
 )
 
 # -----------------------------------------------------------------------------

lib/mayaUsd/base/tokens.cpp

@@ -0,0 +1,22 @@
+//
+// Copyright 2020 Autodesk
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#include "tokens.h"
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+TF_DEFINE_PUBLIC_TOKENS(MayaUsdOptionVars, MAYA_USD_OPTIONVAR_TOKENS);
+
+PXR_NAMESPACE_CLOSE_SCOPE

lib/mayaUsd/base/tokens.h

@@ -0,0 +1,43 @@
+//
+// Copyright 2020 Autodesk
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#pragma once
+
+#include "api.h"
+
+#include <pxr/base/tf/staticTokens.h>
+#include <pxr/pxr.h>
+
+#include <maya/MString.h>
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+// Tokens that are used as optionVars in MayaUSD
+//
+#define MAYA_USD_OPTIONVAR_TOKENS                        \
+    /* Always target a session layer on a mayaUsdProxy*/ \
+    (mayaUsd_ProxyTargetsSessionLayerOnOpen)
+
+TF_DECLARE_PUBLIC_TOKENS(MayaUsdOptionVars, MAYAUSD_CORE_PUBLIC, MAYA_USD_OPTIONVAR_TOKENS);
+
+// Convenience to convert a TfToken to MString
+//
+static inline MString toMString(const TfToken& token)
+{
+    return MString(token.data(), token.size());
+}
+
+PXR_NAMESPACE_CLOSE_SCOPE

lib/mayaUsd/nodes/proxyShapeBase.cpp

@@ -16,6 +16,7 @@
 #include "proxyShapeBase.h"
 
 #include <mayaUsd/base/debugCodes.h>
+#include <mayaUsd/base/tokens.h>
 #include <mayaUsd/listeners/proxyShapeNotice.h>
 #include <mayaUsd/nodes/stageData.h>
 #include <mayaUsd/utils/query.h>
@@ -541,15 +542,27 @@ MStatus MayaUsdProxyShapeBase::computeInStageDataCached(MDataBlock& dataBlock)
 
                 if (SdfLayerRefPtr rootLayer = SdfLayer::FindOrOpen(fileString)) {
                     SdfLayerRefPtr sessionLayer = computeSessionLayer(dataBlock);
-                    if (sessionLayer) {
+
+                    bool targetSession
+                        = MGlobal::optionVarIntValue(
+                              toMString(MayaUsdOptionVars->mayaUsd_ProxyTargetsSessionLayerOnOpen))
+                        == 1;
+                    targetSession = targetSession || !rootLayer->PermissionToEdit();
+
+                    if (sessionLayer || targetSession) {
+                        if (!sessionLayer)
+                            sessionLayer = SdfLayer::CreateAnonymous();
                         usdStage = UsdStage::Open(
                             rootLayer, sessionLayer, ArGetResolver().GetCurrentContext(), loadSet);
                     } else {
                         usdStage = UsdStage::Open(
                             rootLayer, ArGetResolver().GetCurrentContext(), loadSet);
                     }
-
-                    usdStage->SetEditTarget(usdStage->GetRootLayer());
+                    if (sessionLayer && targetSession) {
+                        usdStage->SetEditTarget(sessionLayer);
+                    } else {
+                        usdStage->SetEditTarget(usdStage->GetRootLayer());
+                    }
                 } else {
                     // Create a new stage in memory with an anonymous root layer.
                     usdStage = UsdStage::CreateInMemory(kAnonymousLayerName, loadSet);

lib/mayaUsd/nodes/stageNode.cpp

@@ -15,6 +15,7 @@
 //
 #include "stageNode.h"
 
+#include <mayaUsd/base/tokens.h>
 #include <mayaUsd/nodes/stageData.h>
 #include <mayaUsd/utils/stageCache.h>
 
@@ -34,6 +35,7 @@
 #include <maya/MFnPluginData.h>
 #include <maya/MFnStringData.h>
 #include <maya/MFnTypedAttribute.h>
+#include <maya/MGlobal.h>
 #include <maya/MObject.h>
 #include <maya/MPlug.h>
 #include <maya/MPxNode.h>
@@ -107,9 +109,21 @@ MStatus UsdMayaStageNode::compute(const MPlug& plug, MDataBlock& dataBlock)
         if (SdfLayerRefPtr rootLayer = SdfLayer::FindOrOpen(usdFile)) {
             const bool           loadAll = true;
             UsdStageCacheContext ctx(UsdMayaStageCache::Get(loadAll));
-            usdStage = UsdStage::Open(rootLayer, ArGetResolver().GetCurrentContext());
 
-            usdStage->SetEditTarget(usdStage->GetRootLayer());
+            bool targetSession = MGlobal::optionVarIntValue(toMString(
+                                     MayaUsdOptionVars->mayaUsd_ProxyTargetsSessionLayerOnOpen))
+                == 1;
+            targetSession = targetSession || !rootLayer->PermissionToEdit();
+
+            if (targetSession) {
+                SdfLayerRefPtr sessionLayer = SdfLayer::CreateAnonymous();
+                usdStage
+                    = UsdStage::Open(rootLayer, sessionLayer, ArGetResolver().GetCurrentContext());
+                usdStage->SetEditTarget(sessionLayer);
+            } else {
+                usdStage = UsdStage::Open(rootLayer, ArGetResolver().GetCurrentContext());
+                usdStage->SetEditTarget(usdStage->GetRootLayer());
+            }
         }
 
         SdfPath primPath;