EMSUSD-1758 author metadata only on anonymous root layers

- Only author the up-axis and units metadata when saving a root layer on-disk.
- Update the unit test to verify this.

lib/mayaUsd/nodes/layerManager.cpp

@@ -732,8 +732,6 @@ MStatus addLayerToBuilder(
     auto fileFormatIdToken = layer->GetFileFormat()->GetFormatId();
     fileFormatIdHandle.setString(UsdMayaUtil::convert(fileFormatIdToken.GetString()));
 
-    MayaUsd::utils::setLayerUpAxisAndUnits(layer);
-
     std::string temp;
     if (!stubOnly && ((exportOnlyIfDirty && layer->IsDirty()) || !exportOnlyIfDirty)) {
         if (!layer->ExportToString(&temp)) {
@@ -984,6 +982,10 @@ void LayerDatabase::convertAnonymousLayers(
     //       to convertAnonymousLayersRecursive
     root = stage->GetRootLayer();
     if (root->IsAnonymous()) {
+        // Only set up-axis and units metadata on the root layer
+        // and only if it is anonymous before being saved.
+        MayaUsd::utils::setLayerUpAxisAndUnits(root);
+
         const bool wasTargetLayer = (stage->GetEditTarget().GetLayer() == root);
         PXR_NS::SdfFileFormat::FileFormatArguments args;
         std::string newFileName = MayaUsd::utils::generateUniqueFileName(proxyName);

lib/mayaUsd/utils/utilSerialization.cpp

@@ -394,7 +394,6 @@ bool saveLayerWithFormat(
         = requestedFormatArg.empty() ? usdFormatArgOption() : requestedFormatArg;
 
     UsdMayaUtilFileSystem::updatePostponedRelativePaths(layer, filePath);
-    setLayerUpAxisAndUnits(layer);
 
     if (isCompatibleWithSave(layer, filePath, formatArg)) {
         if (!layer->Save()) {
@@ -471,6 +470,12 @@ SdfLayerRefPtr saveAnonymousLayer(
         return nullptr;
     }
 
+    // Only set up-axis and units metadata on the root layer
+    // and only if it is anonymous before being saved.
+    if (stage->GetRootLayer() == anonLayer) {
+        setLayerUpAxisAndUnits(anonLayer);
+    }
+
     ensureUSDFileExtension(filePath);
 
     const bool wasTargetLayer = (stage->GetEditTarget().GetLayer() == anonLayer);

test/lib/mayaUsd/fileio/testSaveUpAxisAndUnits.py

@@ -54,13 +54,13 @@ def setUp(self):
 
     def testSaveLayerInUSD(self):
         # Save the file. Make sure the USD edits will go to a USD file.
-        self._runTestSaveLayer(1)
+        self._runTestSaveLayer(True)
 
     def testSaveLayerInMaya(self):
         # Save the file. Make sure the USD edits will go to the Maya file.
-        self._runTestSaveLayer(2)
+        self._runTestSaveLayer(False)
 
-    def _runTestSaveLayer(self, saveLocation):
+    def _runTestSaveLayer(self, saveOnDisk):
         '''
         The goal is to create an anonymous sub-layer, verify the up-axis and units
         metadata is present when reloaded.
@@ -88,7 +88,9 @@ def verifyPrims(stage):
 
         verifyPrims(stage)
 
-        # Save the file. Make sure the edit will go where requested by saveLocation.
+        # Save the file. Make sure the edit will go where requested by saveOnDisk.
+        # saveLocation 1 means on-disk, 2 in the Maya scene.
+        saveLocation = 1 if saveOnDisk else 2
         tempMayaFile = 'saveUpAxisAndUnitsTest.ma'
         cmds.optionVar(intValue=('mayaUsd_SerializedUsdEditsLocation', saveLocation))
         cmds.file(rename=tempMayaFile)
@@ -111,13 +113,27 @@ def verifyPrims(stage):
         subLayer = Sdf.Layer.FindOrOpen(subLayerPath)
         self.assertIsNotNone(subLayer)
 
-        self.assertTrue(stage.HasAuthoredMetadata(UsdGeom.Tokens.metersPerUnit))
-        self.assertEqual(UsdGeom.GetStageMetersPerUnit(stage), 0.01)
-        self.assertTrue(stage.HasAuthoredMetadata(UsdGeom.Tokens.upAxis))
-        self.assertEqual(UsdGeom.GetStageUpAxis(stage), UsdGeom.Tokens.y)
+        # Verify metadata if saved on disk or verify the absence of metadata
+        # when saved in the Maya scene. We only author metadata when on-disk,
+        # since when saved in the Maya scene, the layers remain anonymous.
+        if saveOnDisk:
+            self.assertTrue(stage.HasAuthoredMetadata(UsdGeom.Tokens.metersPerUnit))
+            self.assertEqual(UsdGeom.GetStageMetersPerUnit(stage), 0.01)
+            self.assertTrue(stage.HasAuthoredMetadata(UsdGeom.Tokens.upAxis))
+            self.assertEqual(UsdGeom.GetStageUpAxis(stage), UsdGeom.Tokens.y)
+        else:
+            self.assertFalse(stage.HasAuthoredMetadata(UsdGeom.Tokens.metersPerUnit))
+            self.assertFalse(stage.HasAuthoredMetadata(UsdGeom.Tokens.upAxis))
 
         # Verify the two objects are still present.
         verifyPrims(stage)
 
+        # Stage the sub-layer by itself and verify it has no metadata,
+        # since we only author metadata on the root layer.
+        subStage = Usd.Stage.Open(subLayer)
+        self.assertFalse(subStage.HasAuthoredMetadata(UsdGeom.Tokens.metersPerUnit))
+        self.assertFalse(subStage.HasAuthoredMetadata(UsdGeom.Tokens.upAxis))
+
+
 if __name__ == '__main__':
     unittest.main(verbosity=2)