Remove incorrect Populate() on invalidate.

lib/mayaHydra/hydraExtensions/sceneIndex/mhMayaUsdProxyShapeSceneIndex.cpp

@@ -100,7 +100,22 @@ void MayaUsdProxyShapeSceneIndex::_StageInvalidate(const MAYAUSDAPI_NS::ProxySta
 { 
     _usdImagingStageSceneIndex->SetStage(nullptr);
     _populated = false;
-    Populate(); 
+    // Simply mark populate as dirty and do not call
+    // Populate();
+    // here.  Doing so is incorrect for two reasons:
+    // - _StageInvalidate() is a callback called during Maya invalidation.
+    //   Populate() calls MayaUsdProxyShapeBase::getUsdStage(), which calls
+    //   MayaUsdProxyShapeBase::compute(), which should not be done during
+    //   dirty propagation.
+    // - Calling getUsdStage() through Populate() creates an invalidate
+    //   callback dependency between _StageInvalidate() and
+    //   the mayaUsd plugin MayaStagesSubject::onStageInvalidate().  During
+    //   getUsdStage(), MayaStagesSubject::setupListeners() is called, and it
+    //   depends on MayaStagesSubject::onStageInvalidate() being called first,
+    //   otherwise setupListeners() and therefore getUsdStage() will fail.
+    //
+    //   Invalidate callbacks should not have dependencies on one another ---
+    //   it should be possible to call them in random order.
 }
 
 void MayaUsdProxyShapeSceneIndex::_ObjectsChanged(

test/lib/mayaUsd/render/mayaToHydra/cpp/testUsdStageFromFile.py

@@ -17,6 +17,7 @@
 import fixturesUtils
 import mtohUtils
 import usdUtils
+import mayaUsd
 
 import testUtils
 from testUtils import PluginLoaded
@@ -51,11 +52,24 @@ def test_UsdStageFromFile(self):
 
             cmds.setAttr('cubeShape.filePath', usdScenePath, type="string")
 
+            cmds.refresh()
+
             populateCallsPost = cmds.mayaHydraInstruments("MayaUsdProxyShapeSceneIndex:NbPopulateCalls", q=True)
 
             self.assertEqual(populateCallsPre+1, populateCallsPost)
 
             cmds.mayaHydraCppTest(self.CONE_PATH, f="TestHydraPrim.fromAppPath")
 
+    def test_getStage(self):
+
+        # Any file will do.
+        usdScenePath = testUtils.getTestScene('testUsdNativeInstances', 'instancedCubeHierarchies.usda')
+
+        proxyShapePathStr = usdUtils.createStageFromFile(usdScenePath)
+
+        stage = mayaUsd.lib.GetPrim(proxyShapePathStr).GetStage()
+
+        self.assertIsNotNone(stage)
+
 if __name__ == '__main__':
     fixturesUtils.runTests(globals())