Hydra update crash with USDSKELIMAGING_ENABLE_NORMAL_COMPUTATIONS=1 enabled

[cameronwhite]

With the attached example (bug_158640.zip), run:

• USDSKELIMAGING_ENABLE_NORMAL_COMPUTATIONS=1 usdview s0.usda
• In the interpreter, run usdviewApi.stage.GetRootLayer().subLayerPaths = ["s1.usda"]
• Then, run usdviewApi.stage.GetRootLayer().subLayerPaths = ["s2.usda"]

This should crash from attempted member lookup on NULL TfWeakPtr<SdfLayer>. Attached is a stack trace

Tested with USD 26.05 and also reproduced with the dev branch (c58778b)

[spiffmon]

Thanks, @cameronwhite . At first glance it looks like Hydra is not dropping its UsdAttributeQuery’s after the layerStack change makes them invalid.

[cameronwhite]

Yeah, after doing some more investigating this seems related to the changes in 0b159a8
When refreshing a prim in _ProcessPrimsNeedingRefreshAndSendNotices(), _GetResolvedPrimComputations() calls HasNormalsExtComputations() on the old data source which leads to the invalid query for normals

One option might be adding a flag to UsdSkelImagingDataSourceResolvedPointsBasedPrim to remember whether there is a normals computation to avoid querying the data source during a refresh?

[jesschimein]

Filed as internal issue #USD-12481

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