MayaUsdProxyShapeBase: invalidate stage on recomputeLayers attribute changes

[jufrantz]

This PR is a targeted fix for one stale UsdStageMap scenario seen in production with maya-usd v0.35.0.

The crash occurs in workflows like the following:

• The scene contains editAsMaya DAGs, with incoming ProxyAccessor visibility connections from the proxyShape.
• The user performs a file write or export. In some reported cases, this was maya.cmds.exportEdits, which is unrelated to USD content itself.
• Later, the user reloads a Maya-only reference, unrelated to the USD stage.
• A subsequent proxyShape recompute crashes in the ProxyAccessor compute path.

  TdagObject::inRenderLayer(TdnRenderLayer const&, bool) const
  TdagObject::inRenderLayer(TdagPath const&, TdnRenderLayer*)
  TmultiPath::isVisible() const
  Autodesk::Maya::OpenMaya20260000::MDagPath::isVisible(Autodesk::Maya::OpenMaya20260000::MStatus*) const
  MayaUsd_v0::ProxyAccessor::computeOutput(MayaUsd_v0::ProxyAccessor::Item const&, Autodesk::Maya::OpenMaya20260000::MMatrix const&, pxrInternal_v0_25_8__pxrReserved__::TfRefPtr<pxrInternal_v0_25_8__pxrReserved__::UsdStage>, Autodesk::Maya::OpenMaya20260000::MDataBlock&, pxrInternal_v0_25_8__pxrReserved__::UsdGeomXformCache&, MayaUsd_v0::ConverterArgs const&)
  MayaUsd_v0::ProxyAccessor::compute(Autodesk::Maya::OpenMaya20260000::MPlug const&, Autodesk::Maya::OpenMaya20260000::MDataBlock&, bool)
  pxrInternal_v0_25_8__pxrReserved__::MayaUsdProxyShapeBase::compute(Autodesk::Maya::OpenMaya20260000::MPlug const&, Autodesk::Maya::OpenMaya20260000::MDataBlock&)
  MayaUsd_v0::ProxyShape::compute(Autodesk::Maya::OpenMaya20260000::MPlug const&, Autodesk::Maya::OpenMaya20260000::MDataBlock&)
  THsurfaceShape::computePlug(Tplug const&, TdataBlock&)
  TevaluationNode::evaluateDirtyPlugs(TdataBlockDG*, TevaluationNode::TdirtyPlugType)
  TevaluationNode::evaluate(em::SyncMessage const&)
  em::ParallelExecution::evaluateAndVisitChildren(em::EvaluationNode&)
  em::ParallelExecution::evaluateAndSchedule()
  /usr/autodesk/maya2026/lib/libEvaluationManager.so.1.1.27.0(+0x29b42) [0x7f5d24446b42]
  tbb::detail::r1::isolate_within_arena(tbb::detail::d1::delegate_base&, long)
  em::ParallelExecution::evaluateImplementation()
  em::EvaluationGraph::evaluate(bool)
  TevaluationManager::evaluate(TdgContext&, bool)
  /usr/autodesk/maya2026/lib/libDependEngine.so(+0x29070b) [0x7f5d2550c70b]
  TdependGraph::setTime(TanimTime const&, bool)
  [...]

The crash is caused by calling MDagPath::isVisible() on an invalid MDagPath. That invalid path comes from a stale UsdStageMap, which does not yet reflect a new stage created after file I/O. In the scenario above, the export creates a new LayerManager, and the later reference reload triggers a proxyShape recompute by bumping the recomputeLayers attribute (see #4443 ).

This change makes recomputeLayers behave like the other proxyShape attributes that affect outStageData: it now sends MayaUsdProxyStageInvalidateNotice so that stage observers, such as MayaStagesSubject, can invalidate stage-related data like UsdStageMap, and restores the usual notification flow for this code path: first StageInvalidate, then StageSet.

I also included a small unrelated cleanup I came across while investigating this issue: making this attribute non-storable, since it is only used as a runtime trigger.

Note

While investigating this issue, we also tried cherry-picking #4491 which looked like a good candidate for the stale UsdStageMap problem. It effectively fixed the crash for us, but it also caused other issues on our side.

In some scenarios, the map can become incomplete when it is dirtied during its own rebuild. The sequence is:

• UsdStageMap::rebuildIfDirty() indirectly calls MayaUsdProxyShapeBase::getUsdStage()
• getUsdStage() pulls outStageData, which can trigger a compute
• MayaUsdProxyShapeBase::computeOutStageData() sends MayaUsdProxyStageSetNotice
• MayaStagesSubject indirectly calls UsdStageMap::setDirty() on MayaUsdProxyStageSetNotice (since PR4491)
• setDirty() clears the map in the middle of rebuildIfDirty(), so previously processed entries are lost, UsdStageMap is incomplete.

I cannot provide a standalone reproduction for this yet, it is difficult to isolate. However, I hope the proposed change is clear from the context above.