Maya crashes when enabling "Hardware Texturing" after selecting material variant

[jufrantz]

Describe the bug

When selecting a material variant and subsequently enabling viewport "Hardware Texturing", maya crashes with some of our assets. The problem seems to arise if the variantSet is changed on an instance prim (w/ instanced materials) and if the chosen variant alters the material network topology. I've managed to replicate this issue on a relatively simple setup.

Steps to reproduce

1. Ensure viewport's Hardware Texturing option is disabled, e.g. using modelEditor command:

maya.cmds.modelEditor("modelPanel4", e=True, displayTextures=False)
maya.cmds.refresh()

2. Create a mayaUsd proxyShape with this setup

import maya.cmds
import mayaUsd.ufe
import mayaUsd_createStageWithNewLayer
maya.cmds.loadPlugin("mayaUsdPlugin", quiet=True)

proxyShape = mayaUsd_createStageWithNewLayer.createStageWithNewLayer()
shapeStage = mayaUsd.ufe.getStage(proxyShape)

shapeStage.GetRootLayer().ImportFromString('''#sdf 1
def Xform "Asset" (
    prepend variantSets = "materialVariant"
)
{
    def Sphere "Geo" (
        prepend apiSchemas = ["MaterialBindingAPI"]
    )
    {
        rel material:binding = </Asset/Mtl>
        float3[] primvars:myColor = [(0, 1, 0)]
    }

    def Material "Mtl"
    {
        token outputs:surface.connect = </Asset/Mtl/Surf.outputs:surface>

        def Shader "SurfBase"
        {
            uniform token info:id = "UsdPreviewSurface"
        }

        def "Surf" (
            references = </Asset/Mtl/SurfBase>
        )
        {
            token outputs:surface
        }

        def Shader "PrimColor"
        {
            uniform token info:id = "UsdPrimvarReader_float3"
            string inputs:varname = "myColor"
            float3 outputs:result
        }
    }
    variantSet "materialVariant" = {
        "PrimColor" {
            over "Mtl"
            {
                over "Surf"
                {
                    color3f inputs:diffuseColor.connect = </Asset/Mtl/PrimColor.outputs:result>
                }
            }

        }
    }
}

def "Instance" (
    instanceable = true
    references = </Asset>
)
{
}
''')

3. Select the PrimColor variant on the instance prim. This will bring a connection on terminal node's inputs:diffuseColor

shapeStage.GetPrimAtPath("/Instance").GetVariantSet("materialVariant").SetVariantSelection("PrimColor")
maya.cmds.refresh()

4. Enable Hardware Texturing in the viewport

maya.cmds.modelEditor("modelPanel4", e=True, displayTextures=True)
maya.cmds.refresh()

5. It should crash, we get this fatal error:

----------------------------- maya.bin terminated ------------------------------
maya.bin crashed. FATAL ERROR: Failed axiom: ' _primIndex '
in GetSourcePrimIndex at line 104 of build/sources/USD-22.11/pxr/usd/usd/primData.cpp
writing crash report to [ host:/usr/tmp/st_maya.bin.6890 ] ... done.
--------------------------------------------------------------------------------

Specs (if applicable):

• CentOS 7.8
• Maya 2023.3
• maya-usd 0.25
• USD 22.11

[neilrubie]

Just adding that we are frequently getting crashes with this same error, to do with toggling hardware texturing. Would be nice to get this sorted.