MaterialX: HdMtlx shader fails to compile when MaterialX nodedef has filename input

[ashwinbhat]

Description of Issue

If a MaterialX nodedef has a filename type input that is routed to an image node, HdMtlx ShaderGen ignores this input.
I'm looking for some hints on where/how I could fix this.

Here is a nodedef with filename input

  <nodedef name="ND_diagimage" node="diagimage" version="1.0.0" isdefaultversion="true">
    <input name="extra_color" type="color3" value="0.6, 0, 0.6"/>
    <input name="diag_file" type="filename" value="diagnostic.png"/>
    <output name="out" type="surfaceshader" />
  </nodedef>

Here is the corresponding nodegraph.
The nodegraph maps the input filename to the image node.

<nodegraph name="IMPL_diagimage" nodedef="ND_diagimage">
    <texcoord name="texcoord" type="vector2" nodedef="ND_texcoord_vector2"/>
    <image name="image0" type="color3" nodedef="ND_image_color3">
      <input name="file" type="filename" interfacename="diag_file" />
      <input name="texcoord" type="vector2" nodename="texcoord" />
    </image>
    
    <add name="add1" type="color3" >
      <input name="in1" type="color3" interfacename="extra_color"  />
      <input name="in2" type="color3" nodename="image0"  />
    </add>
    <uniform_edf name="uniform_edf" type="EDF" >
      <input name="color" type="color3" nodename="add1" />
    </uniform_edf>
    <surface name="diag_shader" type="surfaceshader" >
      <input name="bsdf" type="BSDF" value="" />
      <input name="edf" type="EDF" nodename="uniform_edf" />
      <input name="opacity" type="float" value="1" />
    </surface>
    <output name="out" type="surfaceshader" nodename="diag_shader" />
</nodegraph>

Generates the following error:

Warning: in _ValidateCompilation at line 213 of pxr\imaging\hdSt\glslProgram.cpp -- Failed to compile shader (FRAGMENT_SHADER): 0(2436) : error C1503: undefined variable "diag_file"
ERROR: Usdview encountered an error while rendering.

Expected result from the referenced mtlx file loads in MaterialXVIew

Steps to Reproduce

1. Extract the files in the zip mtlx_image_nodedef.zip
2. Copy diagtest_def.mtlx from libraries\bxdf to usd\install\debug|release\libraries\bxdf (note this can be also be setup by using env variable)
3. open simplediag.usda
4. Usdview reports error in console
   Warning: in _ValidateCompilation at line 213 of \pxr\imaging\hdSt\glslProgram.cpp -- Failed to compile shader (FRAGMENT_SHADER): 0(2436) : error C1503: undefined variable "diag_file"

System Information (OS, Hardware)

Windows 10

Package Versions

v22.03-rc2

Build Flags

build_scripts\build_usd.py --build-variant debug --materialx

[jilliene]

Filed as internal issue #USD-7244