Flattening produces invalid document

[pablode]

I'm running into an issue where when I flatten following document

Show code

<?xml version="1.0"?>
<materialx version="1.38" xmlns:xi="http://www.w3.org/2001/XInclude">
  <UsdPreviewSurface name="Surface" type="surfaceshader">
    <input name="diffuseColor" type="color3" output="diffuseColor_rgb" nodegraph="prismarine2" colorspace="srgb_texture" />
    <input name="normal" type="vector3" output="normal_out" nodegraph="prismarine2" />
    <input name="roughness" type="float" output="roughness_r" nodegraph="prismarine2" colorspace="lin_rec709" />
    <input name="metallic" type="float" value="0" />
    <input name="opacity" type="float" value="1" />
    <input name="useSpecularWorkflow" type="integer" value="0" />
  </UsdPreviewSurface>
  <surfacematerial name="prismarine" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="Surface" />
  </surfacematerial>
  <nodegraph name="prismarine2">
    <UsdUVTexture name="DiffuseTexture" type="multioutput" nodedef="ND_UsdUVTexture">
      <input name="file" type="filename" value="/Users/pablode/Work/usd-wg-assets/full_assets/McUsd/McUsd_materials/tex/prismarine.png" />
      <input name="wrapS" type="string" value="periodic" />
      <input name="wrapT" type="string" value="periodic" />
      <input name="st" type="vector2" nodename="UVReader" />
    </UsdUVTexture>
    <UsdPrimvarReader name="UVReader" type="vector2" nodedef="ND_UsdPrimvarReader_vector2">
      <input name="fallback" type="vector2" value="0, 0" />
      <input name="varname" type="string" value="st" />
    </UsdPrimvarReader>
    <output name="diffuseColor_rgb" type="color3" output="rgb" nodename="DiffuseTexture" />
    <convert name="convert0" type="vector3" nodedef="ND_convert_color3_vector3">
      <input name="in" type="color3" output="rgb" nodename="NormalTexture" colorspace="lin_rec709" />
    </convert>
    <UsdUVTexture name="NormalTexture" type="multioutput" nodedef="ND_UsdUVTexture">
      <input name="bias" type="color4" value="-1, -1, -1, 0" />
      <input name="file" type="filename" value="/Users/pablode/Work/usd-wg-assets/full_assets/McUsd/McUsd_materials/tex/prismarine_n.png" />
      <input name="scale" type="color4" value="2, 2, 2, 1" />
      <input name="wrapS" type="string" value="periodic" />
      <input name="wrapT" type="string" value="periodic" />
      <input name="st" type="vector2" nodename="UVReader" />
    </UsdUVTexture>
    <output name="normal_out" type="vector3" nodename="convert0" />
    <UsdUVTexture name="RoughnessTexture" type="multioutput" nodedef="ND_UsdUVTexture">
      <input name="file" type="filename" value="/Users/pablode/Work/usd-wg-assets/full_assets/McUsd/McUsd_materials/tex/prismarine_r.png" />
      <input name="wrapS" type="string" value="periodic" />
      <input name="wrapT" type="string" value="periodic" />
      <input name="st" type="vector2" nodename="UVReader" />
    </UsdUVTexture>
    <output name="roughness_r" type="float" output="r" nodename="RoughnessTexture" />
  </nodegraph>
</materialx>

using this code

for (mx::NodeGraphPtr graph : doc->getNodeGraphs())
{
    if (graph->getActiveSourceUri() == doc->getSourceUri())
    {
        graph->flattenSubgraphs();
    }
}

an invalid document is produced:

Show code

<?xml version="1.0"?>
<materialx version="1.38" xmlns:xi="http://www.w3.org/2001/XInclude">
  <UsdPreviewSurface name="Surface" type="surfaceshader">
    <input name="diffuseColor" type="color3" output="diffuseColor_rgb" nodegraph="prismarine2" colorspace="srgb_texture" />
    <input name="normal" type="vector3" output="normal_out" nodegraph="prismarine2" />
    <input name="roughness" type="float" output="roughness_r" nodegraph="prismarine2" colorspace="lin_rec709" />
    <input name="metallic" type="float" value="0" />
    <input name="opacity" type="float" value="1" />
    <input name="useSpecularWorkflow" type="integer" value="0" />
  </UsdPreviewSurface>
  <surfacematerial name="prismarine" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="Surface" />
  </surfacematerial>
  <nodegraph name="prismarine2">
    <image name="image_reader" type="color4">
      <input name="file" type="filename" value="/Users/pablode/Work/usd-wg-assets/full_assets/McUsd/McUsd_materials/tex/prismarine.png" />
      <input name="default" type="color4" value="0, 0, 0, 1" />
      <input name="texcoord" type="vector2" nodename="UVReader" />
      <input name="uaddressmode" type="string" value="periodic" />
      <input name="vaddressmode" type="string" value="periodic" />
    </image>
    <multiply name="image_scale" type="color4">
      <input name="in1" type="color4" nodename="image_reader" />
      <input name="in2" type="color4" value="1, 1, 1, 1" />
    </multiply>
    <add name="image_bias" type="color4">
      <input name="in1" type="color4" nodename="image_scale" />
      <input name="in2" type="color4" value="0, 0, 0, 0" />
    </add>
    <geompropvalue name="primvar" type="vector2">
      <input name="geomprop" type="string" value="st" />
      <input name="default" type="vector2" value="0, 0" />
    </geompropvalue>
    <output name="diffuseColor_rgb" type="color3" output="" nodename="image_bias" />
    <convert name="convert0" type="vector3" nodedef="ND_convert_color3_vector3">
      <input name="in" type="color3" output="" nodename="image_bias2" colorspace="lin_rec709" />
    </convert>
    <image name="image_reader2" type="color4">
      <input name="file" type="filename" value="/Users/pablode/Work/usd-wg-assets/full_assets/McUsd/McUsd_materials/tex/prismarine_n.png" />
      <input name="default" type="color4" value="0, 0, 0, 1" />
      <input name="texcoord" type="vector2" nodename="primvar" />
      <input name="uaddressmode" type="string" value="periodic" />
      <input name="vaddressmode" type="string" value="periodic" />
    </image>
    <multiply name="image_scale2" type="color4">
      <input name="in1" type="color4" nodename="image_reader2" />
      <input name="in2" type="color4" value="2, 2, 2, 1" />
    </multiply>
    <add name="image_bias2" type="color4">
      <input name="in1" type="color4" nodename="image_scale2" />
      <input name="in2" type="color4" value="-1, -1, -1, 0" />
    </add>
    <output name="normal_out" type="vector3" nodename="convert0" />
    <image name="image_reader3" type="color4">
      <input name="file" type="filename" value="/Users/pablode/Work/usd-wg-assets/full_assets/McUsd/McUsd_materials/tex/prismarine_r.png" />
      <input name="default" type="color4" value="0, 0, 0, 1" />
      <input name="texcoord" type="vector2" nodename="primvar" />
      <input name="uaddressmode" type="string" value="periodic" />
      <input name="vaddressmode" type="string" value="periodic" />
    </image>
    <multiply name="image_scale3" type="color4">
      <input name="in1" type="color4" nodename="image_reader3" />
      <input name="in2" type="color4" value="1, 1, 1, 1" />
    </multiply>
    <add name="image_bias3" type="color4">
      <input name="in1" type="color4" nodename="image_scale3" />
      <input name="in2" type="color4" value="0, 0, 0, 0" />
    </add>
    <output name="roughness_r" type="float" output="" nodename="image_bias3" />
  </nodegraph>
</materialx>

The issue here is that the image_reader node still references a UVReader node which does not exist anymore, unlike the other two image nodes.