[EMSUSD-671] Trying to load payloads on Maya USD proxy nodes not working after have been previously disabled

[stephanehoarau]

Discussed in #3351

^{Originally posted by stephanehoarau September 27, 2023}
Hello !
At work, we hit an issue with the Load Payloads option (either in UI or by script) on USD proxy nodes.
I created a simple demo_example.zip file composed of two files to expose you the problem and the steps to reproduce the issue.

• example.usd (ascii USD file)
• geo_payload.usd (binary USD file)
  The example.usd will contain a default prim called "main" that is composed of a payload file (geo_payload.usd).

Steps to reproduce the issue:

    - Load Maya (2023 + USD 0.22.0 plugin)
    - Create a stage with the menu "Edit > Universal Scene Description (USD) > Stage with New Layer" -> the stage node "stage1" is created within an "anonymousLayer1" USD layer.
    - Launch the "USD Layer Editor" by RMB on the "stage1" node in the Outliner.
    - In the "USD Layer Editor", select the option "Load SubLayers..." with the RMB on the "anonymousLayer1*" layer.
    - In the new UI "Load SubLayers in the anonymousLayer1", choose the "Layer Path" option and select in your Browser the provided file "example.usd" .
    - This will show you a wonderfull sphere on your scene.
    - Then select the "stageStage1" shape related to "stage1".
    - in the "Attribute Editor", in the "Extra Attributes" section, find the checkbox called "Load Payloads":
        - If you untick the checkbox, you should see the sphere disappear in the viewport. Actually this is the expected behavior.
        - If you tick again the checkbox, you should see the sphere appear in the viewport. This is the expected behavior.

    - Now save this scene in a maya file (with Save Scene Options: Save all edits (including USD) to the Maya scene file (I saved it under "[stage_example.ma]")
    - Clean the scene with File > New.
    - Load the scene just saved.
    - Select the "stageStage1" shape in the scene. 
    - in the "Attribute Editor", in the "Extra Attributes" section, find the checkbox called "Load Payloads": 
        - If you untick the checkbox, you should see the sphere disappear in the viewport. This is the expected behavior.
        - Now if you tick again the checkbox, unfortunately the sphere won't appear again. THIS IS NOT EXPECTED. Moreover, notice also that the sublayer called "example.usd" under "anonymousLayer1" has been removed too (we do believe this is actually the main issue though).

demo_example.zip

Stephane
TD Lead @ Cinesite

[maya-usd-git-sync]

Issue synced internally to EMSUSD-821

[pierrebai-adsk]

@stephanehoarau you were right that there was a data-loss issue when saving sub-layers inside the Maya scene instead of in separate USD files that happened only after reload. You can track the fix in case you want to rebuild the MayaUSD plugin: #3518