The USDC File Format does not use the AR system unless USDC_USE_ASSET is enabled

[alasdair-foundry]

Description of Issue

The USD_USE_ASSET environment variable has to be explicitly enabled to allow the USDC file format to use the Asset Resolver system when saving/loading files. A commit message for the commit that adds this environment variable mentions that enabling USDC_USE_ASSET causes issues on Windows:

The new testUsdFileFormats_asset test exposed a pre-existing issue with saving .usdc files when
using the ArAsset-based codepath and an ArFilesystemAsset to read .usdc files on Windows. This
codepath isn't typically used; it's meant to handle non-file backed .usdc assets so we hadn't seen
it until now. The test is disabled for now while we look into a fix.

Commit

The behaviour when USD_USE_ASSET is enabled should be the default to allow USDC files to be used with the AR system as users would expect.

Steps to Reproduce

1. Create an AssetResolver & associated ArWritableAsset & ArAsset which handles a given asset path format
2. Export a USD stage using the .usdc file format with the asset path format
3. Observe the export does not call into the Asset Resolver unless USD_USE_ASSET is explicitly enabled, and will instead try to save to the file system

System Information (OS, Hardware)

• Windows, but I can't see why this wouldn't also affect other OSs.

Package Versions

OpenUSD 23.05 - VFX Platform 2023

[jesschimein]

Filed as internal issue #USD-8897