usdGenSchema collapses the authority component of URI asset paths when the associated path resolver is not registered

[nvmkuruc]

Description of Issue

Consider the following schema.usda.

#usda 1.0
(
    subLayers = [
        @usd/schema.usda@
    ]
)

over "GLOBAL" (
    customData = {
        string libraryName = "mySchema"
        string libraryPath = "./"
        string libraryPrefix = "MySchema"
        bool skipCodeGeneration = true
    }
)
{
}

class "MyAPI"
(
    customData = {
        token apiSchemaType = "singleApply"
        string className = "MyAPI"
    }
    doc = "A test asset reference."
    inherits = </APISchemaBase>
)
{
    asset myApi:assetReference= @omni://1.2.3.4/path/to/file.txt@) (
        doc = """"""
    )
}

usdGenSchema uses UsdStage::Flatten and will try to anchor all asset paths. If an asset path refers to a URI resolver that isn't registered, the default resolver will treat the path like a file path and "normalize" away the //. For URIs, this effectively shifts the "authority" component into the "path" component of identifier.

There may be practical reasons why during usdGenSchema the asset and resolver may not be available. usdGenSchema depending on the availability of the resources required to run the resolvers used by attribute values may be unexpected.

If I suppress running SdfComputeAssetPathRelativeToLayer just for the omni: URI prefix in _AnchorAssetPathRelativeToLayer, the // is preserved.

Curiously, if I try to take advantage of "anchored paths" in my schema.usda file, it effectively resolves relative to the source directory. It doesn't seem like there's a use case for for the current anchoring behavior in usdGenSchema.

Steps to Reproduce

1. Save the above layer to schema.usda
2. Run the following code

>>> from pxr import Usd
>>> Usd.Stage.Open("./schema.usda").Flatten().ExportToString()

3. You should see // collapsed into / for myApi:assetReference, effectively erasing the "authority" component.

System Information (OS, Hardware)

Observed on both Linux and Windows

Package Versions

Build Flags

--tests -no-imaging --no-materialx

[jesschimein]

Filed as internal issue #USD-8410