UsdUtils.ExtractExternalReferences requires editable Layers

[ColinKennedy]

Description of Issue

A follow-up from this usd-interest forum post

UsdUtils.ExtractExternalReferences requires the Layer to be editable which is seems odd since theoretically the function should only need to query things on the Layer.

This has caused issues for us in Houdini because Houdini frequently sets Layers to be un-editable if they are created by LOPs or even if it's an external Layer, brought in via sublayer / referencing. We're getting around the issue by using a Python context to temporarily call SetPermissionToEdit(True) and reverting but if UsdUtils could be improved to not require edit permissions, it'd remove a lot of our boilerplate code.

Steps to Reproduce

1. Run this below

referencee = Usd.Stage.CreateInMemory()
prim = referencee.DefinePrim("/root")
referencee.SetDefaultPrim(prim)
referencer = Usd.Stage.CreateInMemory()
prim = referencer.DefinePrim("/root")
prim.GetReferences().AddReference(referencee.GetRootLayer().identifier)
referencer.GetRootLayer().SetPermissionToEdit(False)

UsdUtils.ExtractExternalReferences(referencer.GetRootLayer().identifier)

Assuming file.usda is not empty and has at least one dependency, that code will error with

Error in 'pxrInternal_v0_19__pxrReserved__::Sdf_ListOpListEditor<TypePolicy>::_UpdateListOp [with TypePolicy = pxrInternal_v0_19__pxrReserved__::SdfPayloadTypePolicy]' at line 118 in file /path/to/pxr/usd/lib/sdf/listOpListEditor.h : 'Layer is not editable.'

System Information (OS, Hardware)

CentOS 7.4

Package Versions

USD 19.11

[jilliene]

Filed as internal issue #USD-6808