Relative asset paths with \ separators resolve inside a .usdz but not as loose files #149

Summary

On Linux, DefaultResolver keeps \ as part of a file name when it anchors a loose
relative asset path. A package-relative path is split on both / and \. The same
Windows-authored layer therefore resolves when it is packaged as a .usdz and fails
when it sits beside its files, for asset attributes and for composition arcs alike.

Reproduction

Checked against main at 933aa8f, on Linux. tex/stone.png and sub/layer.usda
(which defines /S with int x = 7) sit beside each layer.

The packaged case is the one that works: a .usdz whose root layer authors
asset f = @tex\stone.png@, with an entry tex/stone.png, resolves to
pkg.usdz[tex/stone.png].

use openusd::{sdf, usd::Stage, usdz::ArchiveWriter};
use std::fs;

fn main() {
    let d = std::path::Path::new("run");
    fs::create_dir_all(d.join("tex")).unwrap();
    fs::write(d.join("tex/stone.png"), b"x").unwrap();
    fs::write(d.join("back.usda"), "#usda 1.0\ndef \"Q\"\n{\n    asset f = @tex\\stone.png@\n}\n").unwrap();
    let s = Stage::open(d.join("back.usda").to_str().unwrap()).unwrap();
    let a = s.attribute("/Q.f").unwrap().get::<sdf::AssetPath>().unwrap().unwrap();
    println!("loose: {:?}", a.resolved_path()); // None

    let mut w = ArchiveWriter::create(d.join("pkg.usdz")).unwrap();
    w.add_layer("root.usda", b"#usda 1.0\ndef \"Q\"\n{\n    asset f = @tex\\stone.png@\n}\n").unwrap();
    w.add_layer("tex/stone.png", b"x").unwrap();
    w.finish().unwrap();
    let s = Stage::open(d.join("pkg.usdz").to_str().unwrap()).unwrap();
    let a = s.attribute("/Q.f").unwrap().get::<sdf::AssetPath>().unwrap().unwrap();
    println!("usdz: {:?}", a.resolved_path()); // Some("…/pkg.usdz[tex/stone.png]")
}

Where it comes from

In crates/openusd/src/ar.rs, DefaultResolver::create_identifier takes two routes:

• Package-relative anchors go through join_packaged_path (~L1017). It splits the
  reference with rel.split(['/', '\\']), so \ is a separator.
• Filesystem anchors use dir.join(path) (~L591), followed by canonical_identifier
  or lexically_normalize. On Unix, Path treats \ as an ordinary byte.

What C++ USD does

C++ normalizes the full path only on Windows. _AnchorRelativePath in
pxr/usd/ar/defaultResolver.cpp replaces \ with / in the anchor alone. The asset
path is then passed through TfNormPath → ArchNormPath. In pxr/base/arch/fileSystem.cpp
the ARCH_OS_WINDOWS version of ArchNormPath begins with
std::replace(path.begin(), path.end(), '\\', '/'). The POSIX version does not.

C++ on Linux therefore fails the same way openusd does, and openusd's loose-file
behaviour is at parity. A layer authored on Windows resolves for its author in C++,
which is why such files exist in the wild.

Where it shows up

Intel's New Sponza USD export, written by "USD for Autodesk 3ds Max", authors all 98
texture inputs as @textures\<name>.png@. Opened with openusd on Linux, none of them
resolves.

Proposal

Treat \ as a separator when anchoring a relative filesystem path, as
join_packaged_path already does. Equivalently, apply on every platform what
ArchNormPath does on Windows.

• Scope: relative paths only, and only after the authored spelling has failed. If a
  file whose name really contains \ exists, it still wins, so no path that resolves
  today changes meaning. A cheaper alternative with the same practical effect is to
  normalize unconditionally; a Unix file name containing \ is rare.
• Drive-letter and UNC paths (C:\…, \\server\…): Path::is_relative() reports
  these as relative on Unix, so today they get joined under the anchor directory. They
  cannot resolve on Unix either way. It may be worth leaving them alone, or treating
  them as absolute, rather than producing …/dir/C:/….
• Coverage: because this lives in create_identifier, it covers asset attributes,
  sublayers, references and payloads together.

link to doubleailes/crust-render#263