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monado/doc/packaging-notes.md

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Packaging Notes

The SONAME and library location question

AKA, "Why does libopenxr_monado.so not have a versioned SONAME? Can I move it to a subdirectory of the library path?"

To understand this, it helps to keep in mind how OpenXR works from an application point of view. Applications are intended to be runtime-independent and portable, using the user's current runtime wherever they are executed. The OpenXR Loader is the library that applications link against to achieve this: it provides a layer of indirection. It also allows for the use of API layers with runtimes, and handles runtime finding. Follow that link for more information from the design documents.

The TL;DR version is this: libopenxr_monado not having a versioned SONAME yet being installed in the default library path is not an error or oversight, but a carefully-reasoned technical decision. Do not patch this or change it in distribution packages without understanding what design choices and features you break by doing so.

My package linter is complaining that libopenxr_monado.so does not have a versioned SONAME

Monado (as well as OpenXR API layers) are like Vulkan drivers (technically ICDs) and API layers, in that they are only used behind a loader doing runtime dynamic linking/late loading. A versioned SONAME makes no real sense for them, but sometimes you do want them in the default dynamic library load path.

You never link against the OpenXR runtime interface of Monado directly, it's loaded dynamically at runtime by the OpenXR loader after reading configurations and manifest files. The OpenXR loader does have a versioned SONAME on Linux, where it might be system installed. This is why it makes no sense to have a SONAME for the runtime SO: the compatibility of that SO is defined by the manifest that is installed and the OpenXR spec itself.

At some point we might give up and set a versioned SONAME for libopenxr_monado, but it would not improve anything about the software other than its compliance with external rules that do not consider how the software is designed or used. Furthermore, it would likely lead to increased misunderstandings in how OpenXR and Monado are intended to be used by developers. The only public interface of that libopenxr_monado.so is strictly specified by the OpenXR specification in general, with its exported symbols specified by the Runtime Interface Negotiation section.

Note: libmonado, which does have a public API as a management interface for software serving as a Monado front-end or dashboard, does have a versioned SONAME.

My package linter is complaining that without a versioned SONAME, libopenxr_monado.so should move to a private subdirectory of /usr/lib/...

Why can we not move that library off of the main dynamic loader path? It is true that OpenXR API layers can live in a subdirectory of the dynamic library path, because the loader will try to load layers from all the manifests and ignore the ones whose library it cannot load, resolving multi-arch issues by trying all architectures. However, until a change to the OpenXR loader in OpenXR SDK 1.0.29 (from late 2023), the only way to get multi-arch support with an OpenXR runtime on Linux was to:

  • Place the binary in the correct library directory for the architecture
  • Put only the library name in the runtime manifest (rather than a relative or absolute path) - turn off both XRT_OPENXR_INSTALL_ABSOLUTE_RUNTIME_PATH and XRT_OPENXR_INSTALL_ABSOLUTE_RUNTIME_PATH for this to happen.

When this is done, the loader uses the normal library search path to find the runtime, which gives you the right architecture for the application. Essentially, it lets the loader use the same argument to dlopen regardless of architecture, and get the right library. This allows, e.g. a 32-bit application to use the 32-bit version of libopenxr_monado, typically to talk to the 64-bit monado-service instance.

At some point it would be nice to generate the architecture-specific runtime manifests for Monado. However, it makes selecting runtimes system-wide using alternatives or similar more complex, as you would need to update multiple symlinks.