Goals

Hold a super set of the semantic spaces that OpenXR requires, local, stage, unbounded, etc...
Allow devices to be attached to other devices.
Generating events when a space is moved, dealing with the requirement that the move only happens after a certain time in the future.
Make the internal graph be simple and non-cyclic.
- No bi-directional links.
- Always have a root object.

Overall design

The xrt_space object is fairly straight forward, it's a space. Some have semantic meaning like local, stage, unbounded. The xrt_space_overseer object is where most of the interesting things happens. By having a separate object where functionality is implemented we can handle things like: different sessions with different spaces, have a object that space events comes from. It also makes it easier for the IPC layer to implement it.

Issues & questions

Where do space events come from?
- UNRESOLVED: Probably will come from the space overseer.
Do we map all spaces one to one, like the each XrSpace with an offset gets backed with an xrt_space. There is going to be a limit on spaces from the point of view of the IPC layer, do we create some of them locally?
- RESOLVED: While you can create a offset xrt_space, the offsets from from the XrSpaces are not expressed as an xrt_space this is to reduce the number of spaces created.
For the IPC gap do we mirror the entire space graph between the app side and the service side. Or is it enough to only expose the spaces/nodes a app needs and keep all of the links on the service side only.
- RESOLVED: The graph is kept opaque from the application (and all other parts of Monado, an space overseer has greater freedom in how to configure the space graph).