In Geometry Nodes a geometry is represented by a `GeometrySet`. This is a
container that can contain one geometry of each of the supported types (mesh,
curves, volume, grease pencil, pointcloud, instances). It's possible for a
`GeometrySet` to contain e.g. a mesh and a point cloud.
This patch creates a Python wrapper for the built-in `GeometrySet`. For now,
it's main purpose is to consume the complete evaluated geometry of an object
without having to go through complex hoops via `depsgraph.object_instances`. It
also also allows retrieving instances that have been created with legacy
instancing systems such as dupli-verts or particles.
In the future, the `GeometrySet` API could also be used for more kinds of
geometry processing from Python, similar to how we use `GeometrySet` internally
as generic geometry storage.
Since we can't really have constness guarantees in Python currently, it's
enforced that the `GeometrySet` wrapper always has its own copy of each geometry
type (so e.g. it does not share a `Mesh` data-block pointer with any other place
in Blender). Without the copy, changes to the mesh in the geometry set would
also affect the evaluated geometry that Blender sees. The copy has a small cost,
but typically the overhead should be low, because attributes and other run-time
data can still be shared. This should be entirely thread-safe, assuming that no
code modifies implicitly shared data, which is forbidden. For historic reasons
there are still cases like #132423 where this assumption does not hold in all
cases. Those cases should be fixed. To my knowledge, this patch does not
introduce any new such issues or makes existing issues worse.
Pull Request: https://projects.blender.org/blender/blender/pulls/135318
29fddf4710