A mesh wrapper might be being accessed for read-only from one thread
while another thread converts the wrapper type to something else.
The proposes solution is to defer assignment of the mesh wrapper
type until the wrapper is fully converted. The good thing about this
approach is that it does not introduce extra synchronization (and,
potentially, evaluation pipeline stalls). The downside is that it
might not work with all possible wrapper types in the future. If a
wrapper type which does not clear data separation is ever added in
the future we will re-consider the threading safety then.
Unfortunately, some changes outside of the mesh wrapper file are
to be made to allow "incremental" construction of the mesh prior
changing its wrapper type.
Unfortunately, there is no simplified file which demonstrates the
issue. It was investigated using Heist production file checked at
the revision 1228: `pro/lib/char/einar/einar.shading.blend`. The
repro case is simple: tab into edit mode, possibly few times.
The gist is that there several surface deform and shrinkwrap
modifiers which uses the same target. While one of them is building
BVH tree (which changes wrapper type) the other one accesses it for
read-only via `BKE_mesh_wrapper_vert_coords_copy_with_mat4()`.
Differential Revision: https://developer.blender.org/D15424
It was looking up the last modifier in the stack, ignoring visibility, instead
of mesh->runtime.subsurf_runtime_data set by the modifier evaluation and used by
the drawing code.
Instancing with geometry nodes uses just the evaluated Mesh, and ignores the
Object that it came from. That meant that it would try to look up the subsurf
modifier on the instancer object which does not have the subsurf modifier.
Instead of storing a session UUID and looking up the modifier data, store a
point to the subsurf modifier runtime data. Unlike the modifier data, this
runtime data is preserved across depsgraph CoW. It must be for the subdiv
descriptor contained in it to stay valid along with the draw cache.
As a bonus, this moves various Mesh_Runtime variables into the subsurf runtime
data, reducing memory usage for meshes not using subdivision surfaces.
Also fixes T98693, issues with subdivision level >= 8 due to integer overflow.
Differential Revision: https://developer.blender.org/D15184
The crash is caused as the subdivision wrapper does not have loop
normals, which are generally computed at the end of the modifier stack
evaluation via `mesh_calc_modifier_final_normals`. (Note that they are
initially computed, but deleted by the subdivision wrapper creation.)
This records in the mesh runtime whether loop normals should have been
computed and computes them alongside the subdivision wrapper.
Differential Revision: https://developer.blender.org/D14489
