Fix T95839: Data race when lazily creating mesh normal layers

Currently, when normals are calculated for a const mesh, a custom data
layer might be added if it doesn't already exist. Adding a custom data
layer to a mesh is not thread-safe, so this can be a problem in some
situations.

This commit moves derived mesh normals for polygons and
vertices out of `CustomData` to `Mesh_Runtime`. Most of the
hard work for this was already done by rBcfa53e0fbeed7178.
Some changes to logic elsewhere are necessary/helpful:
- No need to call both `BKE_mesh_runtime_clear_cache` and
  `BKE_mesh_normals_tag_dirty`, since the former also does the latter.
- Cleanup/simplify mesh conversion and copying since normals are
  handled with other runtime data.

Storing these normals like other runtime data clarifies their status
as derived data, meaning custom data moves more towards storing
original/editable data. This means normals won't automatically benefit
from the planned copy-on-write refactor (T95845), so it will have to be
added manually like for the other runtime data.

Differential Revision: https://developer.blender.org/D14154

1 files changed, 11 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_mesh.h b/source/blender/blenkernel/BKE_mesh.h
index 2b32c6a5420..72a1303fc6b 100644
--- a/source/blender/blenkernel/BKE_mesh.h
+++ b/source/blender/blenkernel/BKE_mesh.h
@@ -428,6 +428,17 @@ float (*BKE_mesh_vertex_normals_for_write(struct Mesh *mesh))[3];
 float (*BKE_mesh_poly_normals_for_write(struct Mesh *mesh))[3];
 
 /**
+ * Free any cached vertex or poly normals. Face corner (loop) normals are also derived data,
+ * but are not handled with the same method yet, so they are not included. It's important that this
+ * is called after the mesh changes size, since otherwise cached normal arrays might not be large
+ * enough (though it may be called indirectly by other functions).
+ *
+ * \note Normally it's preferred to call #BKE_mesh_normals_tag_dirty instead,
+ * but this can be used in specific situations to reset a mesh or reduce memory usage.
+ */
+void BKE_mesh_clear_derived_normals(struct Mesh *mesh);
+
+/**
  * Mark the mesh's vertex normals non-dirty, for when they are calculated or assigned manually.
  */
 void BKE_mesh_vertex_normals_clear_dirty(struct Mesh *mesh);