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, 2 deletions

diff --git a/source/blender/makesdna/DNA_mesh_types.h b/source/blender/makesdna/DNA_mesh_types.h
index bf50100b302..24f3d1c95ed 100644
--- a/source/blender/makesdna/DNA_mesh_types.h
+++ b/source/blender/makesdna/DNA_mesh_types.h
@@ -132,14 +132,23 @@ typedef struct Mesh_Runtime {
    */
   char wrapper_type_finalize;
 
+  int subsurf_resolution;
   /**
    * Settings for lazily evaluating the subdivision on the CPU if needed. These are
    * set in the modifier when GPU subdivision can be performed.
    */
   char subsurf_apply_render;
   char subsurf_use_optimal_display;
-  char _pad[2];
-  int subsurf_resolution;
+
+  /**
+   * Caches for lazily computed vertex and polygon normals. These are stored here rather than in
+   * #CustomData because they can be calculated on a const mesh, and adding custom data layers on a
+   * const mesh is not thread-safe.
+   */
+  char vert_normals_dirty;
+  char poly_normals_dirty;
+  float (*vert_normals)[3];
+  float (*poly_normals)[3];
 
   void *_pad2;