BMesh: improve handling of custom-data flag (Mesh.cd_flag)

Code that handled merging & initializing custom-data from other
meshes sometimes missed checks for this flag, causing bevel weights to
lost when the mesh was converted to a BMesh.

The following changes are a more general fix for T94197.

- Add BM_mesh_copy_init_customdata_from_mesh_array which initializes
  custom-data from multiple meshes at once.
  As well as initializing custom-data layers from Mesh.cd_flag.

  This isn't essential for boolean, however it avoids the overhead of
  resizing custom-data layers.

- Loading mesh data into a BMesh now respects Mesh.cd_flag
  instead of only checking if the BMesh custom-data-layer exists.
  Without this, the order of meshes passed to BM_mesh_bm_from_me could
  give different (incorrect) results.

- Copying mesh data now copies `cd_flag` too. This is a precaution
  as in my tests evaluating modifiers these values always matched.
  Nevertheless it's correct to copy this value as custom-data it's
  self is being copied.

1 files changed, 8 insertions, 7 deletions

diff --git a/source/blender/modifiers/intern/MOD_boolean.cc b/source/blender/modifiers/intern/MOD_boolean.cc
index 2a303c7741c..bb05ae3e1b3 100644
--- a/source/blender/modifiers/intern/MOD_boolean.cc
+++ b/source/blender/modifiers/intern/MOD_boolean.cc
@@ -248,13 +248,14 @@ static BMesh *BMD_mesh_bm_create(
   BMeshCreateParams bmesh_create_params{};
   BMesh *bm = BM_mesh_create(&allocsize, &bmesh_create_params);
 
-  /* Needed so active layers are set based on `mesh` not `mesh_operand_ob`,
-   * otherwise the wrong active render layer is used, see T92384. */
-  BM_mesh_copy_init_customdata_from_mesh(bm, mesh, &allocsize);
-
-  /* NOTE(@campbellbarton): Handle in #BM_mesh_bm_from_me, this is a local fix for T94197. */
-  BM_mesh_cd_flag_apply(bm,
-                        mesh->cd_flag | mesh_operand_ob->cd_flag | BM_mesh_cd_flag_from_bmesh(bm));
+  /* Keep `mesh` first, needed so active layers are set based on `mesh` not `mesh_operand_ob`,
+   * otherwise the wrong active render layer is used, see T92384.
+   *
+   * NOTE: while initializing customer data layers the is not essential,
+   * it avoids the overhead of having to re-allocate #BMHeader.data when the 2nd mesh is added
+   * (if it contains additional custom-data layers). */
+  const Mesh *mesh_array[2] = {mesh, mesh_operand_ob};
+  BM_mesh_copy_init_customdata_from_mesh_array(bm, mesh_array, ARRAY_SIZE(mesh_array), &allocsize);
 
   BMeshFromMeshParams bmesh_from_mesh_params{};
   bmesh_from_mesh_params.calc_face_normal = true;