Fix: Assert in legacy mesh conversion

When setting the old face set custom data type, do it in the
"layers to write" vector instead of the mesh's poly custom
data layers, which weren't "prepared" yet. Otherwise this
could make the mesh's custom data typemap invalid.

3 files changed, 15 insertions, 4 deletions

diff --git a/source/blender/blenkernel/BKE_mesh_legacy_convert.h b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
index d3e582ff197..92182f8045b 100644
--- a/source/blender/blenkernel/BKE_mesh_legacy_convert.h
+++ b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
@@ -10,6 +10,11 @@
 #include "BLI_utildefines.h"
 
 #ifdef __cplusplus
+#  include "BLI_span.hh"
+#  include "DNA_customdata_types.h"
+#endif
+
+#ifdef __cplusplus
 extern "C" {
 #endif
 
@@ -17,10 +22,13 @@ struct CustomData;
 struct Mesh;
 struct MFace;
 
+#ifdef __cplusplus
+
 /**
  * Move face sets to the legacy type from a generic type.
  */
-void BKE_mesh_legacy_face_set_from_generic(struct Mesh *mesh);
+void BKE_mesh_legacy_face_set_from_generic(
+    Mesh *mesh, blender::MutableSpan<CustomDataLayer> poly_layers_to_write);
 /**
  * Copy face sets to the generic data type from the legacy type.
  */
@@ -74,6 +82,8 @@ void BKE_mesh_legacy_convert_material_indices_to_mpoly(struct Mesh *mesh);
  */
 void BKE_mesh_legacy_convert_mpoly_to_material_indices(struct Mesh *mesh);
 
+#endif
+
 /**
  * Recreate #MFace Tessellation.
  *
diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index dfd023b548b..4d98efde5f9 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -252,7 +252,7 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
       BKE_mesh_legacy_convert_selection_layers_to_flags(mesh);
       BKE_mesh_legacy_convert_material_indices_to_mpoly(mesh);
       BKE_mesh_legacy_bevel_weight_from_layers(mesh);
-      BKE_mesh_legacy_face_set_from_generic(mesh);
+      BKE_mesh_legacy_face_set_from_generic(mesh, poly_layers);
       BKE_mesh_legacy_edge_crease_from_layers(mesh);
       /* When converting to the old mesh format, don't save redundant attributes. */
       names_to_skip.add_multiple_new({".hide_vert",
diff --git a/source/blender/blenkernel/intern/mesh_legacy_convert.cc b/source/blender/blenkernel/intern/mesh_legacy_convert.cc
index b68117b7eb2..df3057d9592 100644
--- a/source/blender/blenkernel/intern/mesh_legacy_convert.cc
+++ b/source/blender/blenkernel/intern/mesh_legacy_convert.cc
@@ -922,10 +922,11 @@ void BKE_mesh_add_mface_layers(CustomData *fdata, CustomData *ldata, int total)
 /** \name Face Set Conversion
  * \{ */
 
-void BKE_mesh_legacy_face_set_from_generic(Mesh *mesh)
+void BKE_mesh_legacy_face_set_from_generic(Mesh *mesh,
+                                           blender::MutableSpan<CustomDataLayer> poly_layers)
 {
   using namespace blender;
-  for (CustomDataLayer &layer : MutableSpan(mesh->pdata.layers, mesh->pdata.totlayer)) {
+  for (CustomDataLayer &layer : poly_layers) {
     if (StringRef(layer.name) == ".sculpt_face_set") {
       layer.type = CD_SCULPT_FACE_SETS;
     }