[Bf-blender-cvs] [b642dc7bc7d] master: Fix: Incorrect forward-compatible saving of face sets

[Hans Goudey]

Commit: b642dc7bc7d8db7a2fc3f6c8c58d69429c57d338
Author: Hans Goudey
Date:   Sun Feb 5 18:09:22 2023 -0500
Branches: master
https://developer.blender.org/rBb642dc7bc7d8db7a2fc3f6c8c58d69429c57d338

Fix: Incorrect forward-compatible saving of face sets

There were two errors with the function used to convert face sets
to the legacy mesh format for keeping forward compatibility:
- It was moved before `CustomData_blend_write_prepare` so it
  operated on an empty span.
- It modified the mesh when it's only supposed to change the copy
  of the layers written to the file.

Differential Revision: https://developer.blender.org/D17210

===================================================================

M	source/blender/blenkernel/BKE_mesh_legacy_convert.h
M	source/blender/blenkernel/intern/mesh.cc
M	source/blender/blenkernel/intern/mesh_legacy_convert.cc

===================================================================

diff --git a/source/blender/blenkernel/BKE_mesh_legacy_convert.h b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
index 5b72d85a433..ee33d535179 100644
--- a/source/blender/blenkernel/BKE_mesh_legacy_convert.h
+++ b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
@@ -36,7 +36,7 @@ void BKE_mesh_legacy_convert_uvs_to_generic(Mesh *mesh);
  * Move face sets to the legacy type from a generic type.
  */
 void BKE_mesh_legacy_face_set_from_generic(
-    Mesh *mesh, blender::MutableSpan<CustomDataLayer> poly_layers_to_write);
+     blender::MutableSpan<CustomDataLayer> poly_layers_to_write);
 /**
  * Copy face sets to the generic data type from the legacy type.
  */
diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index 71275444028..dfdad20cc48 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -270,7 +270,6 @@ 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, poly_layers);
       BKE_mesh_legacy_edge_crease_from_layers(mesh);
       BKE_mesh_legacy_sharp_edges_to_flags(mesh);
       BKE_mesh_legacy_attribute_strings_to_flags(mesh);
@@ -292,6 +291,7 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
 
     if (!BLO_write_is_undo(writer)) {
       BKE_mesh_legacy_convert_uvs_to_struct(mesh, temp_arrays_for_legacy_format, loop_layers);
+      BKE_mesh_legacy_face_set_from_generic(poly_layers);
     }
   }
 
diff --git a/source/blender/blenkernel/intern/mesh_legacy_convert.cc b/source/blender/blenkernel/intern/mesh_legacy_convert.cc
index c380a61dce2..b789fbcc0a0 100644
--- a/source/blender/blenkernel/intern/mesh_legacy_convert.cc
+++ b/source/blender/blenkernel/intern/mesh_legacy_convert.cc
@@ -1224,23 +1224,26 @@ void BKE_mesh_tessface_ensure(struct Mesh *mesh)
 /** \name Face Set Conversion
  * \{ */
 
-void BKE_mesh_legacy_face_set_from_generic(Mesh *mesh,
-                                           blender::MutableSpan<CustomDataLayer> poly_layers)
+void BKE_mesh_legacy_face_set_from_generic(blender::MutableSpan<CustomDataLayer> poly_layers)
 {
   using namespace blender;
-  void *faceset_data = nullptr;
+  bool changed = false;
   for (CustomDataLayer &layer : poly_layers) {
     if (StringRef(layer.name) == ".sculpt_face_set") {
-      faceset_data = layer.data;
-      layer.data = nullptr;
-      CustomData_free_layer_named(&mesh->pdata, ".sculpt_face_set", mesh->totpoly);
+      layer.type = CD_SCULPT_FACE_SETS;
+      layer.name[0] = '\0';
+      changed = true;
       break;
     }
   }
-  if (faceset_data != nullptr) {
-    CustomData_add_layer(
-        &mesh->pdata, CD_SCULPT_FACE_SETS, CD_ASSIGN, faceset_data, mesh->totpoly);
+  if (!changed) {
+    return;
   }
+  /* #CustomData expects the layers to be sorted in increasing order based on type. */
+  std::stable_sort(
+      poly_layers.begin(),
+      poly_layers.end(),
+      [](const CustomDataLayer &a, const CustomDataLayer &b) { return a.type < b.type; });
 }
 
 void BKE_mesh_legacy_face_set_to_generic(Mesh *mesh)