Fix #113386: separating by loose parts removes attribute data

The issue was that custom data layers were modified without checking
that the data was not shared. Now the data is unshared first.

I also added an assert to catches this kind of issue more directly.

source/blender/blenkernel/BKE_customdata.h

@@ -379,7 +379,8 @@ void CustomData_copy_layer_type_data(const struct CustomData *source,
                                      int count);
 
 /**
- * Frees data in a #CustomData object.
+ * Frees data in a #CustomData object. This is only expected to be called if the data layers are
+ * not shared (#CustomData_ensure_layers_are_mutable).
  */
 void CustomData_free_elem(struct CustomData *data, int index, int count);
 
@@ -434,6 +435,7 @@ void CustomData_swap_corners(struct CustomData *data, int index, const int *corn
  * function makes sure that the layer is unshared if it was shared, which makes it mutable.
  */
 void CustomData_ensure_data_is_mutable(struct CustomDataLayer *layer, int totelem);
+void CustomData_ensure_layers_are_mutable(struct CustomData *data, int totelem);
 
 /**
  * Retrieve a pointer to an element of the active layer of the given \a type, chosen by the

source/blender/blenkernel/intern/customdata.cc

@@ -2414,11 +2414,26 @@ static void ensure_layer_data_is_mutable(CustomDataLayer &layer, const int totel
   }
 }
 
+static bool layer_is_mutable(CustomDataLayer &layer)
+{
+  if (layer.sharing_info == nullptr) {
+    return true;
+  }
+  return layer.sharing_info->is_mutable();
+}
+
 void CustomData_ensure_data_is_mutable(CustomDataLayer *layer, const int totelem)
 {
   ensure_layer_data_is_mutable(*layer, totelem);
 }
 
+void CustomData_ensure_layers_are_mutable(struct CustomData *data, int totelem)
+{
+  for (const int i : IndexRange(data->totlayer)) {
+    ensure_layer_data_is_mutable(data->layers[i], totelem);
+  }
+}
+
 void CustomData_realloc(CustomData *data, const int old_size, const int new_size)
 {
   BLI_assert(new_size >= 0);
@@ -3292,6 +3307,8 @@ void CustomData_copy_data_layer(const CustomData *source,
 {
   const LayerTypeInfo *typeInfo;
 
+  BLI_assert(layer_is_mutable(dest->layers[dst_layer_index]));
+
   const void *src_data = source->layers[src_layer_index].data;
   void *dst_data = dest->layers[dst_layer_index].data;
 
@@ -3407,6 +3424,7 @@ void CustomData_free_elem(CustomData *data, const int index, const int count)
 
     if (typeInfo->free) {
       size_t offset = size_t(index) * typeInfo->size;
+      BLI_assert(layer_is_mutable(data->layers[i]));
 
       typeInfo->free(POINTER_OFFSET(data->layers[i].data, offset), count, typeInfo->size);
     }

source/blender/editors/mesh/mesh_data.cc

@@ -997,6 +997,7 @@ static void mesh_remove_verts(Mesh *mesh, int len)
   if (len == 0) {
     return;
   }
+  CustomData_ensure_layers_are_mutable(&mesh->vert_data, mesh->totvert);
   const int totvert = mesh->totvert - len;
   CustomData_free_elem(&mesh->vert_data, totvert, len);
   mesh->totvert = totvert;
@@ -1007,6 +1008,7 @@ static void mesh_remove_edges(Mesh *mesh, int len)
   if (len == 0) {
     return;
   }
+  CustomData_ensure_layers_are_mutable(&mesh->edge_data, mesh->totedge);
   const int totedge = mesh->totedge - len;
   CustomData_free_elem(&mesh->edge_data, totedge, len);
   mesh->totedge = totedge;
@@ -1017,6 +1019,7 @@ static void mesh_remove_loops(Mesh *mesh, int len)
   if (len == 0) {
     return;
   }
+  CustomData_ensure_layers_are_mutable(&mesh->loop_data, mesh->totloop);
   const int totloop = mesh->totloop - len;
   CustomData_free_elem(&mesh->loop_data, totloop, len);
   mesh->totloop = totloop;
@@ -1027,6 +1030,7 @@ static void mesh_remove_faces(Mesh *mesh, int len)
   if (len == 0) {
     return;
   }
+  CustomData_ensure_layers_are_mutable(&mesh->face_data, mesh->faces_num);
   const int faces_num = mesh->faces_num - len;
   CustomData_free_elem(&mesh->face_data, faces_num, len);
   mesh->faces_num = faces_num;