source/blender/blenkernel/BKE_customdata.h

@@ -430,6 +430,12 @@ void CustomData_swap_corners(struct CustomData *data, int index, const int *corn
  */
 void CustomData_swap(struct CustomData *data, int index_a, int index_b, const int totelem);
 
+/**
+ * Custom data layers can be shared through implicit sharing (`BLI_implicit_sharing.h`). This
+ * 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);
+
 /**
  * Retrieve a pointer to an element of the active layer of the given \a type, chosen by the
  * \a index, if it exists.

source/blender/blenkernel/intern/customdata.cc

@@ -2422,6 +2422,11 @@ static void ensure_layer_data_is_mutable(CustomDataLayer &layer, const int totel
   }
 }
 
+void CustomData_ensure_data_is_mutable(CustomDataLayer *layer, const int totelem)
+{
+  ensure_layer_data_is_mutable(*layer, totelem);
+}
+
 void CustomData_realloc(CustomData *data, const int old_size, const int new_size)
 {
   BLI_assert(new_size >= 0);

source/blender/makesrna/intern/rna_attribute.c

@@ -305,6 +305,7 @@ static void rna_Attribute_data_begin(CollectionPropertyIterator *iter, PointerRN
       break;
   }
 
+  CustomData_ensure_data_is_mutable(layer, length);
   rna_iterator_array_begin(iter, layer->data, struct_size, length, 0, NULL);
 }