Fix T98237: Double free with curve object conversion to mesh

In some cases (when there is an evaluated curve), the conversion code
would try to free the evaluated data-block twice, because freeing the
object would free it from `data_eval` and then the data-block was freed
again explicitly. Now check if the data-block is stored in `data_eval`
before freeing `object.data` manually. This is another area that's made
more complex by the fact that we change the meaning of `object.data`
for evaluated objects. The solution is more complicated than it should
be, but it works whether or not an evaluated mesh or curve exists.

1 files changed, 8 insertions, 1 deletions

diff --git a/source/blender/blenkernel/intern/mesh_convert.cc b/source/blender/blenkernel/intern/mesh_convert.cc
index 0038a2aa806..0d34c8ad6b1 100644
--- a/source/blender/blenkernel/intern/mesh_convert.cc
+++ b/source/blender/blenkernel/intern/mesh_convert.cc
@@ -889,8 +889,15 @@ static void object_for_curve_to_mesh_free(Object *temp_object)
     curve.editnurb = nullptr;
   }
 
-  BKE_id_free(nullptr, temp_object->data);
+  /* Only free the final object data if it is *not* stored in the #data_eval field. This is still
+   * necessary because #temp_object's data could be replaced by a #Curve data-block that isn't also
+   * assigned to #data_eval. */
+  const bool object_data_stored_in_data_eval = final_object_data == temp_object->runtime.data_eval;
+
   BKE_id_free(nullptr, temp_object);
+  if (!object_data_stored_in_data_eval) {
+    BKE_id_free(nullptr, final_object_data);
+  }
 }
 
 /**