Fix T81854: crash when undoing switch between sculpt and edit mode.

The logic of `BKE_sculpt_update_object_for_edit` was not correct. such
low-level functions should typically never preform depsgraph evaluation
themselves, they should be able to rely on getting a fully evaluated
depsgraph and just get needed data from there.

Supporting that required fixing other broken code higher in the
callstack, namely:
* `ED_object_sculptmode_enter_ex` was freeing evaluated data, for no
  valid reason it would seem.
* `sculpt_undosys_step_decode` was ensuring an evaluated depsgraph
  **before** calling `ED_object_mode_generic_exit`, which would
  invalidate a lot of evaluated data.

Note that it is fairly difficult to track down all code paths leading to
`BKE_sculpt_update_object_for_edit`, so there may be still cases where
this gets called with improperly evaluated depsgraph.

Reviewed By: sergey

Maniphest Tasks: T81854

Differential Revision: https://developer.blender.org/D9270

1 files changed, 2 insertions, 4 deletions

diff --git a/source/blender/editors/sculpt_paint/sculpt.c b/source/blender/editors/sculpt_paint/sculpt.c
index bd381a7b60c..fafa87e910c 100644
--- a/source/blender/editors/sculpt_paint/sculpt.c
+++ b/source/blender/editors/sculpt_paint/sculpt.c
@@ -8133,6 +8133,7 @@ static void SCULPT_OT_symmetrize(wmOperatorType *ot)
 
 /**** Toggle operator for turning sculpt mode on or off ****/
 
+/** \warning Expects a fully evaluated depsgraph. */
 static void sculpt_init_session(Depsgraph *depsgraph, Scene *scene, Object *ob)
 {
   /* Create persistent sculpt mode data. */
@@ -8197,6 +8198,7 @@ void ED_object_sculptmode_enter_ex(Main *bmain,
 
   if (flush_recalc) {
     DEG_id_tag_update(&ob->id, ID_RECALC_GEOMETRY);
+    BKE_scene_graph_evaluated_ensure(depsgraph, bmain);
   }
 
   /* Create sculpt mode session data. */
@@ -8204,10 +8206,6 @@ void ED_object_sculptmode_enter_ex(Main *bmain,
     BKE_sculptsession_free(ob);
   }
 
-  /* Make sure derived final from original object does not reference possibly
-   * freed memory. */
-  BKE_object_free_derived_caches(ob);
-
   /* Copy the current mesh visibility to the Face Sets. */
   BKE_sculpt_face_sets_ensure_from_base_mesh_visibility(me);