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, 5 insertions, 6 deletions

diff --git a/source/blender/blenkernel/intern/paint.c b/source/blender/blenkernel/intern/paint.c
index 42749a93988..e46c91e4bf4 100644
--- a/source/blender/blenkernel/intern/paint.c
+++ b/source/blender/blenkernel/intern/paint.c
@@ -1734,17 +1734,16 @@ void BKE_sculpt_color_layer_create_if_needed(struct Object *object)
   DEG_id_tag_update(&orig_me->id, ID_RECALC_GEOMETRY);
 }
 
+/** \warning Expects a fully evaluated depsgraph. */
 void BKE_sculpt_update_object_for_edit(
     Depsgraph *depsgraph, Object *ob_orig, bool need_pmap, bool need_mask, bool need_colors)
 {
-  /* Update from sculpt operators and undo, to update sculpt session
-   * and PBVH after edits. */
-  Scene *scene_eval = DEG_get_evaluated_scene(depsgraph);
-  Object *ob_eval = DEG_get_evaluated_object(depsgraph, ob_orig);
-  Mesh *me_eval = mesh_get_eval_final(depsgraph, scene_eval, ob_eval, &CD_MASK_BAREMESH);
-
   BLI_assert(ob_orig == DEG_get_original_object(ob_orig));
 
+  Object *ob_eval = DEG_get_evaluated_object(depsgraph, ob_orig);
+  Mesh *me_eval = BKE_object_get_evaluated_mesh(ob_eval);
+  BLI_assert(me_eval != NULL);
+
   sculpt_update_object(depsgraph, ob_orig, me_eval, need_pmap, need_mask, need_colors);
 }