Add a callback to `IDTypeInfo` to allow preservation of some data accross memfile undos

This is essentially adding that new callback, and using it only for already
existing Scene's 3DCursor.

Note that the place where this is called has been moved again, after all
have been lib-linked, such that those callbacks may also work on ID pointers.

Maniphest Tasks: T71759

Differential Revision: https://developer.blender.org/D9237

1 files changed, 17 insertions, 9 deletions

diff --git a/source/blender/blenloader/intern/readfile.c b/source/blender/blenloader/intern/readfile.c
index ae640891eab..c2a18033b44 100644
--- a/source/blender/blenloader/intern/readfile.c
+++ b/source/blender/blenloader/intern/readfile.c
@@ -5672,15 +5672,6 @@ static void read_libblock_undo_restore_at_old_address(FileData *fd, Main *main,
 
   const short idcode = GS(id->name);
 
-  /* XXX 3DCursor (witch is UI data and as such should not be affected by undo) is stored in
-   * Scene... So this requires some special handling, previously done in `blo_lib_link_restore()`,
-   * but this cannot work anymore when we overwrite existing memory... */
-  if (idcode == ID_SCE) {
-    Scene *scene_old = (Scene *)id_old;
-    Scene *scene = (Scene *)id;
-    SWAP(View3DCursor, scene_old->cursor, scene->cursor);
-  }
-
   Main *old_bmain = fd->old_mainlist->first;
   ListBase *old_lb = which_libbase(old_bmain, idcode);
   ListBase *new_lb = which_libbase(main, idcode);
@@ -5692,6 +5683,11 @@ static void read_libblock_undo_restore_at_old_address(FileData *fd, Main *main,
    * process). So we can pass NULL for the Main pointer parameter. */
   BKE_lib_id_swap_full(NULL, id, id_old);
 
+  /* Special temporary usage of this pointer, necessary for the `undo_preserve` call after
+   * lib-linking to restore some data that should never be affected by undo, e.g. the 3D cursor of
+   * #Scene. */
+  id_old->orig_id = id;
+
   BLI_addtail(new_lb, id_old);
   BLI_addtail(old_lb, id);
 }
@@ -6123,6 +6119,18 @@ static void lib_link_all(FileData *fd, Main *bmain)
     }
 
     id->tag &= ~LIB_TAG_NEED_LINK;
+
+    /* Some data that should be persistent, like the 3DCursor or the tool settings, are
+     * stored in IDs affected by undo, like Scene. So this requires some specific handling. */
+    if (id_type->blend_read_undo_preserve != NULL && id->orig_id != NULL) {
+      id_type->blend_read_undo_preserve(&reader, id, id->orig_id);
+    }
+  }
+  FOREACH_MAIN_ID_END;
+
+  /* Cleanup `ID.orig_id`, this is now reserved for depsgraph/COW usage only. */
+  FOREACH_MAIN_ID_BEGIN (bmain, id) {
+    id->orig_id = NULL;
   }
   FOREACH_MAIN_ID_END;