LibOverrides: Refactor how diffing of RNA collections is handled.

Original implementation was a quick prototype which should have never
landed as-is in master. It had very limiting constraints and did not
allow for any real further development.

This commit fixes the internal implementation to make more sensible,
maintainable and evolutive.

NOTE: This commit introduces another forward-incompatibility in the
Blender file format: Files saved after this commit won't open properly
in older versions of blender regarding local inserted constraints or
modifiers into overrides of linked data.

NOTE: Technical details: The 'anchor' item name/index is now stored in
`subitem_reference_` members, and the actual 'source' item name/index is
stored in `subitem_local_` members of the override property operation
data.
Previously, only the `subitem_local_` members were used, storing the
anchor item name/index, and assuming the 'source' item was always the
next in the list.

Milestone I of T82160.

Maniphest Tasks: T82160

Differential Revision: https://developer.blender.org/D13282

1 files changed, 6 insertions, 4 deletions

diff --git a/source/blender/makesrna/intern/rna_animation.c b/source/blender/makesrna/intern/rna_animation.c
index 52c25bae45a..9068fdb6e72 100644
--- a/source/blender/makesrna/intern/rna_animation.c
+++ b/source/blender/makesrna/intern/rna_animation.c
@@ -761,8 +761,8 @@ bool rna_NLA_tracks_override_apply(Main *bmain,
   /* This is not working so well with index-based insertion, especially in case some tracks get
    * added to lib linked data. So we simply add locale tracks at the end of the list always, order
    * of override operations should ensure order of local tracks is preserved properly. */
-  if (opop->subitem_local_index >= 0) {
-    nla_track_anchor = BLI_findlink(&anim_data_dst->nla_tracks, opop->subitem_local_index);
+  if (opop->subitem_reference_index >= 0) {
+    nla_track_anchor = BLI_findlink(&anim_data_dst->nla_tracks, opop->subitem_reference_index);
   }
   /* Otherwise we just insert in first position. */
 #  else
@@ -773,9 +773,11 @@ bool rna_NLA_tracks_override_apply(Main *bmain,
   if (opop->subitem_local_index >= 0) {
     nla_track_src = BLI_findlink(&anim_data_src->nla_tracks, opop->subitem_local_index);
   }
-  nla_track_src = nla_track_src ? nla_track_src->next : anim_data_src->nla_tracks.first;
 
-  BLI_assert(nla_track_src != NULL);
+  if (nla_track_src == NULL) {
+    BLI_assert(nla_track_src != NULL);
+    return false;
+  }
 
   NlaTrack *nla_track_dst = BKE_nlatrack_copy(bmain, nla_track_src, true, 0);