Fix T81218: Crash in pose mode using a driver on bendy bone Segment

The example file in T81218 has a driver that maps a bone's X-location to
the number of BBone segments. This caused a dependency cycle, which
resulted in bad thread serialisation, which caused the crash.

This patch breaks the dependency cycle `BONE_LOCAL` →
`DRIVER(bones["Bone"].bbone_segments)` → `BONE_LOCAL`. The 'Driver Data'
relation now points to `BONE_SEGMENTS` when the driven property starts
with `bbone_`.

Differential Revision: https://developer.blender.org/D9122

1 files changed, 6 insertions, 2 deletions

diff --git a/source/blender/depsgraph/intern/builder/deg_builder_relations.cc b/source/blender/depsgraph/intern/builder/deg_builder_relations.cc
index 4fb893b350c..182404a848b 100644
--- a/source/blender/depsgraph/intern/builder/deg_builder_relations.cc
+++ b/source/blender/depsgraph/intern/builder/deg_builder_relations.cc
@@ -1453,6 +1453,9 @@ void DepsgraphRelationBuilder::build_driver_data(ID *id, FCurve *fcu)
      * which will affect the evaluation of corresponding pose bones. */
     Bone *bone = (Bone *)property_entry_key.ptr.data;
     if (bone != nullptr) {
+      const char *prop_identifier = RNA_property_identifier(property_entry_key.prop);
+      const bool driver_targets_bbone = STRPREFIX(prop_identifier, "bbone_");
+
       /* Find objects which use this, and make their eval callbacks
        * depend on this. */
       for (IDNode *to_node : graph_->id_nodes) {
@@ -1462,8 +1465,9 @@ void DepsgraphRelationBuilder::build_driver_data(ID *id, FCurve *fcu)
           if (object->data == id_ptr && object->pose != nullptr) {
             bPoseChannel *pchan = BKE_pose_channel_find_name(object->pose, bone->name);
             if (pchan != nullptr) {
-              OperationKey bone_key(
-                  &object->id, NodeType::BONE, pchan->name, OperationCode::BONE_LOCAL);
+              OperationCode target_op = driver_targets_bbone ? OperationCode::BONE_SEGMENTS :
+                                                               OperationCode::BONE_LOCAL;
+              OperationKey bone_key(&object->id, NodeType::BONE, pchan->name, target_op);
               add_relation(driver_key, bone_key, "Arm Bone -> Driver -> Bone");
             }
           }