Depsgraph: execute all COPY_ON_WRITE nodes first as a separate stage.

COW nodes in the graph are mostly connected via a relation type
that doesn't propagate the update flags. Unfortunately, due to
the scheduling implementation that means the relations don't
actually guarantee execution order for indirect dependencies.
Relations also don't guarantee order in case of cycles.

As mentioned in IRC, the simplest way to fix possible problems
is to execute all COW nodes as a separate execution stage. This
seems to fix crashes with Data Transfer modifier in a cycle.

Staging works by simply delaying actual scheduling of tasks for
non-COW nodes until the second run of schedule_graph.

Reviewers: sergey

Differential Revision: https://developer.blender.org/D4027

1 files changed, 3 insertions, 0 deletions

diff --git a/source/blender/depsgraph/intern/depsgraph.cc b/source/blender/depsgraph/intern/depsgraph.cc
index 664d30fdaf6..f3f4d788da2 100644
--- a/source/blender/depsgraph/intern/depsgraph.cc
+++ b/source/blender/depsgraph/intern/depsgraph.cc
@@ -405,6 +405,9 @@ DepsRelation *Depsgraph::add_new_relation(OperationDepsNode *from,
 		rel->flag |= flags;
 		return rel;
 	}
+	/* COW nodes can only depend on other COW nodes. */
+	BLI_assert(to->owner->type != DEG_NODE_TYPE_COPY_ON_WRITE ||
+	           from->owner->type == DEG_NODE_TYPE_COPY_ON_WRITE);
 	/* Create new relation, and add it to the graph. */
 	rel = OBJECT_GUARDED_NEW(DepsRelation, from, to, description);
 	rel->flag |= flags;