Depsgraph: clear update flags when skipping through no-op nodes.

As an optimization, dependency graph evaluation skips through
no-op nodes at the scheduling stage. However, that leaves update
flags enabled on the node, and the most problematic one is the
USER_MODIFIED flag, which get flushed to children every time
the no-op node is tagged.

This in turn can cause simulation caches downstream to be
permanently stuck in an outdated state until the depsgraph
is rebuilt and the stuck flag cleared out.

Differential Revision: https://developer.blender.org/D16868

source/blender/depsgraph/intern/eval/deg_eval.cc

@@ -105,8 +105,7 @@ void evaluate_node(const DepsgraphEvalState *state, OperationNode *operation_nod
    * times.
    * This is a thread-safe modification as the node's flags are only read for a non-scheduled nodes
    * and this node has been scheduled. */
-  operation_node->flag &= ~(DEPSOP_FLAG_DIRECTLY_MODIFIED | DEPSOP_FLAG_NEEDS_UPDATE |
-                            DEPSOP_FLAG_USER_MODIFIED);
+  operation_node->flag &= ~DEPSOP_FLAG_CLEAR_ON_EVAL;
 }
 
 void deg_task_run_func(TaskPool *pool, void *taskdata)
@@ -270,6 +269,10 @@ void schedule_node(DepsgraphEvalState *state,
   bool is_scheduled = atomic_fetch_and_or_uint8((uint8_t *)&node->scheduled, uint8_t(true));
   if (!is_scheduled) {
     if (node->is_noop()) {
+      /* Clear flags to avoid affecting subsequent update propagation.
+       * For normal nodes these are cleared when it is evaluated. */
+      node->flag &= ~DEPSOP_FLAG_CLEAR_ON_EVAL;
+
       /* skip NOOP node, schedule children right away */
       schedule_children(state, node, schedule_fn);
     }

source/blender/depsgraph/intern/node/deg_node_operation.h

@@ -224,6 +224,10 @@ enum OperationFlag {
 
   /* Set of flags which gets flushed along the relations. */
   DEPSOP_FLAG_FLUSH = (DEPSOP_FLAG_USER_MODIFIED),
+
+  /* Set of flags which get cleared upon evaluation. */
+  DEPSOP_FLAG_CLEAR_ON_EVAL = (DEPSOP_FLAG_DIRECTLY_MODIFIED | DEPSOP_FLAG_NEEDS_UPDATE |
+                               DEPSOP_FLAG_USER_MODIFIED),
 };
 
 /* Atomic Operation - Base type for all operations */