Print function for new depsgraph nodes for debug.

The NODE_DEPSGRAPH_UPDATED flag indicates that a node has triggered a
depsgraph update. This can be accessed in a depsgraph "pre" handler
to find nodes which have changed through user actions.
The depsgraph clears this flag for the next update.

source/blender/blenkernel/intern/node.cc

@@ -4500,6 +4500,7 @@ void nodeUpdate(bNodeTree *ntree, bNode *node)
   }
   ntree->is_updating = true;
 
+  node->flag |= NODE_DEPSGRAPH_UPDATED;
   if (node->typeinfo->updatefunc) {
     node->typeinfo->updatefunc(ntree, node);
   }

source/blender/depsgraph/intern/builder/deg_builder_nodes.cc

@@ -1711,6 +1711,14 @@ void DepsgraphNodeBuilder::build_nodetree(bNodeTree *ntree)
   /* General parameters. */
   build_parameters(&ntree->id);
   build_idproperties(ntree->id.properties);
+  add_operation_node(&ntree->id,
+                     NodeType::PARAMETERS,
+                     OperationCode::NODETREE_UPDATE,
+                     [ntree](::Depsgraph *depsgraph) {
+                       LISTBASE_FOREACH (bNode *, node, &ntree->nodes) {
+                         node->flag &= ~NODE_DEPSGRAPH_UPDATED;
+                       }
+                     });
   /* Animation, */
   build_animdata(&ntree->id);
   /* Shading update. */

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

@@ -204,6 +204,9 @@ const char *operationCodeAsString(OperationCode opcode)
       return "DUPLI";
     case OperationCode::SIMULATION_EVAL:
       return "SIMULATION_EVAL";
+    /* Node tree*/
+    case OperationCode::NODETREE_UPDATE:
+      return "NODETREE_UPDATE";
   }
   BLI_assert_msg(0, "Unhandled operation code, should never happen.");
   return "UNKNOWN";

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

@@ -206,6 +206,9 @@ enum class OperationCode {
 
   /* Simulation. ---------------------------------------------------------- */
   SIMULATION_EVAL,
+
+  /* Nodes. --------------------------------------------------------------- */
+  NODETREE_UPDATE,
 };
 const char *operationCodeAsString(OperationCode opcode);
 

source/blender/editors/space_node/node_draw.cc

@@ -177,6 +177,8 @@ void ED_node_tag_update_nodetree(Main *bmain, bNodeTree *ntree, bNode *node)
     if (!node_connected_to_output(bmain, ntree, node)) {
       do_tag_update = false;
     }
+
+    node->flag |= NODE_DEPSGRAPH_UPDATED;
   }
 
   /* Look through all datablocks to support groups. */

source/blender/makesdna/DNA_node_types.h

@@ -361,6 +361,9 @@ typedef struct bNode {
 /* A preview for the data in this node can be displayed in the spreadsheet editor. */
 #define __NODE_ACTIVE_PREVIEW (1 << 18) /* deprecated */
 
+/* Read-only flag to indicate that changes to the node have triggered a depsgraph update. */
+#define NODE_DEPSGRAPH_UPDATED (1 << 19)
+
 /* node->update */
 /* XXX NODE_UPDATE is a generic update flag. More fine-grained updates
  * might be used in the future, but currently all work the same way.

source/blender/makesrna/intern/rna_nodetree.c

@@ -11483,6 +11483,12 @@ static void rna_def_node(BlenderRNA *brna)
   RNA_def_property_ui_text(prop, "Show Texture", "Display node in viewport textured shading mode");
   RNA_def_property_update(prop, 0, "rna_Node_update");
 
+  prop = RNA_def_property(srna, "depsgraph_updated", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", NODE_DEPSGRAPH_UPDATED);
+  RNA_def_property_clear_flag(prop, PROP_EDITABLE | PROP_ANIMATABLE);
+  RNA_def_property_flag(prop, PROP_HIDDEN | PROP_SKIP_SAVE);
+  RNA_def_property_ui_text(prop, "Depsgraph Updated", "Changes to the node have triggered a depsgraph update");
+
   /* generic property update function */
   func = RNA_def_function(srna, "socket_value_update", "rna_Node_socket_value_update");
   RNA_def_function_ui_description(func, "Update after property changes");