Geometry Nodes: add mutex for node ui storage

Previously, multiple threads adding information to node ui storage
at the same time resulted in memory corruption. The lock prevents
that, but might potentially become a bottleneck in the future.
For now favour correctness over a potential performance bottleneck.

source/blender/blenkernel/BKE_node_ui_storage.hh

@@ -95,8 +95,16 @@ struct AvailableAttributeInfo {
 };
 
 struct NodeUIStorage {
+  std::mutex mutex;
   blender::Vector<NodeWarning> warnings;
   blender::Set<AvailableAttributeInfo> attribute_hints;
+
+  NodeUIStorage() = default;
+  /* Needed because the mutex can't be moved or copied. */
+  NodeUIStorage(NodeUIStorage &&other)
+      : warnings(std::move(other.warnings)), attribute_hints(std::move(other.attribute_hints))
+  {
+  }
 };
 
 struct NodeTreeUIStorage {

source/blender/blenkernel/intern/node_ui_storage.cc

@@ -152,6 +152,7 @@ void BKE_nodetree_error_message_add(bNodeTree &ntree,
   node_error_message_log(ntree, node, message, type);
 
   NodeUIStorage &node_ui_storage = node_ui_storage_ensure(ntree, context, node);
+  std::lock_guard lock{node_ui_storage.mutex};
   node_ui_storage.warnings.append({type, std::move(message)});
 }
 
@@ -163,6 +164,7 @@ void BKE_nodetree_attribute_hint_add(bNodeTree &ntree,
                                      const CustomDataType data_type)
 {
   NodeUIStorage &node_ui_storage = node_ui_storage_ensure(ntree, context, node);
+  std::lock_guard lock{node_ui_storage.mutex};
   node_ui_storage.attribute_hints.add_as(
       AvailableAttributeInfo{attribute_name, domain, data_type});
 }