Add a pimpl inside rclcpp::Node for future distro backports (ros2#2228)

* Add a pimpl inside rclcpp::Node for future distro backports

Signed-off-by: Emerson Knapp <emerson.b.knapp@gmail.com>
Co-authored-by: Chris Lalancette <clalancette@gmail.com>

rclcpp/include/rclcpp/node.hpp

@@ -1603,6 +1603,12 @@ class Node : public std::enable_shared_from_this<Node>
   const rclcpp::NodeOptions node_options_;
   const std::string sub_namespace_;
   const std::string effective_namespace_;
+
+  class NodeImpl;
+  // This member is meant to be a place to backport features into stable distributions,
+  // and new features targeting Rolling should not use this.
+  // See the comment in node.cpp for more information.
+  std::shared_ptr<NodeImpl> hidden_impl_{nullptr};
 };
 
 }  // namespace rclcpp

rclcpp/src/rclcpp/node.cpp

@@ -111,6 +111,22 @@ create_effective_namespace(const std::string & node_namespace, const std::string
 
 }  // namespace
 
+/// Internal implementation to provide hidden and API/ABI stable changes to the Node.
+/**
+ * This class is intended to be an "escape hatch" within a stable distribution, so that certain
+ * smaller features and bugfixes can be backported, having a place to put new members, while
+ * maintaining the ABI.
+ *
+ * This is not intended to be a parking place for new features, it should be used for backports
+ * only, left empty and unallocated in Rolling.
+ */
+class Node::NodeImpl
+{
+public:
+  NodeImpl() = default;
+  ~NodeImpl() = default;
+};
+
 Node::Node(
   const std::string & node_name,
   const NodeOptions & options)
@@ -260,7 +276,8 @@ Node::Node(
   node_waitables_(other.node_waitables_),
   node_options_(other.node_options_),
   sub_namespace_(extend_sub_namespace(other.get_sub_namespace(), sub_namespace)),
-  effective_namespace_(create_effective_namespace(other.get_namespace(), sub_namespace_))
+  effective_namespace_(create_effective_namespace(other.get_namespace(), sub_namespace_)),
+  hidden_impl_(other.hidden_impl_)
 {
   // Validate new effective namespace.
   int validation_result;