added lock to prevent simultaneous access of on_shutdown_callbacks_ w…

…hich led to a race condition (seg fault) when creating multiple executors (multithreaded)

Signed-off-by: Joshua Hampp <[email protected]>

rclcpp/src/rclcpp/context.cpp

@@ -344,6 +344,7 @@ Context::shutdown(const std::string & reason)
 rclcpp::Context::OnShutdownCallback
 Context::on_shutdown(OnShutdownCallback callback)
 {
+  std::lock_guard<std::mutex> lock(on_shutdown_callbacks_mutex_);
   on_shutdown_callbacks_.push_back(callback);
   return callback;
 }