Document issue with shutdown

[Yadunund]

Address #170.

[clalancette]

I think we need to be careful with this advice.

In particular, if you are writing composable nodes, you basically never want to call rclcpp::shutdown. That's because rclcpp::shutdown is a global operation, and will shut down your node and all other nodes that happen to be composed with your node at the same time. That's probably not what is intended.

The composable container should call rclcpp::shutdown when it is going down, but that is a different thing.

Overall, I'm not saying that you are wrong, but there is subtle distinction here.

[Yadunund]

Wouldn't composable nodes be created as shared libraries without a main function and hence lack the opportunity to invoke rclcpp::init() and rclcpp::shutdown()? Only executables that implement compile-time composition would include these statements and the advise above should be valid in this scenario?

The default main impl for composable nodes includes an rclcpp::shutdown() https://github.com/ros2/rclcpp/blob/a78d0cbd33b8fe0b4db25c04f7e10017bfca6061/rclcpp_components/src/node_main.cpp.in#L78

[Yadunund]

Happy to massage the wording in any other way!

[clalancette]

Wouldn't composable nodes be created as shared libraries without a main function and hence lack the opportunity to invoke rclcpp::init() and rclcpp::shutdown()?

I mean, they shouldn't call it, but that doesn't mean they won't. rclcpp::shutdown, in particular, is often (erroneously) called when a node is done with its work; even our demos are guilty of this: https://github.com/ros2/demos/blob/0f2afe53be38b71c01d43f0900e18187d2e36082/demo_nodes_cpp/src/services/add_two_ints_client_async.cpp#L65

I'm going to suggest some wording to point out that composable nodes shouldn't generally do this.