resetting subscriber leads to crash

[Karsten1987]

given the following code: https://gist.github.com/Karsten1987/82837b87fd11ef23353754b9fa31cf7a

the situation:

1.) node spawns a subscriber and a publisher
2.1) main thread spins the executor, having sub and pub talking
2.2) second thread randomly stops the subscription (resetting the shared_ptr of it)
3.) program crashes with the backtrace at the bottom of the gist.

quickly discussed the problem with @wjwwood, when resetting the subscription, the executor still has a raw handle on it which leads to race conditions. The problem can be addressed by turning the handle into a shared pointer.

concretely speaking, this function should return a shared_ptr instead of a raw pointer.

[dhood]

this is the permalink to the function @Karsten1987 is referencing in his post (in case those lines move around):

rclcpp/rclcpp/include/rclcpp/subscription.hpp

Lines 81 to 83 in b906768

	RCLCPP_PUBLIC
	const rcl_subscription_t *
	get_subscription_handle() const;

[deng02]

We have been hitting this issue when using the ROS2-to-ROS1 ros1_bridge with bridge_all_2to1_topics set to false. When the ROS1 subscriber for a topic disappers the ROS2 subscriber is destroyed and sometimes this happens while the wait set is still holding a raw pointer to a member of that object. The result is various segfault errors within rmw_wait.

[wjwwood]

I believe #431 closes this, please comment here if this is still an issue.