Make transition event topic reliable to avoid topic lost

[kjjpc]

This PR is related to #1166.
Lifecycle action of launch_ros occasionally fails to transit because of topic event lost.
This PR make transition event topic reliable.
After the marge of this PR, I will make a PR for launch_ros.

[fujitatomoya]

rmw_qos_profile_default already sets this RMW_QOS_POLICY_RELIABILITY_RELIABLE, so no need to overwrite this.

[kjjpc]

Thanks, I've removed the line.

[fujitatomoya]

Suggested change

	publisher_options.qos.durability = RMW_QOS_POLICY_DURABILITY_TRANSIENT_LOCAL;
	publisher_options.qos.depth = 1;
	// transition event topic needs to be latched for the subscription joins later.
	publisher_options.qos.durability = RMW_QOS_POLICY_DURABILITY_TRANSIENT_LOCAL;
	publisher_options.qos.depth = 1;

changing publisher's QoS to transient local should not break any downstream subscriptions, https://docs.ros.org/en/rolling/Concepts/Intermediate/About-Quality-of-Service-Settings.html#id6.

IMO this transition event topic needs to be latched to deliver the latest state for the subscribers that join later. probably adding comments why we use transient local with depth 1 would be useful.

besides, Managed Node Design also says the following.

A topic should be provided to broadcast the new life cycle state when it changes. This topic must be latched.

[mjcarroll]

@kjjpc Thanks for reporting. We discussed this at the recent PMC maintainers' meeting and came to the following conclusion.

In this case, changing the durability of the transition event could have far reaching impacts on the system that are undesirable. The main concern is that if you had a late-joining lifecycle node, that it could be transitioned inadvertently by the latched topic. This would be pretty un-intuitive and could cause issues in larger systems.

The real issue here stems from the fact that we are triggering the transition only when the process becomes available, rather than waiting for the node to be in the unconfigured state.

I think that the real solution here would be detect when the node is started in the unconfigured state in your launch file, like:

    emit_active = RegisterEventHandler(
        launch_ros.event_handlers.OnStateTransition(
            target_lifecycle_node=lc_node,
            goal_state='unconfigured',
            ...
        ))

The other alternative would be to add on OnNodeReady event to launch_ros so that we are more intelligently waiting for the node to start.

[fujitatomoya]

@mjcarroll

i might be mistaken, but i was thinking opposite...

The main concern is that if you had a late-joining lifecycle node, that it could be transitioned inadvertently by the latched topic. This would be pretty un-intuitive and could cause issues in larger systems.

either that is lifecycle node or just a subscription, delivering the latest state change event via latched topic to the subscription that does specify the QoS with durability by the user application makes sense?
this is something user application wants to do that, so is specified by user.
if they do not want to receive the latest state change event via latched topic, they just do not need to specify the QoS durability on the subscription side? without this change, system cannot provide the choice for the user application.
besides, this does not break the communication between them, https://docs.ros.org/en/rolling/Concepts/Intermediate/About-Quality-of-Service-Settings.html#qos-compatibilities

[mjcarroll]

either that is lifecycle node or just a subscription, delivering the latest state change event via latched topic to the subscription that does specify the QoS with durability by the user application makes sense?

Hmm, maybe my understanding of the issue here was wrong. I thought we were making the subscription transient_local, which would cause the issues I described.

[kjjpc]

@mjcarroll
Thanks for the discussion. The issue is that the transition event topic is volatile in the current implementation.
launch_ros occasionally fails to receive the change event topic because the topic is not latched.