Automatically transition lifecycle entities when node transitions

[ivanpauno]

Fixes #1846.
Similar to what was done in rclpy: ros2/rclpy#865.
Demo: ros2/demos#552.

[ivanpauno]

The interface in rclpy is a bit different (see here):

• on_activate()/on_deactivate() receives the previous state as a parameter and can fail.
  • I think in most of the cases on_activate()/on_deactivate() shouldn't fail, the on_shutdown()/on_configure() transitions should be doing the fallible work.
  • Receiving the previous state is not important for this transitions, as it's always 'inactive' for on_activate() and 'activated' for on_deactivate().
• The rclpy interface allows you to also provide on_shutdown()/on_configure()/on_error() transitions.
  • LifecyclePublisher is not using it, though entities could lazely create resources on on_configure() and destroy them on on_shutdown() if those methods were available.
  • The default on_configure/on_shutdown() implementation in Node might be less useful, as it's not clear what to do if some of the callbacks fail (see this and this).
    In practice, I think people usually want to manually write on_configure()/on_shutdown().

We could either modify this PR to do exactly what rclpy does or reduce rclpy flexibility to match more closely what this PR is adding.
I prefer a bit this less flexible version, which is what we actually seem to need.

[jacobperron]

I think in most of the cases on_activate()/on_deactivate() shouldn't fail

Having less failure points sounds nice to me. I haven't thought through scenarios where someone may want to fail during activation/deactivation, but we could add it if the need arises.

The default on_configure/on_shutdown() implementation in Node might be less useful, as it's not clear what to do if some of the callbacks fail

If one or more of the callbacks fail, I would think we would want to transition all entities to the unconfigued state (as indicated by the design doc diagram). What do you think?

I agree that in most cases people will probably provide their own on_configure/on_shutdown logic. I think that it still might be nice if we call the transitions on all managed entities though. E.g. call any user-defined callback for the node and then, if that is successful, call the transition callbacks for managed entities. Or do you see something wrong with that behavior?

[ivanpauno]

If one or more of the callbacks fail, I would think we would want to transition all entities to the unconfigued state (as indicated by the design doc diagram). What do you think?

I think we could do that.
The logic would be:

• configure
  • Try to transition all managed entities to configured
  • If transition fails (TRANSITION_FAILURE), shut down all the managed entities that already transitioned to go back to the unconfigured state.
  • If any callback errors (TRANSITION_ERROR), then we immediately return error without doing any more transitions.
    This applies when a on_configure callback errors, or when an on_shutdown callback errors after a previous failure.
• shutdown
  • Similar, but on failures we do nothing, and continue trying to move the rest of the entities to the shutdown state.

The only problem I see is that the logic is a bit complex and might not be actually useful in practice (we don't have any lifecycle entity that do work on configure/shutdown currently).

I agree that in most cases people will probably provide their own on_configure/on_shutdown logic. I think that it still might be nice if we call the transitions on all managed entities though. E.g. call any user-defined callback for the node and then, if that is successful, call the transition callbacks for managed entities. Or do you see something wrong with that behavior?

I think this has been resolved in the discussion here.

[jacobperron]

Fair enough. I'm fine keeping the implementation simple for now; we can add complexity if the need arises.

[jacobperron]

I think in most of the cases on_activate()/on_deactivate() shouldn't fail

Having less failure points sounds nice to me. I haven't thought through scenarios where someone may want to fail during activation/deactivation, but we could add it if the need arises.

The default on_configure/on_shutdown() implementation in Node might be less useful, as it's not clear what to do if some of the callbacks fail

If one or more of the callbacks fail, I would think we would want to transition all entities to the unconfigued state (as indicated by the design doc diagram). What do you think?

I agree that in most cases people will probably provide their own on_configure/on_shutdown logic. I think that it still might be nice if we call the transitions on all managed entities though. E.g. call any user-defined callback for the node and then, if that is successful, call the transition callbacks for managed entities. Or do you see something wrong with that behavior?

[ivanpauno]

CI:

• Linux
• Linux-aarch64
• Windows

[ivanpauno]

• Windows