avoid lifecycle node transition exception

[fujitatomoya]

addresses https://github.com/ros2/rclpy/issues/1209

replaces https://github.com/ros2/rclpy/pull/1317

[fujitatomoya]

@mjforan @Barry-Xu-2018 @chrisbitter if any of you can do the review or test, that will be really appreciated.

[Barry-Xu-2018]

About __change_state(), my understanding is

• Before calling this function, the state machine should be in a primary state (unconfigured, inactive, active or finalized). After calling this function, it should switch to another primary state.
• The argument transition_id for __change_state() should be one of configure, cleanup, activate, deactivate, unconfigured_shutdown, inactive_shutdown and active_shutdown

So, when returning from __change_state(), we need to ensure that the state machine is in a primary state, not a transition state (such as configuring, cleaningup, shuttingdown, etc). If trigger_transition_by_id/trigger_transition_by_label throw exception, it should call on_error and then switch to unconfigured or finalized state.

Is my understanding correct?

Besides, __change_state() may be executed concurrently (the program modifies the state while an external process modifies the state through change_state service). This function does not have lock protection

[mjforan]

Testing the basic lifecycle_py demo shows the change working properly. Before: After: I had to use the Rolling branch of rclpy because lifecycle_py doesn't work under Jazzy.

I agree with @Barry-Xu-2018; the exception should not be handled the same way for all three instances. My opinion:	x	exception caused by	how to handle
1	invalid transition request	write to log and return error to caller
2	unable to transition out of transition state	enter ErrorProcessing
3	unable to transition out of ErrorProcessing	raise an exception and crash