Inconsistency caused by combined_robot_hw interface filtering

[RobertWilbrandt]

Over in this ros_control_boilerplate PR we found an inconsistency in the handling of a RobotHWs prepareSwitch() and doSwitch() function between normal operation (i.e. just using a controller_manager with a single RobotHW) and usage inside a combined_robot_hw.

A scenario where the inconsistency is visible is the loading of a controller that does not claim any resources (e.g. a joint_state_controller):

• If a controller_manager directly handles a RobotHW, it will call the RobotHWs prepareSwitch and doSwitch functions with a start_list containing the controller info about the controller.
• If a controller_manager handles multiple RobotHWs through a combined_robot_hw, the combined_robot_hw will filter the start_list and stop_list based on the claimed resources. As no hardware interface has any resources that are claimed by the controller (obviously), the same functions get called with empty start_list and stop_list.

I can't think of any use case where this would be a real problem - and changing the behavior of either combined_robot_hw or controller_manager would surely break some working setups. That's why i think adding some documentation for this special quirk of combined_robot_hw would be best.

I'd be happy to provide a PR with this documentation, but hope to first get some feedback on this - Am i just overlooking something really obvious / is there any obvious change that would remove this inconsistency?

---

For completeness, here are some examples from the linked PR showing the described behavior:

1. Loading a hardware interface directly in a controller_manager:

[...]
Loaded 'joint_state_controller'
Loaded 'position_trajectory_controller'
[ INFO] [1608059009.336061045]: prepareSwitch (nh: /rrbot):
[ INFO] [1608059009.336108133]:   + joint_state_controller
[ INFO] [1608059009.339490340]: doSwitch (nh: /rrbot):
[ INFO] [1608059009.339531124]:   + joint_state_controller
Started ['joint_state_controller'] successfully
[ INFO] [1608059009.343537246]: prepareSwitch (nh: /rrbot):
[ INFO] [1608059009.343579910]:   + position_trajectory_controller
[ INFO] [1608059009.346152074]: doSwitch (nh: /rrbot):
[ INFO] [1608059009.346195179]:   + position_trajectory_controller
Started ['position_trajectory_controller'] successfully
[...]

2. Two hardware interfaces are managed through a combined_robot_hw:

[...]
Loaded 'joint_state_controller'
Loaded 'position_trajectory_controller'
[ INFO] [1608063130.022795073]: prepareSwitch (nh: /rrbot/rrbot_combined_hardware_interface/joint_1_hw):
[ INFO] [1608063130.022841212]: prepareSwitch (nh: /rrbot/rrbot_combined_hardware_interface/joint_2_hw):
[ INFO] [1608063130.026187157]: doSwitch (nh: /rrbot/rrbot_combined_hardware_interface/joint_1_hw):
[ INFO] [1608063130.026236738]: doSwitch (nh: /rrbot/rrbot_combined_hardware_interface/joint_2_hw):
Started ['joint_state_controller'] successfully
[ INFO] [1608063130.029357874]: prepareSwitch (nh: /rrbot/rrbot_combined_hardware_interface/joint_1_hw):
[ INFO] [1608063130.029399325]:   + position_trajectory_controller
[ INFO] [1608063130.029440723]: prepareSwitch (nh: /rrbot/rrbot_combined_hardware_interface/joint_2_hw):
[ INFO] [1608063130.029463272]:   + position_trajectory_controller
[ INFO] [1608063130.029528641]: doSwitch (nh: /rrbot/rrbot_combined_hardware_interface/joint_1_hw):
[ INFO] [1608063130.029575905]:   + position_trajectory_controller
[ INFO] [1608063130.029625720]: doSwitch (nh: /rrbot/rrbot_combined_hardware_interface/joint_2_hw):
[ INFO] [1608063130.029667801]:   + position_trajectory_controller
Started ['position_trajectory_controller'] successfully
[...]