Single threaded executor used to process reentrant callback group as well as multiple callback groups

[sayali-purdue]

In file attacher_action.py, a) the 'GraspingActionServer' node is setup to use reentrant callback group (at line 140), and b) one more node, e.g. 'Detach', is added to the global executor in addition to the 'GraspingActionServer' node; that means the single threaded global executor is responsible to process callbacks of the callback groups of two nodes.

As per the ROS documentation, callbacks of reentrant callback group can execute in parallel, and callbacks from different callback groups can execute in parallel. However, here, callbacks cannot execute in parallel due to the usage of single threaded executor. A multi threaded executor would be more appropriate for this use case.

Would appreciate if you could share your thoughts/reasoning on this.