Proper use of ros_control's RealtimeBuffer

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For your first question, the RealtimeBuffer is intended for synchronization of data shared by one non-rt and one rt thread, where the non-rt thread writes into the shared data, and the rt thread reads it. When writing, the non-rt thread is subject to waiting for the shared resource to become free, while the rt thread never waits for reading. Internally, RealtimeBuffer keeps two versions of the shared resource, one which can always be read, and another one which can be safely written into.

For your second question, the implementation of both ForwardCommandController and ForwardJointGroupCommandController is missing proper synchronization between the non-rt thread that executes the subscriber callback and the rt thread that runs the controller update method. Good catch!, I just opened a ticket to track this issue. effort_controllers::JointPositionController, on the other hand, handles concurrency correctly.

Finally, for the third question. The RealtimeBuffer::initRT() is likely assuming that it can only be called in circumstances in which there is no active writer to the shared resource. This is of course undocumented, and I'm judging from the implementation. The fact is, RealtimeBuffer is not well suited for usecases in which the rt-thread writes into the shared resource. For such cases, you could try using a regular lock, possibly around a lightweight object like a pointer. realtime_tools::RealtimeBox is a trivial implementation of this (in fact, there is nothing rt-specific in the implementation).

It has been on my mind for ros_control to leverage better synchronization primitives, but so far the resources have not been there to properly take on the task.

i hope to have answered your questions.