MultiThreadedExecutor does not process callbacks asynchronously

[jacobperron]

Each callback is guarded by mutex, so even though they are happening in different threads, they are processed in series:

ros2_java/rcljava/src/main/java/org/ros2/rcljava/executors/MultiThreadedExecutor.java

Lines 85 to 87 in ae5edaa

	synchronized (mutex) {
	this.spinOnce();
	}

I proposed a patch to actually execute callbacks in parallel by default (osrf#86), but @ivanpauno pointed out that it would be more consistent with other client libraries to introduce the concept of callback groups. I agree with this as it would not only be consistent, but make it more flexible to configure the executor.

[scpeters]

Is there a ticket yet about supporting callback groups?

[jacobperron]

Is there a ticket yet about supporting callback groups?

That's a negative.