If a subscriber callback hasn't finished before another message is recieved, is the next call queued?
In my Melodic Python node I am maintaining a list of messages from different sources with information to be used later. Lets say sensor 1 publishes some data that the node has to handle and update it's global list of sensor data. This may take a small time to handle, but there is another message on the same topic that was published very shortly after by sensor 2.
In this circumstance, would the callback finish it's work from when sensor 1 published before being called again to work with sensor 2, or would another thread open up, if available, and do the same work concurrently? Does this change between ROS versions?