Hi, we've found the implementation of demux is quite slow when switching topics. Or at least the topic connections that need to be re-created during every switch are slow. The result is the switching happens quickly, but any subscribers start receiving messages a few seconds later (up to 15 seconds in my measurement).

Steps to reproduce:

rosrun topic_tools demux a b c

rostopic echo b

rostopic echo c

rostopic pub  -r 10 /a stamped_msgs/Int32 "header:
  seq: 0
  stamp:
    secs: 0
    nsecs: 0
  frame_id: ''
data: 0"    

rosservice call /demux/select b # (or c)

Try calling the select service a few times, and you'll see the long delay between a topic gets selected and the rostopics start printing the new values.

What about giving demux a parameter that would tell it to keep all the publishers running all the time? Or even throwing away the old behavior and having the new one for all? I don't see any drawbacks, since only one of the publishers is doing some real work at a time...