How to remap a topic to sub namespaces

edit

Let's say I have a node, io_node, that provides a service, service, and publishes on a topic, topic. I also have nodes of type client_node and client_node2 (both named client_node) that use service and subscribe to topic. The client_nodes are in two different namespaces, a and b for various reasons.

So rosnode list would show:

/io_node
/a/client_node
/b/client_node

The problem is that I can't get remapping to work with my launch files no matter where I put the remap tags. I want the two client nodes to subscribe to /topic and use /service, but they subscribe to their namespace instead (for example, /a/client_node subscribes /a/topic and tries to use /a/service).

rostopic list shows

/topic
/a/topic
/b/topic

Here's one version of the top level launch file:

<launch>
   <include file=$(find io_node)/launch/io.launch"/>

   <group>
     <remap from="topic" to="a/topic"/>
     <include file=$(find client_node)/launch/client.launch" ns="a"/>
   </group>

   <group>
     <remap from="topic" to="b/topic"/>
     <include file=$(find client_node2)/launch/client.launch" ns="b"/>
   </group>
</launch>

And here's a client.launch (client_node2 is similar):

<launch>
        <node pkg="client_node" name="client_node" type="client_node" output="screen" respawn="true"/>
</launch>

The whole top level launch file is going to be used in yet another launch file, with multiple instances of the three nodes being used in one system only with different namespaces, so I don't think I can use global namespaces. I've tried

• putting the ns tag in a group tag in the client launch files
• putting the ns tag in the launch tag in the client launch files and putting ns in the node tag
• putting the ns tag in the group tag in the top level launch file
• putting the remap tag in the node tag in the client launch file
• putting the remap tag in the group tag in the client launch file
• changing the remap to be from="a/topic" to="topic"

What am I missing here?

EDIT

Guess I should say that I filed an enhancement request. In the meantime, my quick fix was to ditch the namespaces and use the names client_node_a and client_node_b to make them both in the same namespace, and thus both able to use /topic. This isn't really the solution I wanted, though, and isn't really all that flexible...

I tried Boris's suggestion, too, and it works, but I have a bunch of launch files that makes argument passing tedious.