Roslaunch blocking message publication

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I have two simple Python nodes, a publisher outputting data at /mypub/data and a subscriber consuming that data.

When I run them using rosrun, everything works perfectly, but when I launch each of them using roslaunch, the subscriber never receives any topics being output by the publisher.

Logging inside the publisher shows messages are being published and rosmsg list|grep -i mypackage shows my message type has been generated, but when my publisher is running and publishing, rostopic echo /mypub/data shows nothing and appears to hang.

My publisher's launch file is trivial:

<launch>
   <node name="mypub" pkg="mypackage" type="mypub_node.py" output="screen">
   </node>
</launch>

And the node seems to start up cleanly with the output:

started roslaunch server http://localhost:60558/

SUMMARY
========

PARAMETERS
 * /rosdistro: indigo
 * /rosversion: 1.11.16

NODES
  /
    mypub (mypackage/mypub_node.py)

ROS_MASTER_URI=http://localhost:11311

core service [/rosout] found
process[mypub-1]: started with pid [12609]
[DEBUG] [WallTime: 1458106329.532694] init_node, name[/mypub], pid[12609]
[DEBUG] [WallTime: 1458106329.535395] binding to 0.0.0.0 0
[DEBUG] [WallTime: 1458106329.537606] bound to 0.0.0.0 49259
[DEBUG] [WallTime: 1458106329.540770] ... service URL is rosrpc://localhost:49259
[DEBUG] [WallTime: 1458106329.542789] [/mypub/get_loggers]: new Service instance
[DEBUG] [WallTime: 1458106329.554825] ... service URL is rosrpc://localhost:49259
[DEBUG] [WallTime: 1458106329.556568] [/mypub/set_logger_level]: new Service instance
  self.packet_pub = rospy.Publisher('~data', msgs.Data)
[DEBUG] [WallTime: 1458106329.639749] ... service URL is rosrpc://localhost:49259

If I start it instead with rosrun, like:

rosrun mypackage mypub_node.py

it again seems to start cleanly, with output:

[DEBUG] [WallTime: 1458107532.209104] init_node, name[/mypub], pid[13349]
[DEBUG] [WallTime: 1458107532.211273] binding to 0.0.0.0 0
[DEBUG] [WallTime: 1458107532.212995] bound to 0.0.0.0 37078
[DEBUG] [WallTime: 1458107532.215700] ... service URL is rosrpc://localhost:37078
[DEBUG] [WallTime: 1458107532.217320] [/mypub/get_loggers]: new Service instance
[DEBUG] [WallTime: 1458107532.228779] ... service URL is rosrpc://localhost:37078
[DEBUG] [WallTime: 1458107532.230445] [/mypub/set_logger_level]: new Service instance
[DEBUG] [WallTime: 1458107532.320980] ... service URL is rosrpc://localhost:37078
[DEBUG] [WallTime: 1458107532.323380] [/mypub/packet_write]: new Service instance

and then rostopic echo /mypub/data successfully shows the messages.

What's the difference between these two calls? Why is roslaunch preventing messages from being visible outside the node?