Two nodes subscribed to each other that should count do not start unless I force a publisher

edit

I assume the missing indentation in /second for bar.publish(count+1) is just formatting error since you got the nodes to work.

#!/usr/bin/env python
import rospy
from std_msgs.msg import Int32
rospy.init_node('second')
bar = rospy.Publisher('first_channel', Int32, queue_size=10)
count = 0
def callback(msg):
     count = msg.data
     print count
     bar.publish(count+1)  #EDIT: Indented to be inside callback function.

bar.publish(count)
rospy.spin()

What happens if you start /first in a terminal

$ rosrun PACKAGE_NAME first

and then start /second afterwards?

$ rosrun PACKAGE_NAME second

If you are starting both nodes together (via roslaunch perhaps), then then /second is publishing before /first starts "listening in" since ROS Subscribers take some > 0 time to register with the ROS Master.

Check out this thread for more.

Two possible solutions:
1) Use latching - this will cache the last message sent and resend it to new Subscribers that connect to the Topic.

bar = rospy.Publisher('first_channel', Int32, queue_size=10, latch=True)

2) Use the get_num_connections method - this will return the number of other connections to your topic.

#!/usr/bin/env python
import rospy
from std_msgs.msg import Int32
rospy.init_node('second')
bar = rospy.Publisher('first_channel', Int32, queue_size=10)
count = 0
def callback(msg):
     count = msg.data
     print count
     bar.publish(count+1)  #EDIT: Indented to be inside callback function.

rate = rospy.Rate(10)  # 10hz
subscriber_connected = False
while not subscriber_connected:
  num_connections = bar.get_num_connections()
  if num_connections > 0:
    subscriber_connected = True
  rate.sleep()

bar.publish(count)
rospy.spin()

You might also need similar code in /first's callback function

Sources:
http://wiki.ros.org/rospy/Overview/Pu...
http://docs.ros.org/api/rospy/html/ro...
http://docs.ros.org/api/rospy/html/ro...