subprocess.popen-process stops running while using it with SMACH

Question

I'm simply trying to start a rosbag-command from python in a SMACH. I figured out that one way to do so is to use subprocesses. My goal is that as soon as the rosbag starts, the state machine transitions to state T2 (and stays there).

However, when starting a rosbag using subprocess.popen inside a SMACH-state and then using rostopic echo 'topic' , the rosbag appears to first properly publishing data, then suddenly stops publishing data and only as soon as I end the SMACH using Ctrl+C, the rosbag continues publishing some more data and before it stops as well.

Is there any reasonable explanation for that (did I maybe miss a parameter or is it just not possible to keep the node running that way)? Or is there maybe a better way to start the rosbag and let in run in the background?

(Btw also some other commands like some roslaunch-commands appear to stop working after they're started via subprocess.popen!)

My code looks as follows:

#!/usr/bin/env python3

import os
import signal
import subprocess
import smach
import smach_ros
import rospy
import time
from gnss_navigation.srv import *

class t1(smach.State):
    def __init__(self, outcomes=['successful', 'failed', 'preempted']):
        smach.State.__init__(self, outcomes)

    def execute(self, userdata):
        if self.preempt_requested():
            self.service_preempt()
            return 'preempted'
        try:
            process1 = subprocess.Popen('rosbag play /home/faps/bags/2020-05-07-11-18-18.bag', stdout=subprocess.PIPE,
                                        shell=True, preexec_fn=os.setsid)
        except Exception:
            return 'failed'
        return 'successful'

class t2(smach.State):
    def __init__(self, outcomes=['successful', 'failed', 'preempted']):
        smach.State.__init__(self, outcomes)

    def execute(self, userdata):
        #time.sleep(2)
        if self.preempt_requested():
            self.service_preempt()
            return 'preempted'
        return 'successful'


if __name__=="__main__":
    rospy.init_node('test_state_machine')

    sm_1 = smach.StateMachine(outcomes=['success', 'error', 'preempted'])
    with sm_1:
        smach.StateMachine.add('T1', t1(), transitions={'successful': 'T2', 'failed': 'error'})
        smach.StateMachine.add('T2', t2(), transitions={'successful': 'T2', 'failed': 'error', 'preempted':'preempted'})

    # Execute SMACH plan
    outcome = sm_1.execute()
    print('exit-outcome:' + outcome)
    # Wait for ctrl-c to stop the application
    rospy.spin()

Answer 1

As explained in the answer's comment section of this thread the problem appears when using subprocess.PIPE as stdout.

Therefore, the two possible solutions I used to solve the problem are:

1. If you don't care about print-outs and stuff -> use devnull as output:

   FNULL = open(os.devnull, 'w')
   process = subprocess.Popen('your command', stdout=FNULL, stderr=subprocess.STDOUT,
                                        shell=True, preexec_fn=os.setsid)
   

2. If you do need print-outs and stuff -> create a log-file and use it as output:

   log_file = open('path_to_log/log.txt', 'w')
   process = subprocess.Popen('your command', stdout=log_file, stderr=subprocess.STDOUT,
                                       shell=True, preexec_fn=os.setsid)