[launch] not wiring python prints to stdout in realtime

Bug report

Required Info:

• Operating System:
  • Ubuntu 18.04
• Installation type:
  • binaries
• Version or commit hash:
  • Crystal
• DDS implementation:
  • Default
• Client library (if applicable):
  • rclpy

Steps to reproduce issue

• Put a cout equivalent of the logged message in demo_nodes_cpp/src/topics/talker.cpp
• Put a print equivalent of the logged message in demo_nodes_py/demo_nodes_py/talker.py

Add them both to a launcher:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import launch
import launch_ros.actions

def generate_launch_description():
    launch_description = launch.LaunchDescription()
    launch_description.add_action(
        launch_ros.actions.Node(
            package='demo_nodes_cpp', node_executable='talker', output='screen'),
    )
    launch_description.add_action(
        launch_ros.actions.Node(
            package='demo_nodes_py', node_executable='talker', output='screen'),
    )
    return launch_description

and call ros2 launch ... on it.

Expected behavior

Both logged messages and cout/print messages make it in realtime to stdout.

Actual behavior

All make it there except the python print statements. They only show on stdout after interrupting the launch with CTRL-C.

Start by reproducing the issue with the provided ros2 launch description, comparing demo_nodes_cpp/src/topics/talker.cpp with demo_nodes_py/demo_nodes_py/topics/talker.py. Trace how the launch command handles each process's output; done means Python print messages appear on stdout in realtime alongside the logged and C++ messages.