Discovery fails if daemon is started as root

Bug report

Required Info:

• Operating System: Ubuntu 20.04
• Installation type: official foxy docker image
• Version or commit hash:
• DDS implementation: rmw_fastrtps_cpp
• Client library (if applicable):

Steps to reproduce issue

Open two terminal sessions. In the first terminal run:
$ docker run -it --rm --name test ros:foxy
root@f536811845fe:/# apt update -y && apt upgrade -y
...
root@f536811845fe:/# useradd test -m
root@f536811845fe:/# sudo -u test bash
test@f536811845fe:/$  . /opt/ros/foxy/setup.bash
test@f536811845fe:/$ ros2 topic pub /test std_msgs/String "data: Hello"
publisher: beginning loop
publishing #1: std_msgs.msg.String(data='Hello')
...

In second terminal run:
$ docker exec -it test bash
root@f536811845fe:/# . /opt/ros/foxy/setup.bash
root@f536811845fe:/# ros2 topic list
/parameter_events
/rosout

Expected behavior

Topic /test should be visible on topic list

Actual behavior

Topic /test is not present.

Additional information

Even after stopping root-owned _ros2_daemon and starting a new one as “test” user, topics in existing nodes are not discovered.

This issue only appears when ros packages are upgraded to the latest version from apt repository.

[MiguelCompany]

Shared memory transport is active by default since Fast DDS v2.0.2, so I think this is related to eProsima/Fast-DDS#1750

We are currently developing a fix that should avoid this issue. It will basically detect the situation and fallback to UDP communication in that case.

[fujitatomoya]

https://github.com/eProsima/Fast-DDS/releases/tag/v2.0.2

CC: @MiguelCompany @IkerLuengo any thoughts?