get_publishers_info_by_topic does not return qos depth for publishers with fastDDS

[eschembor-irobot]

Bug report

Required Info:

• Operating System:
  • Ubuntu 20.04
• Installation type:
  • From source
• Version or commit hash:
  • 5EBF27D810C6AB68EEF7D42937CD421D85E50509AE96883239979A1B3A2F4F82
• DDS implementation:
  • fastDDS
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

Noticed this when using foxglove_bridge. They compute the qos depth to use for a subscriber by summing the qos depths of all publishers on that topic. This is not working with fastDDS as get_publishers_info_by_topic reports a qos depth of 0 for any publisher.

Tracing this through the code, I see the comment here which I think indicates the source of the issue

rmw_fastrtps/rmw_fastrtps_shared_cpp/include/rmw_fastrtps_shared_cpp/qos.hpp

Line 129 in 901339f

* Since WriterQos or ReaderQos do not have information about history and depth,

Expected behavior

I would expect get_publishers_info_by_topic to report the actual qos depth of publishers

Actual behavior

get_publishers_info_by_topic reports all publishers as having a qos depth of 0

Additional information

[fujitatomoya]

technically, with rmw_fastrtps history depth is unknown, we can see this using ros2 topic info -v. rmw_fastrtps does not store the history depth from discovery information, that leads to unknown (zero). (rmw_cyclonedds can get the actual history depth, but rmw_connextdds, which tries to store the data but it is unknown.)

see details ros2/rclpy#849

to address this issue, we need Fast-DDS to get the history depth information via discovery process to store the graph.