Document which QoS policies are correctly read by rmw_get_publishers/subscriptions_info_by_topic

[ivanpauno]

Based on ros2/rclcpp#1626 (comment).

[jacobperron]

LGTM

I guess some RMW implementations should probably be updated to match this documentation. In that case, this may be considered an API change and perhaps we should hold until after we branch for Galactic.

[ivanpauno]

LGTM

I guess some RMW implementations should probably be updated to match this documentation. In that case, this may be considered an API change and perhaps we should hold until after we branch for Galactic.

The rmw implementations already match this behavior, so this is only documentation.

I don't remember if they set the policies that are not updated to RMW_QOS_POLICY_*_UNKNOWN, so I can delete that line of the docs.
But I think they should be doing that, if not it's hard/impossible to understand the output.

[ivanpauno]

I don't remember if they set the policies that are not updated to RMW_QOS_POLICY_*_UNKNOWN, so I can delete that line of the docs.
But I think they should be doing that, if not it's hard/impossible to understand the output.

Fastrtps was already doing that https://github.com/ros2/rmw_fastrtps/blob/7bb3563bebd20c3cae03231c2e321bf132651449/rmw_fastrtps_shared_cpp/include/rmw_fastrtps_shared_cpp/custom_participant_info.hpp#L149.
Cyclonedds shares through discovery all qos policies.

I send a patch to connextdds to fix this ros2/rmw_connextdds#28.

[ivanpauno]

PR checker is happy, this a docs only PR so no need of more CI.