topic statistics doesn't publish "message_age" data if QoS is Keep last.

[hsgwa]

Description

topic statistics doesn't publish "message_age" data if subscription's QoS is Keep last.

Expected Behavior

$ ros2 topic echo /topic_statistics # in another terminal
~~
metrics_source: message_age
~~
metrics_source: message_period
~~
metrics_source: message_age
~~
metrics_source: message_period

Both message_age and message_period are published.

Actual Behavior

$ ros2 topic echo /topic_statistics # in another terminal
~~
metrics_source: message_period
~~
metrics_source: message_period

Only message_period are published.

To Reproduce

I've implemented test case.

$ cd ~/ros2_ws/
$ git clone -b qos_history_1 https://github.com/hsgwa/intra_process_ping_latency_test.git src/intra_process_ping_latency_test
$ colcon build --packages-select intra_process_ping_latency_test
$ . ~/ros2_ws/install/local_setup.bash
$ ros2 run intra_process_ping_latency_test ping_test
$ ros2 topic echo /topic_statistics # in another terminal

System

• OS: Ubuntu 20.04.1 LTS (Focal Fossa)
• ROS 2 Distro: Rolling

Additional context

As far as I read source code, this issues seems be caused by following reason.

1. topic statistic's publisher takes over subscriber's QoS.
   https://github.com/ros2/rclcpp/blob/master/rclcpp/include/rclcpp/create_subscription.hpp#L88
2. publish_message() publishes "message_age" and then "message_period" almost simultaneously.
   Here, second message (message_period) overwrites first message (message_age) if QoS depth = Keep Last.
   https://github.com/ros2/rclcpp/blob/master/rclcpp/include/rclcpp/topic_statistics/subscription_topic_statistics.hpp#L152

--

I posted this issue to libsatistics_collector because this bug is related to topic statistics, but I probably should have published it to rclcpp.
If you have any comments, I will republish this issue to rclcpp.

[emersonknapp]

Will try to reproduce today - if this is the case, it probably also affects Foxy and any fix will need to be backported