use rclcpp serialized messages to write data

[Karsten1987]

Next on the list towards a user-friendly C++ API. Sits on top of "defaults" branch.

The implementation here is pretty straightforward even though we have to iterate over the existing data structures used here at some point, namely the rosbag2_storage::SerializedBagMessage. We're dealing with shared pointers here in order to ease the cleanup of the underlying uint8 buffer. However, this prevents any stack allocations and makes it pretty cumbersome to integrate with the existing rclcpp::SerializedMessage API.

As a second point, I personally don't see a way around the rclcpp dependency at this point. Even though I'd like to keep the number of dependencies here fairly small, in order to eventually come up with a template<class MsgT> void write(MsgT & msg) API, we'd need to have that dependency.

I'll open this PR as a draft to start a discussion.

[emersonknapp]

Maybe I'm missing some context - now that rclcpp::SerializedMessage exists (and if we pull a dependency on rclcpp) - do we still need the rosbag2 SerializedBagMessage structure? Or do they provide the same functionality

[Karsten1987]

fair point. I am a big fan of making the serialized bag message a POD, essentially removing the shared pointer and only having the actual buffer pointer.

We could get around rclcpp if the rosbag2 api only introduces API which takes a serialized data buffer (uint8_t*), but yes, if we introduce rclcpp, I think it makes sense to remove the serialized bag message altogether.

[Karsten1987]

oh, also to be clear. The rclcpp dependency will only be introduced at rosbag2_cpp, not below. So we still need to keep the SerializedBagMessage struct around. It should really only be a container for the data buffer IMO.

[emersonknapp]

Makes sense. I think it's fine to add the rclcpp dependency. We were trying to limit it to just rosbag2_transport, right? However, it does seem reasonable to say "you're using a ROS2-related C++ API, it requires the C++ client library"

[ros-discourse]

This pull request has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/ros-2-tsc-meeting-minutes-2020-07-16/15468/1

[emersonknapp]

@Karsten1987 did this already get implemented in a different PR? Any need to keep this draft open?

[Karsten1987]

@emersonknapp please have a look at this. I've just finished the reader and writer API in conjunction with the rclcpp addition. The API should now look relatively close to the ROS1 cookbook snippets.

[emersonknapp]

This LGTM - it is a clean interface

[Karsten1987]

I never run CI on this:

• Linux
• Linux-aarch64
• macOS
• Windows

[Karsten1987]

• Linux
• Linux-aarch64
• macOS
• Windows