Recursively Search for .msg files

[aditya2592]

We were trying to use mcap on a large ROS2 project and ran into an issue where .msg files were not being found in the install/<package_name>/share/<package_name>/msg/*.msg path. We got errors like below and the produced mcap files were empty.

[node-2] [WARN] [1668788082.046164202] [rosbag2_storage_mcap]: no .msg definition for AAA/BBB.msg, falling back to IDL

On closer look we saw that our package is structured to create .msg files in paths like install/<package_name>/share/<package_name>/<sub_directory_name>/*.msg. This is purely for organizational purposes and ROS2 doesn't have issue in finding these. So this PR modifies the MCAP plugin code to look for .msg files in any sub-directory within install/<package_name>/share/<package_name>/. It resolves our problem

[jtbandes]

Can you look at the contents of /opt/ros/<distro>/share/ament_index/resource_index/rosidl_interfaces/<pkg_name> ? What does it look like?

I suspect that using the resource index for rosidl_interfaces is the more "correct" way of doing this.

[aditya2592]

Looks like that file contains relative paths to msg and idl files. So would suggested solution be to read that file, form paths, match to desired .msg filename and check for existence?

[jtbandes]

I think so. @emersonknapp do you know more about the ament index and would you be able to validate this proposal?

[jhurliman]

We need to make this fix in foxglove_bridge as well. I created a ticket with a description of what I think is the correct way to use the ament_index_cpp library to resolve a datatype (package name plus type name) to an absolute path in the filesystem. foxglove/ros-foxglove-bridge#58

[james-rms]

This codebase is moving over to ros2/rosbag2, so i've created an issue there capturing this. Lets move discussion around what to do to that issue.