gazebo_plugins: export plugin path in package.xml

[lopsided98]

gazebo_plugins does not have a gazebo_ros export in its package.xml, so it is not picked up by gazebo_ros_paths_plugin. This PR adds the necessary export tag and adds gazebo_ros as a dependency so that pluginlib processes this tag correctly.

[chapulina]

Thank you for the PR. I'm wondering how this hasn't come up before. What plugin were you trying to use when you ran into this?

[jonbinney]

Is this change actually needed? That exported path will only be correct in install space anyway, right? In devel space ${prefix} will (i think) resolve to the package's location in source space, and the path won't lead to the devel/lib folder.

I almost added something similar to a package because it suddenly stopped finding plugins, but the cause turned out to be that cmake 3.0.2 didn't prepend lib to library filenames (cmake 2.8.6 did).

[chapulina]

@jonbinney , you may be interested in this conversation: https://bitbucket.org/osrf/gazebo_tutorials/pull-requests/552/

I recently started recommending that people use env-hooks instead of exporting paths through package.xml.

[jonbinney]

@chapulina env hooks do sound better than exporting from package.xml. But in my experience you don't need either for the plugin path (at least in devel space). I think this is because the setup.sh that catkin generates adds devel/lib to your LD_LIBRARY_PATH. I think this is true for install space as well, since /opt/ros/melodic/setup.sh adds /opt/ros/melodic/lib to LD_LIBRARY_PATH.

$ . /opt/ros/melodic/setup.sh 

$ env|grep LD_LIBRARY_PATH     
LD_LIBRARY_PATH=/opt/ros/melodic/lib

$ ls /opt/ros/melodic/lib|grep "libgazebo.*\.so"
libgazebo_ros_api_plugin.so
libgazebo_ros_block_laser.so
libgazebo_ros_bumper.so
libgazebo_ros_camera.so
libgazebo_ros_camera_utils.so
libgazebo_ros_control.so
libgazebo_ros_depth_camera.so
libgazebo_ros_diff_drive.so
libgazebo_ros_elevator.so

And in gazebo it falls back to just calling dlOpen on the library name if it doesn't find anything by that name in the plugin path: https://bitbucket.org/osrf/gazebo/src/84ae5943bf02843b91a10f82ba084db0d753aee7/gazebo/common/Plugin.hh#lines-171

So i don't think this PR needed to add plugin_path="${prefix}/../../lib" in the package.xml (or do anything to env hooks). gazebo_media_path is a different story I assume.

[chapulina]

Yeah that makes sense, Gazebo will look into LD_LIBRARY_PATH too.

the setup.sh that catkin generates adds devel/lib to your LD_LIBRARY_PATH

Do you know if that's also true for catkin_tools and colcon? I'm pretty sure we need either the env-hooks or the exported paths for ROS 2. I haven't used ROS 1 much lately though.

[jonbinney]

I just did a test with catkin tools, and it does add devel/lib to LD_LIBRARY_PATH. I tried out colcon both with and without --symlink-install, and it adds install/<package_name>/lib to LD_LIBRARY_PATH for every package in the workspace.

I mean, they have to, right? Otherwise ROS nodes wouldn't be able to find the libraries they want to link against. Unless they used rpath or something to get around LD_LIBRARY_PATH completely.

[jonbinney]

Oh! I just realized who @chapulina is :-P

[chapulina]

I just realized who @chapulina is :-P

Hi there 🙋

Thanks for the quick tests. I'd have to investigate a bit more to find out why the env hooks / export paths were necessary in the first place. I'm 90% sure Gazebo won't find plugins without them.