[ros-users] questions about nodelets

[Daniel Stonier]

I think Josh answered some of your more nodelet related questions, so
I'll just respond to your roslaunch queries since I've been running
similar scenarios (1 nodelet manager, about 10 nodelets loading into
it).

On 9 November 2010 23:54, Jack O'Quin <jack.oquin at gmail.com> wrote:
> I am experimenting with using nodelets for Velodyne 3D LIDAR support
> on C-turtle. So far, converting the code has been relatively
> straightforward. But, there are some questions I can't answer.
>
>  * How do I launch a nodelet manager and several nodelets with roslaunch?
>
> The example in the tutorial seems to have no interdependencies or timing issues.
>
>  http://www.ros.org/wiki/nodelet/Tutorials/Running%20a%20nodelet#Using_withing_roslaunch_files
>
> But, when I try to launch my driver nodelet along with a point cloud
> nodelet that converts the raw scan data into ROS
> sensor_msgs/PointCloud format, I seem to get startup sequencing
> problems depending on the order in which things initialize. Sometimes
> it works and sometimes no point cloud gets published. Here is a simple
> example:
>
> <launch>
>  <node pkg="nodelet" type="nodelet" name="velodyne_nodelet_manager"
>        args="manager" />
>  <node pkg="nodelet" type="nodelet" name="driver_nodelet"
>        args="load velodyne_common/DriverNodelet velodyne_nodelet_manager" />
>  <node pkg="nodelet" type="nodelet" name="pointcloud_nodelet"
>        args="load velodyne_common/PointCloudNodelet
> velodyne_nodelet_manager" />
> </launch>
>
> I guess a "nodelet load" command sometimes arrives before the "nodelet
> manager" is ready. Since roslaunch makes no guarantees about order, I
> wonder how to solve it. I see no similar problems when dealing with
> the node-based versions of these programs.

No prioritised sequencing in roslaunch causes me a headache too,
though I've never had a situation when the nodelet manager didn't
launch before all my nodelets (I guess because its first in line in my
launcher).

I do have other sequencing problems though and the only way around it
for now that I know of is to have multiple stage launchers. That
starts getting pretty messy though. It would be more efficient and
simpler if we had a mechanism in roslaunch for this.

> Shutting down this nodelet manager process has also been unreliable.
> Sometimes the threads with go away with ^C (INTERRUPT), but they often
> hang and require a ^\ (TERMINATE). This may also be an issue with the
> order in which the signals get delivered and processed. Shutting down
> the node-based version works reliably.

That one's actually not a critical bug, but its a bit annoying having
to wait around for the terminate to arrive. Problem is the nodelets
try and unload themselves on completion, but oft times roslaunch has
killed off the manager, so they can't communicate. You can fix it by
hacking the nodelet source nodelet/src/nodelet.cpp. Cturtle and latest
have similar code (last time I checked), and down near the end in main
there's a block of code that looks like:

  else if (command == "load")
  {
    ros::init (argc, argv, arg_parser.getDefaultName (),
ros::init_options::NoSigintHandler);
    NodeletInterface ni;
    ros::NodeHandle nh;
    std::string name = ros::this_node::getName ();
    std::string type = arg_parser.getType();
    std::string manager = arg_parser.getManager();
    ni.loadNodelet (name, type, manager, arg_parser.getMyArgv ());
    signal (SIGINT, nodeletLoaderSigIntHandler);
    // Spin our own loop
    while (nh.ok ())
    {
      // ros::spinOnce ();
      if (request_shutdown)
      {
        //ni.unloadNodelet (name, manager);
        ros::shutdown ();
        break;
      }
      usleep(100000);
      sleep(2);
    }
  }

You'll notice I commented out the

//ni.unloadNodelet (name, manager);

line. It doesn't have any side effects as the manager automatically
makes sure the loaded nodelets destruct gracefully anyway.  Should
probably send in a patch for this one.