Topic remapping from YAML file

[oysstu]

Feature request

Feature description

Currently, the only way of remapping node topics is through node arguments, e.g. NodeOptions or launch arguments. This means that remapping must either change the node implementation, launch file implementation, or pass remap arguments when starting. The ability to declare the topics to be remapped together with the node parameters in the YAML file would make it possible to keep as much of the configuration in the same place.

/ns:
  node1:
    ros__remap:
      /topic/to/remap: /new/topic/path
    ros__parameters:
      foo: "bar"
      bar: "foo"

Alternative syntax where topics are not keys but an array:

/ns:
  node1:
    ros__remap:
      - from: /topic/to/remap1
        to:  /new/topic/path1
      - from: /topic/to/remap2
        to: /new/topic/path2
    ros__parameters:
      foo: "bar"
      bar: "foo"

Implementation considerations

I'm thinking the above should have the same order of precedence as the parameters if a topic is both remapped in NodeOptions, node arguments, or yaml file. There's also an ongoing discussion about dynamic/runtime remapping (#1072), but the above should just set the initial value, same as parameters.

[clalancette]

I personally think this would be a fine feature to have; my feeling is that anything we can do on the command-line we should also be able to do in a config file.

That said, getting this working is probably going to be a bunch of work. This will probably need work in rcl_yaml_param_parser, rcl, rclcpp, rclpy, launch_ros, and ros2cli to work properly (there may be more). I'm going to mark this as a "feature request", and put it on the backlog.