In ROS2 galactic, fixed-size Array msg type plotting broken

[mwcondino]

Title summarizes issue, but it looks like fixed-size array visualization is not working correctly.

Steps to reproduce

In one terminal window, run:

ros2 topic pub /foo sensor_msgs/msg/NavSatFix

In a separate terminal, run:

ros2 run rqt_plot rqt_plot /foo/position_covariance[0]

Observed error text:

PluginManager._load_plugin() could not load plugin "rqt_plot/Plot":
Traceback (most recent call last):
  File "/opt/ros/galactic/lib/python3.8/site-packages/qt_gui/plugin_handler.py", line 102, in load
    self._load()
  File "/opt/ros/galactic/lib/python3.8/site-packages/qt_gui/plugin_handler_direct.py", line 55, in _load
    self._plugin = self._plugin_provider.load(self._instance_id.plugin_id, self._context)
  File "/opt/ros/galactic/lib/python3.8/site-packages/qt_gui/composite_plugin_provider.py", line 72, in load
    instance = plugin_provider.load(plugin_id, plugin_context)
  File "/opt/ros/galactic/lib/python3.8/site-packages/qt_gui/composite_plugin_provider.py", line 72, in load
    instance = plugin_provider.load(plugin_id, plugin_context)
  File "/opt/ros/galactic/lib/python3.8/site-packages/rqt_gui_py/ros_py_plugin_provider.py", line 69, in load
    return super(RosPyPluginProvider, self).load(plugin_id, ros_plugin_context)
  File "/opt/ros/galactic/lib/python3.8/site-packages/qt_gui/composite_plugin_provider.py", line 72, in load
    instance = plugin_provider.load(plugin_id, plugin_context)
  File "/opt/ros/galactic/lib/python3.8/site-packages/rqt_gui/ros_plugin_provider.py", line 107, in load
    return class_ref(plugin_context)
  File "/opt/ros/galactic/lib/python3.8/site-packages/rqt_plot/plot.py", line 64, in __init__
    self._widget.switch_data_plot_widget(self._data_plot)
  File "/opt/ros/galactic/lib/python3.8/site-packages/rqt_plot/plot_widget.py", line 229, in switch_data_plot_widget
    self.add_topic(topic_name)
  File "/opt/ros/galactic/lib/python3.8/site-packages/rqt_plot/plot_widget.py", line 345, in add_topic
    for topic_name in get_plot_fields(self._node, topic_name)[0]:
  File "/opt/ros/galactic/lib/python3.8/site-packages/rqt_plot/plot_widget.py", line 125, in get_plot_fields
    if index >= current_type.maximum_size:
AttributeError: 'Array' object has no attribute 'maximum_size'

Proposed fix

Looking at https://github.com/ros2/rosidl/blob/master/rosidl_parser/rosidl_parser/definition.py#L313, it seems that both Array and BoundedSequence types in the IDL should have the size member, but only BoundedSequences have maximum_size. It looks like maximum size exclusively is used here in rqt_plot.

If we switch to using size instead of maximum_size, would this fix things?