rostopic echo of array indices fails

[trainman419]

This is part of the root cause for ros-visualization/rqt_common_plugins#261

When publishing an array message (on topic /array):

Header header
float32[] values

rostopic echo /array/values[0] fails, but rostopic echo /array/values succeeds.

This worked on Hydro, and fails on Indigo. It looks like the underlying bug here is a change somewhere in rostopic's _get_topic_type()

[trainman419]

I'm able to reproduce this with the test_pub.py node in this package: https://github.com/trainman419/test_publishers

[dirk-thomas]

Please try #481 and check if it also fixes the problem you have with rqt_plot.

[trainman419]

I'm not near my Indigo machine at the moment; I'll try to test this when I get home tonight.

[trainman419]

Confirmed; this fixes rostopic echo for array members, and rqt_plot for the same cases, although I do get some rather ugly stacktraces whlie typing that don't happen in the hydro version:

Traceback (most recent call last):
  File "/home/htpc/catkin_ws/src/rqt_common_plugins/rqt_plot/src/rqt_plot/plot_widget.py", line 208, in on_topic_edit_textChanged
    plottable, message = is_plottable(topic_name)
  File "/home/htpc/catkin_ws/src/rqt_common_plugins/rqt_plot/src/rqt_plot/plot_widget.py", line 108, in is_plottable
    fields, message = get_plot_fields(topic_name)
  File "/home/htpc/catkin_ws/src/rqt_common_plugins/rqt_plot/src/rqt_plot/plot_widget.py", line 49, in get_plot_fields
    topic_type, real_topic, _ = topic_helpers.get_topic_type(topic_name)
  File "<string>", line 421, in get_topic_type
  File "<string>", line 404, in _get_topic_type
  File "<string>", line 305, in msgevalgen
AssertionError: Topic name 'values[0' contains '[' but does not end with ']'

[dirk-thomas]

What exactly did you try when you got the stack trace? From the assertion it looks like you passed a topic name with an open bracket but without a closing one.

[trainman419]

Yep. I got that stack while typing the full topic name.

[dirk-thomas]

Can you please post a reproducible example?

[trainman419]

I used the topic entry box to interactively select the topic in question, using the test_pub.py node from my test_publishers package: https://github.com/trainman419/test_publishers

With the test_pub.py node running, run rqt_plot (no arguments), and then type the topic name in the Topic box: /array/values[0] - upon entering the [ and the 0, you should see errors on the console.

[dirk-thomas]

So rqt_plot is calling the API with an invalid topic name. Before it was silently ignored - now it prints the assertion. Do you want to modify rqt_plot to not call the API with temporary (invalid) user input? Or should I change the patch to silently ignore invalid input again?

[trainman419]

rqt_plot is calling get_topic_type to determine what is and is not a valid topic/field name. The previous version of get_topic_type handled this by returning None for invalid topics, and rqt_plot then indicated that the topic was invalid.

I don't expect to modify rqt_plot so that it doesn't call get_topic_type with an invalid topic name.

I think it makes sense to wrap the call to get_topic_type in rqt_plot with a try/catch so that this error
can be passed to the user more gracefully.

[dirk-thomas]

Sorry, but the last two paragraphs seem to contradict each other:

I don't expect to modify rqt_plot...

...wrap the call to get_topic_type in rqt_plot...

I have modified the pull request to not change API behavior - for an invalid topic it will return None as the msg eval function.