Enforce ASCII characters on string and char fields

[hidmic]

This pull request adds ASCII codification and de-codification steps whenever dealing with string and char message fields.

Fixes ros2/ros2cli#176 (which is actually one instance of an issue that is Python support wide).

[hidmic]

@dirk-thomas I believe you're the best person to review this. Not sure how this plays with (or if it's already fixed in) the refactored IDL code.

[dirk-thomas]

The patch looks fine to me. Please run CI for the change and maybe add a test to check what happens when passing non-ASCII strings to it.

[hidmic]

Running CI (again, including rclpy test):

• Linux
• Linux-aarch64
• macOS
• Windows

[hidmic]

@dirk-thomas I think that writing a test for this is going to be somewhat involved. The conversions only take place within rclpy C extension code. I'd have to call publish on a message with a non-ascii string field to trigger a check when going from Python to C, and then take a message with a non-ascii string as sent by code that doesn't have such validations (e.g. a C++ node) to trigger a check when going from C to Python. Maybe a candidate for a test_rclpy package in system_tests? An easier alternative might be to write the test in C, using Python's C API.

[dirk-thomas]

I'd have to call publish on a message with a non-ascii string field to trigger a check when going from Python to C

That part sounds fairly straight forward?

and then take a message with a non-ascii string as sent by code that doesn't have such validations (e.g. a C++ node) to trigger a check when going from C to Python.

If the test only covers the Python-to-C case but not this one that would be sufficient for me.

[hidmic]

That part sounds fairly straight forward?

That's the easy part, yes.

If you're OK with just testing Python-to-C, so am I. But I think we could add the test in rclpy, to avoid adding it as a dependency here.

[dirk-thomas]

👍 with passing CI.

[hidmic]

CI's green now! We're good to go.