Unclear when failure occurs due to lack of `dload` shim?

[EricCousineau-TRI]

In Anzu, we had an issue where (somehow?) a node could run without a dload shim, but it would not communicate with other processes (e.g. because ROS_LOCALHOST_ONLY was a non-default value). See Anzu 9280.

There should be some way to warn about this, ideally? (not sure if via Starlark via deps analysis, runtime check, etc.)

\cc @IanTheEngineer

[EricCousineau-TRI]

Per DM w/ Shane, simple analysis can check with "@ros2//:" (in whatever project). However, this won't catch transitive deps.

[sloretz]

Reproduction in https://github.com/RobotLocomotion/drake-ros/pull/114

[EricCousineau-TRI]

My suggestion, if we were to check transitive dependencies via a rule() method, we should (ideally?) be able to do something like:

• Add a custom rule() which will take deps, and have some way to crawl through the information and see if dload shim is present (ideally, have option to check if project-specific shim is present). Not sure if we'd need C++ / Python for it.
• Add that as a simple linting rule, e.g. add_ros_target_lint_tests() (see Drake or Anzu linting for example)

Alternatively, we have some crazy non-Bazel friendly thing that uses bazel query. I don't like this option, though.

I would recommend reviewing Bazel docs on available mechanisms for querying within the analysis phase...

[sloretz]

I think this was resolved in Anzu using a Bazel aspect that looked for ROS 2 dependencies. At analysis phase it errors if ROS 2 is depended upon without using one of the ros_*() bazel rules (determined by checking if the rule target has a specific "ros" tag).

There are Anzu specific carve outs in the aspect's implementation there that make me think it wouldn't be able to use an upstreamed version of that check. Do you still want it upstreamed here?

[EricCousineau-TRI]

Hm... what aspects seem like it'd be hard to modularize? And yup, I'd like at least an example of it carved out and used in drake_ros_examples. (We find it to be a good convention, have used it for a few months, would be nice to share.)